Jabez Hogg

Microscopic Forms of Life—Thallophytes—Pteridophyta, PhanerogamÆ—Structure and Properties of the Cell.

The time has long since passed by since the value of the microscope as an instrument of scientific research might have been called in question. By its aid the foundation of mycology has been securely laid, and cryptogamic botany in particular has, during the last quarter of a century, made surprising progress in the hands of those devoted to pursuits which confer benefits upon mankind.

Little more than thirty years ago practically nothing was known of the life history of a fungus, nothing of parasitism, of infectious diseases, or even of fermentation. Our knowledge of the physiology of nutrition was in its infancy; even the significance of starches and sugars in the green plant was as yet not understood, while a number of the most important facts relating to plants and the physiology of animals were unknown and undiscovered. When we reflect on these matters, and remember that bacteria were regarded merely as curious animalculÆ, that rusts and smuts were supposed to be emanations of diseased states, and that spontaneous generation still-survived among us, some idea may be formed of the condition of cryptogamic botany and the lower forms of animal life some eight or ten years after my book on the microscope made its first appearance (1854).

Indeed, long prior to this time, dating from that of even the earliest workers with the microscope, it was known that the water of pools and ditches, and especially infusions of plants and animals of all kinds, teem with living organisms, but it was not recognised definitely that vast numbers of these microscopic living beings (and even actively moving ones) are plants, growing on and in the various solid and liquid matters examined, and as truly as visible and accepted plants grow on soil and in the air and water. Perhaps the most important discovery in the history of cryptogamic botany was initiated here. The change, then, that has come over our knowledge of microscopic plant life during this last busy quarter of a century has been almost entirely due to the initiation and improvement, first in methods of growing them, and in the methods of “Microscopic Gardening”; and secondly, to the greater knowledge gained in the use of the microscope.

“If we look at the great groups of plants from a broad point of view, it is remarkable that the fungi and the phanerogams occupy attention on quite other grounds than do the algÆ, mosses, and ferns. AlgÆ are especially a physiologist’s group, employed in questions on nutrition, reproduction, and cell division and growth; the Bryophyta and Pteridophyta are, on the other hand, the domain of the morphologist. Fungi and Phanerogams, while equally or even more employed by specialists in morphology and physiology, appeal widely to general interest on the ground of utility.

“It is very significant that a group like the fungi should have attracted so much scientific attention, and aroused so general an interest at the same time. But the fact that fungi affect our lives directly has been driven home; and whether as poisons or foods, destructive moulds or fermentation agents, parasitic mildews or disease germs, they occupy more interest than all other cryptogams put together, the flowering plants alone rivalling them in this respect. A marked feature of the period in which we live will be the great advances made in our knowledge of the uses of plants, for, of course, this development of economic botany has gone hand in hand with the progress of geological botany, the extension of our planting, and the useful applications of botany to the processes of home industries.”49

The intimate organic structure of the vegetable world is seen to consist of a variety of different materials indeterminable by unassisted vision, and for the most part requiring high magnification for their discrimination. Chemical analysis had, however, shown that vegetables are composed of a few simple substances, water, carbonic acid gas, oxygen, nitric acid, and a small portion of inorganic salts. Out of these simple elements the whole of the immense variety of substances produced by the vegetable kingdom are constructed. No part of the plant contains fewer than three of these universally distributed elements, hence the greater uniformity in their chemical constituents. It will be seen, then, that the methods of plant chemistry are of supreme interest both to the chemist and the physiologist, or biologist. Plants, while they borrow materials from the inorganic, and powers from the physical world, whereby they are enabled to pass through the several stages of germination, growth, and reproduction, could not accomplish these transformations without the all-important aid of light and heat, the combined functions of which are indispensable to the perfect development of the vegetable world.

Light, then, enables plants to decompose, change into living matter, and consolidate, the inorganic elements of carbonic acid gas, water, and ammonia, which are absorbed by the leaves and roots from the atmosphere and earth; the quantity of carbon consolidated being exactly in proportion to the intensity of the light. Nevertheless, light in its chemical character is a deoxidising agent, by which the numerous neutral compounds common to vegetables are formed. It is the principal agent in preparing the food of plants, and it is during the chemical changes spoken of that the specific heat of plants is slowly evolved, which, though generally feeble, is sometimes very sensibly evolved, especially so when flowers and fruits are forming, on account of the increase of chemical energy at this period.

The action of heat is measurable throughout the whole course of vegetable life, although its manifestations take on various forms—those suited to the period and circumstances of growth. Upon the heat generated depends the formation of protein and nitrogenous substances, which abound more directly in the seed buds, the points of the roots, and in all those organs of plants which are in the greatest state of activity. The whole chemistry of plant life, in fact, is manifest in this production of energy for drawing material from its surroundings; therefore the organising power of plants bears a direct ratio to the amount of light and heat acting upon them.

The living medium, then, which possesses the marvellous property of being primarily aroused into life and energy, and which either forms the whole or the greater portion of every plant, is in its earliest and simplest form nothing more than a microscopic cell, consisting of one or two colourless particles of matter, in closest contact, and wholly immersed in a transparent substance somewhat resembling albumen (white of egg), termed protoplasm, but differing essentially in its character and properties. This nearly colourless organisable matter is the life-blood of the cell. It is sufficiently viscid to maintain its globular form, and under high powers is seen to have a slightly consolidated film enclosing semi-transparent particles, together with vacuoles which are of a highly refractive nature. These small bodies are termed nuclei, and they appear to be furnished with an extremely delicate enveloping film. In a short time the nuclei increase in number and split up the parent body. The protoplasmic mass, however, is undoubtedly the true formative material, and is rightly regarded as “the physical basis of life” of both the vegetable and animal kingdoms.

There are, however, certain members of the vegetable kingdom which somewhat resemble animals in their dependence upon receiving organic compounds already formed for them, being themselves unable to effect the fixation of the carbon needed to effect the first stage in their after chemical transformations. Such is the case with a large class of flowering plants, among Phanerogams, and the leafless parasites which draw their support chiefly from the tissues of their hosts. It is likewise the case with regard to the whole group of fungi; the lower cryptogams, which derive the greater portion of their nutritive materials from organic matter undergoing some form of histolysis; while others belonging to this group have the power of originating decomposition by a fermentative (zymotic) action peculiarly their own. There are many other protophytes which live by absorption, and which appear to take in no solid matter, but draw nourishment from the atmosphere or the water in which they exist.

With regard to motion, this was at one time considered the distinctive attribute of animal life, but many protophytes possess a spontaneity of power and motion, while others are furnished with curious motile organs termed cilia, or whip-like appendages, flagella, by which their bodies are propelled with considerable force through the water in which they live.

Henceforth this protoplasmic substance was destined to take an important position in the physiological world. It is, then, desirable to enter somewhat more fully into the life history of so remarkable a body. It has a limiting membrane, composed of a substance somewhat allied to starch, termed cellulose, one of the group of compounds known as carbo-hydrates. The mode of formation and growth of this cell wall is not yet definitely determined; nevertheless, it is the universal framework or skeleton of the vegetable world, although it appears to play no special part in their vital functions. It merely serves the purpose of a protecting membrane to the globular body called the “primordial cell,” which permanently constitutes the living principle upon which the whole fundamental phenomena of growth and reproduction depend.

Sometimes this protoplasmic material is seen to constitute the whole plant; and so with regard to the simplest known forms of animal life—the amoeba, for example. That so simple and minute an organism should be capable of independent motion is indeed surprising. Dujardin, a French physiologist, termed this animated matter sarcode. On a closer study of the numerous forms of animal life it was found that all were alike composed of this sarcode substance, some apparently not having a cell wall. The same seemed to hold good of certain higher forms of cells, the colourless blood corpuscles for instance, which under high powers of the microscope are seen to change their shape, moving about by the streaming out of this sarcode. At length the truth dawned on histologists that the cell contents, rather than the closing wall, must be the essential structure. On further investigation it became apparent that a far closer similarity existed between vegetables and animals than was before supposed. Ultimately it was made clear that the vegetable protoplasm and the animal sarcode were one and the same structure. Max Schultz found this to be the case, and to all intents and purposes they are identical.

We have now to retrace our steps and look somewhat more closely into the discovery of that important body, the cell-nucleus. It was an English botanist, Dr. Robert Brown, who, in 1833, during his microscopical studies of the epidermis of orchids, discovered in their cells “an opaque spot,” to which soon afterwards he gave the name of nucleus. Schleiden and Schwann’s later researches led them to the conclusion that the nucleus is the most characteristic formative element in all vegetable and animal tissues in the incipient phase of existence. It then began to be taught that there is one universal principle of development for the elementary parts of all organisms, however different, and that is the formation of cells. Thus was enunciated a doctrine which was for all practical purposes absolutely new, and which opened out a wide field of further investigation for the physiologist, and led up to a fuller knowledge of the cell contents. In fact, it became a question as to whether the cell contents rather than the enclosing wall should not be considered the basis of life, since the cell at this time had by no means lost its importance, although it no longer signified the minute cavity it did when originally discovered by Schwann. It now implied, as Schultz defined it, “a small mass of viscid matter, protoplasm, endowed with the attributes of life.” The nucleus was once more restored to its original importance, and with even greater significance. In place of being a structure generated de novo from non-cellular substance, and disappearing as soon as its function of cell formation is accomplished, the nucleus is now known as the central permanent feature of every cell, and indestructible while the cell lives, and the parent, by division of its substance, of other generations of nuclei and cells. The word cell has at the same time received its final definition as “a small mass of protoplasm supplied with a nucleus.” In short, all the activities of plant and animal life are really the product of energy liberated solely through histolysis, or destructive processes, amounting to the combustion that takes place in the ultimate cells of the organisms.

But there are other points of especial interest involved in the question of cell formation beside those already mentioned.

The cell and its contents collectively are termed the endoplasm, or when coloured, as in algÆ, endochrome. With regard to the outer layer of the cell and its growth nothing satisfactory has been clearly determined and finally accepted.

The cell as a whole is a protoplasmic mass, and not an emulsion, as some observers would have us suppose. It is, in fact, a reticulated tissue of the most delicate structure, made up of canaliculate spiral fibrils with hyaline walls capable of expansion and contraction. These fibrils are probably composed of still finer spirals. The visible granulated portion of the protoplasm, the only part that takes a stain under ordinary circumstances, is simply the contents of these canals. It is the chromatin of Flemming, and is capable of motion within the canals. The nucleus, then, is probably nothing more than a granule of the extra-cellular net, and is formed by the junction of the several bands of wall-threads which traverse it in different directions. The cell wall of plants possesses the same structure as protoplasm, and is probably protoplasm impregnated by cellulose.

It is this portion of the protoplasmic mass that is now recognised under the term octoplasm, or primordial utricle, and is of so fine and delicate a nature that it is only brought into view when separated from the cell wall either by further developmental changes, or by reagents and certain stains or dyes. It was, in fact, discovered to be a slightly condensed portion of the protoplasmic layer corresponding to the octosare of the lower forms of animal life. The octoplasm and cell wall can only be distinguished from each other by chemical tests. Both nucleus and nucleoli are only rendered visible in the same way, that is, by staining for several hours in a carmine solution, and washing in a weak acetic acid solution.

With the enlargement of the cell by the imbibition of water, clear spaces, termed vacuoles, are seen to occupy a small portion of the cell, while the nucleus and nucleoli lie close to the parietal layer.

The interesting phenomenon of cyclosis, to which I shall have occasion to refer further on, is now believed to be due to the contractility of certain wall-threads stretching from the nucleus to the outermost layers of the cell. An intimate relationship is thereby established between the nucleus, the nucleolus, and the parietal layer. This much has been made clear by the more scientific methods of investigation pursued in the use of the microscope. Nevertheless a large and important class of cells, forming a kind of borderland between the vegetable and animal kingdoms, still remains to be dealt with, in which the cell contents are only imperfectly differentiated, while numerous other unicellular organisms, owing to their extreme minuteness, tenuity, and want of all colour, are apparently devoid of any nucleus, and when present can only be differentiated by a resort to a specially conducted method of preparation and staining. There is, however, a remarkable feature in connection with many micro-organisms—that certain of these protophytes possess motive organs, cilia or flagella, bodies at one time supposed to be characteristic of, and belonging to, the protozoa.

This being the case, the methods of plant chemistry are of supreme interest, the more so because physiologists are in a position to isolate a single bacterial cell, grow it in certain media, and thus devote special attention to it, and keep it for some time under observation. In this way it has become possible to further grasp facts in connection with cell nutrition and the nature of its waste products. We have, then, arrived at a stage when the history of the chemical changes brought about by bacteria can be more definitely determined, as we have here to do with the vegetable cell in its simplest form. The chemical work performed by these micro-organisms has as yet occupied only a few years; nevertheless, the results have been of the most remarkable and encouraging character.

At an earlier period an interesting discovery in connection with the pathogenic action of these bodies was, by the labours of SchÖenlein, Robin, and others, brought to the notice of the medical profession, viz., that certain diseases affecting the human body were due to vegetable parasites. In 1856 an opportunity offered itself for a thorough investigation, and the microscopical part of the work fell into my hands, with the result that I was able to add considerably to SchÖenlein’s list of parasitic skin diseases. My observations were in the first instance communicated to the medical journals. But the generalisation arrived at was that “If there be any exceptions to the law that parasites select for their sustenance the subjects of debility and decay, such exceptions are rarely to be found among the vegetations belonging to fungi, which invariably derive nutrition from matter in a state of lowered vitality, passing into degeneration, or wherein decomposition has already taken place to a certain extent.... It scarcely admits of a doubt that all diseases observed of late years among plants have been due to parasites of the same class favoured by want of vigour of growth and atmospheric conditions, and that the cause of the various murrains of which so much has been heard is also due to similar causes.”50

Herein, then, is to be found the solution of a difficulty that so long surrounded the question, but which subsequently culminated in the specialisation and scientific development of bacteriology, due to the unceasing labours of Pasteur, whose solid genius enabled him to overcome the prejudices of those who were at work on other lines, and who had no conception of the functions that parasitic organisms fulfil in nature.

Going back to my earlier experimental researches to determine the part taken by saccharomycetes and saprophytes in fermentation, I find, from correspondence in my possession, that in 1859 I demonstrated to the satisfaction of Dr. Bell, F.R.S., the then head of the chemical laboratory of Somerset House, that a very small portion of putrefactive matter taken from an animal body, a parasitic fungus (Achorion SchÖenleinii), a mould (Aspergillus or Penicillium), and a yeast (Torula cerevisiÆ) would in a short time, and indifferently, set up a ferment in sweet-wort and transform its saccharine elements into alcohol, differing only in degree (quantitative), and not in kind or quality. This, then, was the first step in the direction towards proving symbiotic action between these several parasitic organisms. The only apparent difference observed during the fermentative processes was that putrefactive (saprophytic) action commenced at a somewhat earlier stage, and that the percentage of alcohol was also somewhat less.51

In 1856, also, the Ærobic bacteria attracted my attention, and, together with the late Rev. Lord Sidney Godolphin Osborne, I exposed plates of glass (microscopical slides), covered with glycerine and grape sugar, in every conceivable place where we thought it possible to arrest micro-organisms. The result is known, viz., that fungoid bodies (moulds and bacterial) were taken in great numbers, and varying with the seasons. The air of the hospital and sick-room likewise engaged attention, each of which proved especially rich in parasitic bodies. During the cholera visitation of 1858 the air was rich in Ærobic and anÆrobic bacteria, while a blue mist which prevailed throughout the epidemic yielded a far greater number than at any former period (represented in Plate I., No. 13). This blue mist attracted the especial attention of meteorologists. At a somewhat later period a more remarkable fungoid disease, the fungus foot of India, mycetoma, came under my observation, a detailed description of which I contributed to the medical journals, and also, with further details, to the “Monthly Microscopical Journal” of 1871. Interlacing mycelia, ending in hyphÆ, in this destructive form of parasitic disease were seen to pervade the whole of the tissues of the foot, the bony structures being involved, and it was only possible to stay the action of the parasite by amputation.

So far, then, the study of parasitic organisms had at an early period shared largely in my microscopical work, extending over several years, and with the result that these micro-organisms were found to exhibit on occasions great diversity of character, and that different members of the bacteria in particular flourish under great diversity of action, and often under entirely opposite conditions; that they feed upon wholly different materials, and perform an immense variety of chemical work in the media in which they live.

The study of the chemistry (chemotaxis) of bacteria has, however, greatly enlarged our conception of the chemical value and power of the vegetable cell, while it is obvious that no more appropriate or remunerative field of study could engage the attention of the microscopist, as well as the chemist, than that of bacterial life, and which is so well calculated to enlarge our views of created organisms, whether belonging to the vegetable or animal kingdom.

Pathogenic Fungi and Moulds.

It is scarcely necessary to go back to the history of the parasitic fungi to which diseases of various kinds were early attributable. The rude microscopes of two and a half centuries ago revealed the simple fact that all decomposable substances swarmed with countless multitudes of organisms, invisible to ordinary vision. Leuwenhoek, the father of microscopy, and whose researches were generally known and accepted in 1675, tells of his discovery of extremely minute organisms in rain-water, in vegetable infusions, in saliva, and in scrapings from the teeth; further, he differentiated these living organisms by their size and form, and illustrated them by means of woodcuts; and there can be no doubt that his figures are intended to represent leptothrix filaments, vibrios, and spirilla. In other of his writings attempts are made to give an idea of the size of these “animalcules”; he described them as a thousand times smaller than a grain of sand. From his investigations a belief sprung up that malaria was produced by “animalcules,” and that the plague which visited Toulon and Marseilles in 1721 arose from a similar cause. Somewhat later on the natural history of micro-organisms was more diligently studied, and with increasing interest. MÜller, in 1786, pointed out that they had been too much given to occupy themselves in finding new organisms, he therefore devoted himself to the study of their forms and biological characters, and it was on such data he based a classification. Thus the scientific knowledge gained of these minute bodies was considerably advanced, and the subject now entered upon a new phase: the origin of micro-organisms. It further resolved itself into two rival theories—spontaneous generation, and development from pre-existing germs—the discussion over which lasted more than a century. Indeed, it only ended in 1871, when the originator of the Abiogenesis theory withdrew from the contest, and the more scientific investigations of Pasteur (1861) found general acceptance. This indefatigable worker had been investigating fermentation, and studying the so-called diseases of wines and a contagious disease which was committing ravages among silkworms. Pasteur in time was able to confirm the belief that the “muscadine disease” of silkworms was due to the presence of micro-organisms, discernible only by the microscope. The oval, shining bodies in the moth, worm, and eggs had been previously observed and described by NÄgeli and others, but it was reserved for Pasteur to show that when a silkworm whose body contained these organisms was pounded up in a mortar with water, and painted over the leaves of the tree upon which healthy worms were fed, all took the disease and died.

PLATE IX.

AFTER D^R CROOKSHANK J. T. Balcomb. del.

TYPICAL FORMS OF BACTERIA, SCHIZOMYCETES, OR FISSION-FUNGI.

As the contagious particles were transmitted to the eggs, the method adopted for preventing the spread of the disease was as follows:—Each female moth was kept separate from the others, and allowed to deposit her eggs, and after death her body was crushed up in a mortar as before, and a drop of the fluid examined under the microscope. When any trace of muscadine was found present, the whole of the eggs and body were burnt. In this way the disease was combated, and ultimately stamped out.

Pasteur also pointed out that one form or cause of disease must not be confounded with another. For example, muscadine, a true fungus (Botrytis bassiana), should not be confounded with another disease known to attack silkworms, termed pebrin, this being caused by a bacterium, and, according to the more recent researches of Balbiani, by a Psorospermia. Botrytis is a true mould, belonging to the Oomycetes, and allied to the potato fungus, Peronospora. It is propagated by spores, which, falling on a silkworm, germinate and penetrate its body. A mycelium is then developed, which spreads throughout the body. HyphÆ appear through the skin, and bear white chalky-looking spores; these become detached, and float in the air as an impalpable dust-like smoke. Damp further develops the fungus.

Insects suffer much from the ravages of fungi. The house-fly sticking to the window-pane is seen to be surrounded by the mycelia of Penicillium racemosum (Sporendonema muscÆ, or Saprolegnia ferÆ). In other cases Cordiceps attacks certain caterpillars belonging to the genera Cossus and Hepialus when they are buried in the sand and before their metamorphosis into chrysalides; they are killed by the rapid development of hyphÆ and mycelium in their tissues.

SphÆria miletaris, a parasite of Bombyx pilyocarpa, the caterpillar of which is found on pine-trees, is one of the few fungi which may be regarded as beneficial to man, since it aids in the destruction of multitudes of these caterpillars, which otherwise would devour the young shoots and pine needles. Giard specialises other parasites of insects, which he terms EntomophoreÆ. Others, E. rimosa, attack grasshoppers and the diptera, enveloping them in a dense coating of mycelium and spores, which speedily kills the victim.

The study, then, of the life-history of germs, microbes, micro-organisms, or bacteria (as they are indifferently termed), opened up a new science, that of Bacteriology. By the more recent advances in this science we are enabled to understand the very important part these minute organisms fill in the great scheme of Nature, for almost exclusively by their agency the soil is supplied with the requisite nutritive material for plant life. And, as already pointed out, wherever organic matter is present—that is, the dead and useless substances which are the refuse of life—such material is promptly seized upon by micro-organisms, by means of which histolysis is rapidly accomplished.

Bacteria require a power of from 600 to 1,000 diameters or more for the determination of the species to which they belong. The number of species has been so much increased of late that a bulky volume is found to be insufficient for their enumeration. I am, however, by the courtesy of Professor Crookshank, enabled to present my readers with the typical forms of thirty-nine species of Bacteria, Schizomycetes, or fission-fungi, a selection, it will be seen, chiefly taken from among pathogenic organisms—those believed to originate disease. But many of the supposed Saprophytic forms often described as originating disease are merely accidental associates, that is, living in companionship for a time.

Size.—In ordinary terms of measurement, bacteria are on an average from ¹/₂₅₀₀₀th to about ¹/₅₀₀₀th of an inch long. These measurements do not convey a definite impression to the mind. It is calculated that a thousand million of them could be contained in a space of ¹/₂₅th of an inch. The best impression of the size of the bacteria is, perhaps, obtained when it is stated that a ¹/₂₅-inch immersion objective gives a magnification of nearly 2,200 diameters, and that under this power the bacteria appear to be about the size of very small print. The standard of measurement accepted by bacteriologists is the micro-millimeter. One millimeter is equal to about ¹/₂₅₀₀₀th an English inch. The number of micrococci in a milligramme of a culture of Staphylococcus pyogenes aurens has been estimated by Bujwid by counting at eight thousand millions. Not only do various species differ in dimensions, but considerable differences may be noted in a pure culture of the same species. On the other hand, there are numerous species which so closely resemble each other in size and shape that they cannot be differentiated by microscopic examination alone, and we have to look to other characteristics, as colour, growth in various culture media, pathogenic power, chemical products, &c., in order to decide the question of identity.

Reproduction.—The reproduction of bacteria takes place for the most part by fission and by spore formation. Fission is a process of splitting up or division, whereby an organism divides into two or more parts, each of which lives and divides in its turn. If certain organisms are watched under the microscope, a coccus or bacillus will be seen to elongate and at the same time become narrower, until its two halves become free, the two individual organisms again dividing and subdividing in their turn. This kind of reproduction is more readily seen in a higher class of unicellular organisms, the desmids. If, however, the new organisms do not break away from each other, but remain connected in groups or clusters, they are termed Staphylococci; if they remain connected in the form of a chain, or like a string of beads, they are termed Streptococci. If the division takes place in one plane, Diplococci are formed; if in two directions Tetracocci, or Tablet-cocci, are formed. On account of this multiplication by fission, the generic name of Schizomycetes, or fission-fungi, has been given to bacteria.

Spores.—A second method by which bacteria propagate is by spores. These bodies are distinguished by their remarkable power of resistance to the influence of temperature and the action of chemical reagents. Some of them will resist their immersion in strong acid solutions for many hours; also freezing and very high temperatures. Spore formation may take place in two ways: firstly, by “endogenous spores” (internal spores); secondly, by “arthrospores.”

Endogenous Spores.—When the formation of the spores takes place in the mother-cell, the protoplasm is seen to contract, giving rise to one or more highly refractive bodies, which are the spores. The enclosing membrane of the organism then breaks away, leaving the spores free.

Arthrospores.—When the spore is not formed in the parent bacillus, but when entire cells (owing to lack of favourable conditions of growth) become converted into spores, the formation is known as “arthrogenous,” the single individual being called an arthrospore. When the conditions are again favourable, spores germinate, giving rise to new bacilli. The germinating spore becomes elongated, and loses its bright appearance, the outer membrane becomes ruptured, and the young bacillus is set free. Certain conditions, such as the presence of oxygen in the case of the anthrax bacillus, give rise to the formation of spores; while various kinds of bacteria secure continuous existence by developing spores when there is lack of proper food material.

With reference to the incredible rapidity with which the bacteria multiply under conditions favourable to the growth and development, Cohn writes as follows:—“Let us assume that a microbe divides into two within an hour, then again into eight in the third hour, and so on. The number of microbes thus produced in twenty-four hours would exceed sixteen and a half millions; in two days they would increase to forty-seven trillions; and in a week the number expressing them would be made up of fifty-one figures. At the end of twenty-four hours the microbes descended from a single individual would occupy ¹/₄₀th of a hollow cube, with edges ¹/₂₅th of an inch long, but at the end of the following day would fill a space of twenty-seven cubic inches, and in less than five days their volume would equal that of the entire ocean.”

Again, Cohn estimated that a single bacillus weighs about 0·000,000,000,024,243,672 of a grain; forty thousand millions, 1 grain; 289 billions, 1 pound. After twenty-four hours the descendants from a single bacillus would weigh ¹/₂₆₆₆th of a grain; after two days, over a pound; after three days, sixteen and a half million pounds, or 7,366 tons. It is quite unneccessary to state that these figures are purely theoretical, and could only be realised if there were no impediment to such rapid increase.

Fortunately, however, various checks, such as lack of food and unfavourable physical conditions, intervene to prevent unmanageable multiplication of these bodies.

These figures show, however, what a tremendous vital activity micro-organisms do possess, and it will be seen later at what great speed they increase in water, milk, broth, and other suitable media.

The following bacilli, among others, have numerous flagella distributed over the whole of the organism: the bacillus of blue milk (Bacillus cyanogenus)52; the bacillus of malignant oedema; the hay bacillus (Bacillus subtilis); Proteus vulgaris, &c.

The following have only one or two flagella at the poles: the Bacillus pyocyaneus, the Spirillum finkleri, the Spirillum cholerÆ AsiaticÆ, &c.

The Spirillum undala, Spirillum rubrum, Spirillum concentricum, and SarcinÆ, pocket-cocci, have several flagella.

Micrococcus agilis have also several flagella; these possibly arise from one point. As I have already pointed out, the classification of the bacteria is one of great difficulty, since new kinds are being constantly discovered, and at present any attempt made in this direction can only be considered as quite of a provisional nature.

The difficulties which stand in the way may be surmised from the fact that SarcinÆ, pocket-cocci, were originally believed to be a single species, described by me, under the name of Sarcina ventriculi, in the fourth edition of my book, “as remarkable bodies invading the human and animal stomach, and seriously interfering with its functions.”

Fig. 268.—SarcinÆ.

The original woodcut of these curious parasites is reproduced in Fig. 268, also in Plate IX., No. 7, and which evidently belong to a different species, numbering thirty-nine altogether. Quite recently Mr. G. H. Broadbent, M.R.C.S., Manchester, sent me a supply of these interesting bodies lately discovered by him in an infusion of cow manure. On examining a drop with a power of 1500 diameters they were discovered moving over the field of the microscope with a gyrating motion by the aid of flagella projecting from each corner of the pocket. After some days, having attained their full growth of four, eight or sixteen in a pocket, they break up, and recommence the formative process. SarcinÆ are certainly pathogenic in their nature. Cocci in groups, or asso-cocci, are similarly associated. These several forms of spiro-bacteria are enclosed in a transparent cell-wall, and are sometimes described as zooglÆa.

Of bacteria the most characteristic groups are bacillus, bacterium, and a species of clostridium, a bottle-shaped bacillus. It is, however, difficult to draw a sharp line between so-called species.

Spiro-bacteria, or spirilla, possess short or long filaments, rigid or flexible, and their movements are accordingly rotatory, or in the long axis of the filaments. These bodies are again divided into comma bacilli, or vibrios—a name invented by the older microscopists who first described them—some species of which have a flagellate appendage, to which their movements are due.

Anthrax, Splenic Fever, has been long known to be prevalent among cattle at certain seasons of the year, and is believed to originate from peculiar conditions of climate and soil. This view of splenic fever on microscopical examination proved an entire fallacy. Bollinger in 1872 discovered that the blood of the affected animal was still virulent after death, owing to the presence of the spores of the bacillus, and that the soil also became infected and impregnated by the disease germs wherever the fever first broke out. In 1877 Dr. Koch made a more careful investigation into the source of the disease, and was able to give a complete demonstration of the life-history of the splenic fever bacillus, and to offer definite proofs of its pathogenic properties. He pointed out that the rods grew in the blood and tissues by lengthening and by cross division. Further, that they not only grew into long leptothrix filaments but they produced enormous numbers of seeds or spores. He watched the fusion of the rods to the formation of spores and the sprouting of fresh rods. He furthermore inoculated a mouse, watched the effect through several generations, and fully demonstrated that in the blood and swollen spleen of the animal the same rods were always present. Pasteur and Paul Bret pursued the same course of investigations, which were always followed with precisely similar results. It was, however, principally due to the researches of Koch that the doctrine of contagium vivum was placed on a scientific basis.

Subsequently Koch formulated methods of cultivation, and dictated the microscopical apparatus needful. Furthermore, he furnished postulates for proving beyond doubt the existence of specific pathogenic micro-organisms.

“The chain of evidence regarded by Dr. Koch as essential for proving the existence of a pathogenic organism is as follows:—1. The micro-organism must be found in the blood, lymph, or diseased tissue of man or animal suffering from, or dead of the disease. 2. The micro-organism must be isolated from the blood or tissue, and cultivated in suitable media—i.e., outside the animal body. These pure cultivations must be carried on through successive generations of the organism. 3. Pure cultivation thus obtained must, when introduced into the body of a healthy animal, produce the disease in question. 4. In the inoculated animal the same micro-organism must again be found. The chain of evidence will be still more complete if, from artificial culture, a chemical substance is obtained capable of producing the disease quite independently of the living organism. It is not enough to merely detect, or even artificially cultivate, a bacterium associated with disease. An endeavour must be made to establish the exact relationship of the bacteria to disease processes. In many instances disease bacteria regarded as the actual contagia have been found, on a further searching inquiry, to be entirely misleading. It is almost needless to remind the enthusiast that the actual contagion of the disease must be fully demonstrated.”

Fig. 269.—Micro-Photograph of Typhoid Fever Bacteria. Magnified 1000 ×. Taken by Leitz’s oil immersion ¹/₁₂-inch ocular No. 4, and sunlight exposure of one minute.

Typhoid Bacillus (Fig. 269).—Rods 1 to 3µ in length, and ·5 to ·8µ in breadth, and threads. Spore-formation has not been observed, but the protoplasm may be broken up, producing appearances which may be mistaken for spores. Actively motile, provided, some with a single and others with very numerous flagella, which are from three to five times as long as the bacillus itself. They stain readily in aqueous solutions of aniline dyes; and grow rapidly at a temperature of about 60° Fahr. In plate cultivations minute colonies are visible in thirty-six to forty-eight hours; they are circular or oval, with an irregular margin. On agar they form a whitish transparent layer, and they flourish in milk.

Fig. 270.—Plague Bacillus, Bombay, 1897. Magnified 1200 ×.

The Plague (Pestis Bacillus).—The Bombay plague of 1897-98 will ever be remembered as one of the most appalling visitations ever known. The number of deaths will never be accurately determined, as the native population, among whom the disease chiefly prevailed and became so fatal, concealed their dead or carried them away by night. The outbreak from the first proved to be most infectious, its incubation lasting from a few hours to a week only. It prevailed in all the over-crowded native quarters of the city. The rats and mice that infested the dwellings of the poor were found to be equally susceptible with human beings, and these vermin also died by hundreds. Those that survived left their holes and made off, in this way helping to spread the infective virus. On examining the bodies of dead rats, they were found to have swollen legs, the blood being filled by bacilli and curious monads, with whip-like appendages. The bacillus of plague was discovered by Kitasato in 1894; it is characterised by short rods with rounded ends, and a clear space in the middle. The bacilli stain readily with aniline dyes, and when cultivated on agar, white transparent colonies are formed which present an iridescent appearance when examined by reflected light. In addition to the bubonic swellings, the neighbouring lymphatic glands were also swollen and blocked by bacilli.

Fig. 271.—Monads in Rat’s Blood, 1,200 ×. (Crookshank.)

a. Monad threading its way among the blood-corpuscles; b. Another with pendulum movement attached to a corpuscle; c. Angular forms; d. Encysted forms; e and f. The same seen edgeways.

My illustration (Fig. 270) is from a micro-photograph taken in 1897, when the death rate stood very high. The general distribution of the bacilli, together with phagocytes and the contents of swollen lymphatic glands, magnified 1,200 ×, is from a preparation made in hospital. The monads from the rat’s blood, 1200 ×, seen threading their way among the blood corpuscles of a rat, and represented in Fig. 271, are somewhat larger than those found in the Bombay rats, but the flagella in the latter were quite as marked, while the encysted forms were wholly absent and the blood corpuscles less crenated. The white bodies (Fig. 270) were in some preparations, together with the lymphatic bodies, more numerous and more swollen.

With regard to the conditions of life of the bacteria, they may be divided broadly into two classes. When the organisms draw their nourishment from some living body or “host,” they are known as “parasites.” These are further termed “obligate” parasites if they exclusively live on their “host.” If the bacteria draw their nourishment from dead organic matter, they are called “saprophytes.” These are also divided into “obligate” and “facultative” saprophytes. Thus it will be apparent that a parasite under certain circumstances may readily become a saprophyte.

Some of the more important saprophytes are those organisms which play an important and useful part in our every-day life, such as, for instance, in the phenomena associated with fermentation, and putrefaction agents which transform dead and decomposing organic matter into their simpler elements, thus completing the great life cycle, and rendering the dead and effete matter again ready for the vital processes.

Among other life manifestations of certain bacteria may be mentioned those which have the property of generating colouring matter, though not chlorophyll. The bacteria themselves are colourless and transparent, and the pigment is merely formed as a product of their metabolism, especially under the influence of light. Many of the bacteria give rise to various gases and odours, particularly the anÆrobic organisms, which originate those foul putrefactive gases (ammonia, sulphuretted hydrogen, &c.). The blood-rain, Micrococcus prodigiosus, gives off an odour resembling trimethylamin. Micro-organisms have the property of producing various changes in the medium on which they are grown. In many cases albuminous bodies are peptonized and gelatine is liquefied. Many bacteria have the faculty of resolving organic bodies into their simplest elements; others, again, have the property of converting ammonia into nitric and nitrous acid. Certain microbes have the property of becoming phosphorescent in the dark. These phosphorescent bacteria are often seen on decaying plants and wood; sometimes in tropical climates the sea becomes luminous owing to the presence of countless numbers of these organisms. Again, they are frequently seen on the surface of dead fish, particularly mackerel, which often become so bright as to strongly illuminate the cupboard in which they lie.

The particular class of fungi that produce disease in man and the higher animals are generally known as “pathogenic.” These pathogenic organisms may exert their pernicious power in several ways. They may be injurious on account of their abstracting nourishment from the blood or tissues, or for the purely mechanical reason of their stopping up the minute capillaries and blood-vessels by their excessive multiplication. But the poisonous action of most of the pathogenic bacteria is due to the chemical products secreted by the organisms, and it is to the circulation and absorption within the body of these poisons that the disturbances of the animal system, which characterise disease, decay, and dissolution of every organism, must be traced.

Parasitic Diseases of Plants.

The subject of fungoid diseases and fungus epidemics are of worldwide interest, if only because of the annual losses to agriculturists from parasitic diseases of plants, amounting to millions of pounds sterling. The history of wheat-rust, and that of oats and rye, each equally susceptible to the ravages of the same Rufus, can be traced back to Genesis. A description of it was given in 1805 by Sir Joseph Banks. He suggested that the germs entered the stomata, and he warned farmers against the use of rusted litter, and made important experiments on the sowing of rusted wheat-grains. A great discussion on the barberry question followed, Fries particularly insisting on the difference between Æcidium berberidis and Puccinia graminis. Tulasne confirmed the statement made by Henslow that the uredo and puccinia stages belong to the same fungus, and are not mixed species. De Bary’s investigations in 1860-64 proved that the sporidia of some UredinieÆ (e.g., Coleosporium) will not infect the plant which bears the spores, and that the Æcidia of certain other forms are stages in the life-history of species of Uromyces and Puccinia. Furthermore, De Bary in 1864 attacked the question of wheat rust, and by means of numerous sowings of the telentospores on barberry proved that they bring about the infection.

This led to the discovery of the phenomenon of Heteroecism (colonisation), introducing a new idea, and clearing up many difficulties. In 1890 the rust question entered on a new phase: it was taken up by men of science all over the world, and active inquiries were set on foot. The result has been the confirmation of De Bary’s results, but with the further discovery that our four common cereals are attacked by no less than ten different forms of rust belonging to five separate species or “form species,” and with several physiological varieties, capable of turning the table upon the barberry by infecting it. Some of these are found to be strictly confined to one or other of the four common cereals, infecting two or more of them, while others can infect various kinds of our common wild grasses.

Fig. 272.—Puccinia, displaying uredospores and telentospores.

a. Aregma speciosum; b. Xenodochus paradoxus; c. P. AmorphÆ; d. Triphoemium dubens; e. Younger spores; f. P. lateripes; magnified 450 diameters.

The fact is, that what has usually gone by the name of Puccinia graminis is an aggregate of several species, and that varietal forms of this exist so especially adapted to the host, that, although no morphical differences can be detected between them, they cannot be transferred from one cereal to another, pointing to physiological variations of a kind met with among bacteria and yeasts, but hitherto unsuspected in these higher parasitic fungi. It now appears we must be prepared for similar specialisation of varietal forms among UstilagineÆ as well as among UredineÆ.

Moreover, it has been found that different sorts of wheat, oats, barley, and rye are susceptible to their particular rusts in different degrees, at the bottom of which, it is suggested, there must be some complex physiological causes. De Bary gave proof, in 1886, that Peziza (Plate I., Nos. 1, 4, 5, 6) succeeds in becoming parasitic only after saprophytic culture to a strong mycelium, and that its form is altered thereby—probably by the excretion of a poison. Professor Marshall Ward showed that similar results took place in the case of the lily disease. Reinhardt, in 1892, showed that the apical growth of a peziza is disturbed and interrupted if the culture solution is employed concentrated; and BÜsgen, in 1893, showed that Botrytis cinerea excretes poison at the tips of the hyphÆ, thus confirming Professor Ward’s results with the lily disease in 1888, and of later years, that a similar excretion occurs in rust-fungus. He further found that the water contents of the infected plant exercises an influence, as in the case of Botrytis attacking chrysanthemums and other plants in the autumn, and that cold increases the germinating capacity of the spores.

Pfeiffer, in his work on “Chemotaxis,” shows that bacteria will congregate in the neighbourhood of an algal cell evolving oxygen. He also found that many motile antherozoids, zoospores, bacteria, &c., when free to move in a liquid, are attracted towards a point whence a given chemical substance is diffusing. He was concerning himself less with the evolution of oxygen or movements of bacteria than with a fundamental question of stimulation to movement in general. He found the attractive power of different chemical substances vary with the organism, and that various other bodies beside oxygen attract bacteria—peptone, dextrose, potassium salts, &c.; that swarm spores of the fungus Saprolegnia are powerfully attracted towards the muscles of a fly’s leg placed in the water in which they are swimming about; also, that in many cases where the hyphÆ of fungi suddenly and sharply bend out of their original course to enter the body of a plant or animal, the cause of the bending lies in a powerful chemotropic action, due to the attraction of some substance escaping from the body. Professor Ward has seen zoospores of a Pythium suddenly dart out on to the cut surface of a bean-stem, and there fix themselves.

This will be better understood by referring to the course pursued by these bodies generally. When the spore of a parasitic fungus settles on a plant, it frequently behaves as follows:—The spore germinates and forms a slender tube of delicate consistency, blunt at the end, and containing colourless protoplasm, as shown, highly magnified in Fig. 272, and in Figs. 273 and 274 much less magnified. De Bary long ago showed that such a tube—the germinal-hypha—only grows for a short time along the surface of the organ, and its tip soon bends down and enters the plant, either through one of the stomata or by boring its way directly through the cell-walls. Professor Ward says these phenomena suggested to himself that the end of the tube is attracted in some way, and by some force which brings its tip out of the previous direction, and De Bary has suggested that this attraction is due to some chemical substance excreted by the host plant. It is remarkable with what ease the tube penetrates the cell-walls, and which Ward believes to be due to the solvent action of an enzyme, capable of dissolving cellulose.

“Miyoshi carried these observations a step further when, in 1894, he showed that if a leaf is injected with a substance such as ammonium-chloride, dextrine, or cane-sugar (all substances capable of exerting chemotropic attraction on fungus-hyphÆ), and spores of a fungus which is not parasitic are then sown on it, the hyphÆ of the fungus penetrate the stomata and behave exactly as if the fungus were a true parasite.

“So surprising a result lets in a flood of light on many known cases of fungi, which are, as a rule, non-parasitic, becoming so, in fact, only when the host plant is in an abnormal condition, e.g., the entry of species of Botrytis into living tissues when the weather is cold and damp and the light dull; the entry of Mucor into various fruits, tomatoes, apples, pears, &c., when the hyphÆ meet with a slight crack or wound, through which the juices are exposed. It is exceedingly probable that the rapid infection of potato leaves in damp weather in July is traceable not merely to the favouring effect of the moisture on the fungus, but that the state of super-saturation of the cell-walls of the potato leaf—the tissues of which are now unduly filled with water and dissolved sugars, &c., owing to the dull light and diminished transpiration—is the primary factor which determines the easy victory of the parasite, and, as Professor Ward suggested some time ago, that the suppressed life of UstilagineÆ in the stems of grasses is due to the want of particular carbo-hydrates in the vegetative tissues, but which are present in the grain. A year later Miyoshi carried proof to demonstration, and showed that a fungus-hypha is actually so attracted by substances on the other side of a membrane, and that its tip pierces the latter; for the hyphÆ were made to grow through films of artificial cellulose, of collodion, of cellulose impregnated with paraffin, of parchment paper, and even the chitinous coat of an insect, simply by placing the intact films on gelatine impregnated with the attracting substance, and laying the spores on the opposite side of the membrane.

“Now this is obviously a point of the highest importance in the theory of parasitism and parasitic diseases, because it suggests at once that in the varying conditions of the cells, the contents of which are separated only by membranous walls from the fungus-hyphÆ, whose entrance means ruin and destruction, there may be found circumstances which sometimes favour and sometimes disfavour the entrance of the hyphÆ; and it is, at least, a remarkable fact that some of the substances which experiments prove to be highly attractive to such hyphÆ—e.g., sugars, the sap of plums, phosphates, nitrates, &c.—are just the substances found in plants; and the discovery that the action depends upon the nature of the substance as well as on the kind of fungus, and is affected by its concentration, the temperature, and other circumstances, only confirms us in this idea.”

Moreover, there is one other fact which it is important to notice, viz., that there are substances which repel instead of attract the hyphÆ. Is it not, then, asks Professor Ward, natural to conclude that the differences in behaviour of different parasites towards different host-plants, and towards the same host-plant under different conditions, probably depend on the chemotropic irritability of the hyphÆ towards the substance formed in the cells on the other side of the membranous cell-walls? And when, as often happens, the effusion of substances, such as the cells contain, to the exterior is facilitated by over-distension and super-saturation, or by actual wounds, we cannot be surprised at the consequences when a fungus, hitherto unable to enter the plant, suddenly does so. To this proposition my answer is emphatically in the affirmative, since in my investigations into the “fungus-foot disease” (“Mycetoma”), 1871, of India, the entry of the fungus was in almost every case shown to be through an abrasion of the skin or a direct open wound; the majority of the cases reported were among the agricultural classes. When, then, as often happens, the effusion of substances, such as the cells contain, to the exterior is facilitated by over-distension and super-saturation, or by actual wounds, we cannot be surprised at the consequences when a fungus, hitherto unable to enter the plant, suddenly does so. Nevertheless, it must be admitted that the knowledge gained of parasites does not satisfactorily account for epidemic visitations over large areas.

Habitat of Fungi and Moulds.

Fig. 273.—Fungi and Moulds.

Description of Figures.—a. Fungi Spores, taken in a sick chamber; b. Aspergillus glaucus; c. Yeast, recent state; d. Exhausted yeast, budding; e. Penicillium spores more highly magnified; g. Aerobic spores and mould mycelium; h. Aspergillus spore, grown on melon.

Habitat, Specialised Forms of Parasites.

Habitat.—The habitat of vegetable parasitic fungi is extremely variable. Fungi are found everywhere, living and flourishing on all the families of the vegetable and animal kingdoms. They attack our houses, foods, clothes, utensils of every kind, wall papers and books, the paste of which, to my astonishment, affords a sufficient supply of nourishment. Members of the parasitic tribe of bacteria, by a combined effort of countless myriads, have given rise to a sense of supernatural agency. Bacillus prodigiosus, described also as Palmella mirifica and Zoogalactina imetropia, from its attacking milk and other alimentary substances, the spores of which are often of a deep red colour, have been found to cover whole tracts of country in a single night with what is called a “gory dew,” changing in daylight to a deep green colour. This was at one time regarded with superstitious awe as a miracle, as it has been known to attack bread and even the sacred wafer, and which in mediÆval ages was described as the “bleeding-host.” This parasitic plant belongs to anÆrobic bacteria, and is only developed in the dark. The nitrogen required for nutrition must be derived from the air. An algal form gives rise to the red scum seen in ponds and reservoirs in the autumn. The discharge from wounds is coloured blue by Bacterium pyocyanine. There are many other forms, some of which have an orange colour, and the genus is recognised as “chromogenic microbes.”

Fig. 274.—Fungi and Moulds.

Description of Figures.—d. Puccinia graminis on wheat; c. Polycystis spore of rye-smut; f. Alder fungus spores, Microspheria penicellula; g. Dactylium roseum, rose-coloured mould; h. Verticillium distans, whorled mould found on herbaceous plants; i. Botrytis, vine and lily fungus; j, j'. Peronospora infestans, potato fungus; k. P. gangliformis, mould of herbaceous plants; l. Various Penicillium and other spores taken in a bean-field.

A cryptogam belonging to anÆrobic bacteria, described as Protococcus invalis, on being set aside in a bottle, and a little rain water added, was seen to set up spontaneous fermentation, and in a very short time exhibited remarkable activity. The colour of the infusion changed, it assumed a delicate pink hue in direct light, which deepened to a red in reflected light. The fluid contents were now observed to be dichoric, and the spectroscopic appearance subsequently presented was one of much interest. The spectrum was a well-marked one, and might be taken to determine the presence of a nitrogenous element or of glucose.

Among all the various plants known to suffer from the attacks of parasites, the vine has been the greatest sufferer. The oÏdium, or Erysiphe Tuckeri, so called from the name of the discoverer by whom it was first described, has been longest known to the vine grower. This really belongs to the group Ascomycetes, and appears to have been brought from America in 1845, whence it was passed on to France, where it soon threatened to entirely destroy the vineyards. This was followed by another parasite, belonging in this instance to the animal kingdom, Phylloxera vastatrix. This oÏdium appears on the grape in the form of greyish filaments, terminating in an enlarged head, which contains an agglomeration of spores, not free or in a chaplet, as in Aspergillus (Fig. 273). These spores when ripe burst from the capsule as fine dust, and are diffused by the air in all directions, thus spreading the disease far and away. Another of the parasitic moulds, Peronospora viticola, is a kind of mildew, differing from oÏdium. The hyphÆ penetrate more deeply than that of oÏdium. On the upper surface of the leaf brown patches appear; these branch out and ramify as seen in the potato-fungus, P. infestans (Fig. 274). The parasite destroys the tissue of the leaf, and it withers and dies. There are other well-known parasites, the black-rot, Phomauvicola, belonging to the Ascomycetes. This appears in early shoots in the form of round black spots, and gradually spreads over leaves and young fruit. This same rot, one year, devastated the American vineyards.

Fig. 275.—Fungi, Moulds.

a. Clustered Spores, Gonatobotrys simplex; b. Spore of Puccinia coronata, the mildew of grapes; c. Barley smut; d. Puccinia althÆa; e. Penicillium glaucum; m. Ixodes farinÆ, found in damaged flour together with smut.

Cereals, wheats and grasses, suffer from other well-known forms of microscopic fungi termed rusts and smuts, which cover the blades or infect the full ear of the fruit. The name given indicates their colour, and these belong, for the most part, to the genus Uredo and the family of the Basidiomycetes. They have no endogenous spores but as many as four forms of exogenous. This is also the case with wheat and barley, whereby they are distinguished as Uredo or Puccinia graminis (see Figs. 273 and 274, and Plate I., Nos. 19 and 22, Æcidium berberidis). For a long time it was believed that Uredo linearis and Puccinia graminis were so many distinct species, but it is now known that there are only three successive phases of the developmental stages of a single species—that, as a matter of fact, puccinia presents the phenomenon of alternation of generations, that is, that the complete development of the fungus is only effected by its transference from one plant to another. Other uredines, Ustilago and Tilletia smuts, are more apt to affect the ears of wheat, rye, and other grasses than puccinia. Bread made from wheat affected by smut has an acrid and bitter taste, while that made from rye flour often produces a serious form of disease. The propagation of either, then, should be stopped as quickly as possible by destroying all barberry bushes growing near or within the vicinity of corn fields, and by other means. The ergot of rye is due to distinct species of fungi having endogenous spores enclosed in a sac or ascus, hence the name of the family, Ascomycetes or TuberaceÆ, which are reproduced by the spores contained in these asci. Truffles belong to this family. But other members of the same family have several forms of spores, and these again present us with the phenomenon of alternation of generations.

Fig. 276.—Fungi, Moulds.

p. Spores of Tilletia caries; q. Spores of Tilletia caries, when germinating, produce a foetid olive-coloured spore in cereal grains; r. Telentospores of Puccinia graminis; s. Crystopus candidus, spores growing in chains; t. Petronospora infestans, mildew of turnips, &c.; u. A transverse section of ergot of rye, showing spores in masses; v. Claviceps purpurÆ, associated with ergoted rye.

Ergot of rye is used in medicine, but if not used with care it will produce a dangerous disease. This parasitic fungi consists of minute microscopic masses of spores, which cover the young flower of the rye with a white flocculent mass, formerly termed sphacelium. The mycelium formed spreads over the ear of corn in thick felt-like masses, termed sclerotis. The sphacelium changes its form in the following spring. Other changes are brought about, and it seems to pass through a cycle of alternations of generations.

Bread made from rye so infested is known to produce grave consequences, soon to become fatal if not detected in time. The disease is termed ergotism, and gangrene of the extremities takes place among people of the north of France and Russia, who consume bread made from rye flour. Ergot of maize will also cause similar diseases. Fowls and other animals fed upon this cereal become in a short time poisoned, and the cause of death is not rightly suspected. There is another fungus belonging to the same group of Ascomycetes, known as Eurotium repens, which appears upon leather when left in a damp place, and also upon vegetable or animal substances if badly preserved, and gradually destroys it. This mould is of a darkish green colour.

The minute spores display themselves as rows of beads when fully ripe on the erect mycelium. Aspergillus glaucus represents the white exogenous spores of the sphacelium of the ergot of rye; and those subsequently produced in the yellow balls correspond with the asci developed in sclerotis, the endogenous species. Many of the parasitic species belonging to the genera Erysiphe, SphÆria, Sordaria, Penicillium, &c., have a similar mode of propagation, and affect a large number of plants.

Parasitic Fungi of Men and Animals.

In the microscopical examinations especially given to the elucidation of parasitic diseases of the skin, previously referred to, I discovered more varieties of spores and filaments of certain cryptogamic plants associated with a larger number of specific forms of fungi than any previous observer. I did not, however, feel justified in concluding, with KÜchenmeister, Schoenlein, and Robin, that these fungoid growths were the primary cause of the diseases referred to. Indeed, the foremost dermatologists of the period utterly refused to entertain the specific germ theory of the German investigators. Nevertheless, I contended, “the universality of their distribution is in itself a fact of very considerable importance, and one pointing to the belief that they are scavengers ever ready to fasten on decaying matter, and, on finding a suitable soil, spread out their invisible filaments in every direction in so persistent a manner as to arrest growth and overwhelm the plant in destruction.”53

Special forms of fungi are given in Plate I., Nos. 10-14, and those of the ascomycetes in Nos. 17-21.

Fig. 277.—Healthy fresh Yeast, from a large Brewery, in an active stage of formation, × 400.

OÏdium albicans affects both animals and plants. It often attacks the mucous membrane of the mouths of young children. The spores become elongated and converted into hyphÆ, and ramify about in all directions, producing a troublesome form of disease. This parasitic fungus is better known under another name, Saccharomyces mycoderma. OÏdium resemble algÆ in their mode of life, as they are mostly found in a liquid media. The structure of all ferments is very simple: each plant is composed of a single cell, either of a spherical, elliptical, or cylindrical form, varying in size, and filled with protoplasmic and nucleated matter. This grows, and is seen to bud out and divide into two or more parts, all resembling the mother cell.

Fig. 277 represents the healthy cells of yeast, Saccharomyces cerevisiÆ, freshly taken from a brewer’s vat, and in an active stage of growth. The mode of multiplication continues as long as the plant remains in a liquid favourable to its nutrition.

The changes from one stage to another are rapid, as will be noticed on reference to the consecutive formative processes the cells are known to pass through, Fig. 278 (1859).

If the development of the plant is arrested by want of a saccharine or nitrogenous substance, and the liquid dries up, the protoplasm contained in the cell contracts, and the spores, or endogenous reproductive organs, of the plant will remain in a state of rest, become perfectly dry, and yet retain life. They are not easily killed, even when subjected to a very high or low temperature, they do not lose the power of germination when favourable conditions present themselves, and at once take on a new birth.

There are, however, many other ferments besides that of beer-yeasts, such as alcoholic and wine ferments, the commonest of which, according to Pasteur, is Saccharomyces ellipsoideus.

Fig. 278.—Development of Yeast Cells.

1. When first taken; 2. One hour after introducing a few cells into sweet-wort; 3. Three hours after; 4. Eight hours; 5. Forty-eight hours, when the cells become elongated.

But yeast-fungi and mould-fungi, like bacteria or fission-fungi, are micro-organisms, belonging to two specific orders, the Saccharomycetes and the Hyphomycetes, which are intimately related to each other, but quite distinct from bacteria. Their germs occur widely distributed in air, soil, and water. Many species are of hygienic, while others are of pathological interest and importance in being either accidentally associated with, or the cause of, disease processes, while others are fermentations of very essential service in various industrial processes. The making of beers, wines, and spirits, as we understand them, constitutes but a small part of the province of fermentation. The life activities of ferments open out a study of vast importance to mankind, and while they have only been regarded in their worst aspect—that of a bane—they are, nevertheless, a boon to mankind. The first clear view we obtained of this was that of Reess, who in 1870 showed there were several species or forms of the yeast-fungus. Hansen followed up this discovery in 1883, and, taking advantage of the strict methods of culture introduced by bacteriologists, found that by cultivating yeast on a solid media from a single spore it was quite possible to obtain constant types of pure yeasts, each possessing its own peculiar properties. One consequence of Hansen’s labours was that it now became possible for every brewer to work with a yeast of uniform type instead of with haphazard mixtures, in which serious disease forms might predominate and injure the beer. Among other things made clear was that a true yeast may have a mycelial stage of development. Furthermore, there is the influence exercised by the nucleus of the yeast cell. Many other points of interest arose out of these investigations; one was, that many higher fungi can assume a yeast-like stage of development if submerged in fluids, as, for instance, various species of Mucor, Ustilago, Exoascus, and numerous others. Ascomycetes, and Basidiomycetes as well, are known to form budding cells, and it was thought that the yeasts of alcoholic fermentation are merely reduced forms of these higher fungi, which have become habituated to the budding condition—a conclusion supported by Hansen’s discovery that a true Saccharomyces can develop a feeble, but a true, mycelium.

Fig. 279.—Saccharomyces and Moulds.

1. Section from a tomato, showing spores growing from cuticle; 2. Portion detached to show budding-out process; 3. Lateral view of spore sac with oospores issuing forth; 4. Apiculated ferment spores; 6 and 7. Mycoderma cerivisiÆ in different stages of growth, as seen on wine bottles; 8 and 9. TorulÆ diabeticÆ, torulÆ and fission spores.

“This view has been entirely confirmed by an inquiry into the mode of brewing sakÉ by the Japanese, by the aid of the Aspergillus fungus. Further researches established the fact that other forms of fungi, e.g., those on the surface of fruits, developed endogenous spores, which cause alcoholic fermentation. More recently, and by further experimental inquiry, partly by pure cultures of separate forms, and partly by well-devised cultures on ripening fruits still attached to the plant but imprisoned in sterilised glass vessels, it has been found that yeast and moulds are separate forms, not genetically connected, but merely associated in nature, as are so many other forms of yeasts, bacteria, and moulds. Further, Hansen has discovered that several yeasts furnish quite distinct races or varieties in different breweries in various parts of the world, so that we cannot avoid the conclusion that their race characteristics have been impressed on the cells by the continued action of the conditions of culture to which they have so long been exposed—they are, in fact, domesticated races.”

The environments of yeasts are peculiar. Sauer found that a given variety of yeast, whose activity is normally inhibited when the alcohol attains a certain degree of concentration in the liquid, can be induced to go on fermenting until a higher degree is attained by the addition of a certain lactic acid bacterium. The latter, indeed, appears to prepare the way for the yeast. It has been shown, also, that damage may be done to beers and wines by allowing plant germs to gain access with the yeast; there are, too, several forms of yeast that are inimical to the action of the required fermentation. Other researches show that associated yeasts may ferment better than any single yeast, and such symbiotic action of two yeasts of high fermenting power has given better results than either alone. English ginger-beer furnishes a curious symbiotic association of two organisms—a true yeast and a true bacterium—so closely united that the yeast cells become imprisoned in the gelatinous meshes of the bacterium; and it is a curious fact that this symbiotic union of yeast and bacterium ferments is far more energetic than either when used alone, and the product is different, large quantities of lactic and carbonic acids being formed, and little or no alcohol.

Many years ago I gave an account of similar curious symbiotic results obtained by introducing into a wort-infusion a small proportion of German yeast, an artificial product composed of honey, malt, and a certain proportion of spontaneously-fermented wheat flour. This, to my astonishment, produced ten per cent. more alcohol than any of its congeners, and did not so soon exhaust itself as brewer’s yeast.54

In the hephir used in Europe for fermenting milk, another symbiotic association of yeast and a bacterium, it is seen that in this process no less than four distinct organisms are concerned. I have already instanced the fermentation of rice to produce sakÉ, which is first acted upon by an Aspergillus that converts the starch into sugar and an associated yeast, and this is also shown to be a distinct fungus, symbiotically associated in the conversion. “Starting, then, from the fact that the constitution of the medium profoundly affects the physiological action of the fungus, there can be nothing surprising in the discovery that the fungus is more active in a medium which has been favourably altered by an associated organism, whether the latter aids the fungus by directly altering the medium, or by ridding it of products of excretion, or by adding gaseous or other body. It is not difficult to see, then, that natural selection will aid in the perpetuation of the symbiosis, and in cases like that of the ginger-beer plant it is extremely difficult to get the two organisms apart, a difficulty similar to that in the case of the soredia of lichens.”

Buchner discovered that by means of extreme pressure a something can be extracted from yeast which at once decomposes sugar into alcohol and carbon-dioxide. This something is regarded as a kind of incomplete protoplasm—a body, as we have already seen, composed of proteid—and in a structural condition somewhere between that of true soluble enzymes like invertin and a complete living protoplasm. This reminds me of an older experiment of mine, the immediate conversion of cane-sugar into grape-sugar. If we take two parts of white sugar and rub it up in a mortar with one part of a perfectly dry solid, the German yeast before spoken of, it is immediately transformed as if by magic into a flowing liquid mass—a syrup. This process of forming “invert sugar” can be watched under the microscope; the liberation of carbonic acid gas in large bubbles is seen to go on simultaneously with the assimilation of the dextrose, and the breaking up of the crystals of sugar; the cell at the same time increasing in size as well as in refractive power; a curious state of activity appears to be going on in the small mass, which is very interesting to watch throughout.

However, the enzymes of Buchner are probably bits off the protoplasm, as it were, and so the essentials of the theory of fermentation remain, the immediate agent being not that of protoplasm itself, but of something made by or broken off from it. Enzymes, or similar bodies, are known to be very common in plants, and the suspicion that fungi do much work with their aid is abundantly confirmed. It seems, indeed, that there are a whole series of these bodies which have the power of carrying over oxygen to other bodies, and so bringing about oxidations of a peculiar character. These curious enzymes were first observed owing to studies on the changes which wine and plant juice undergo when exposed to the action of the oxygen of the air.

The browning of cut apples is known to be due to the action of an oxydase, that is, an oxygen carrying ferment, and the same is claimed for the deep colouring of certain lacs obtained from the juice of plants, such as AnacardiaceÆ, which are pale and transparent when fresh drawn, but which gradually darken in colour on exposure to the air. Oxydases have been isolated from beets, dahlia, potato-tubers, and several other plants. This fact explains a phenomenon known to botanists, and partly explained by SchÖnbein as far back as 1868, that if certain fungi (e.g., Boletus beridies) are broken or bruised, the yellow or white flesh at once turns blue; this action is now traced to the presence in the cell sap of an oxydase.

It is the diastatic activity of Aspergillus which is utilised in the making of sakÉ from rice, and in the preparation of soy from the soja bean in Japan. Katz has recently tested the diastatic activity of Aspergillus, of Penicillium, and of Bacterium megatherium, in the presence of large and small quantities of sugar, and found all are able to produce not only diastase, but also other enzymes; as the sugar accumulates the diastase formed diminishes, whereas the accumulation of other carbo-hydrates produces no such effect. Harting’s investigation on the destruction of timber by fungi derives new interest from the discovery of an emulsion-like enzyme in many such wood-destroying forms, which splits up glucosides, amygdalin, and other substances into sugar, and that hyphÆ feed on other carbo-hydrates. The fact, also, that Aspergillus can form inverts of the sucrase and maltase types, as well as emulsin, inulate, and diastase, according to circumstances of nutrition, will explain why this fungus can grow on almost any organic substance it may happen to alight upon. The secretion of special enzymes by fungi has a further interest just now, for recent investigations promise to bring us much nearer to an understanding of the phenomena of parasitism than it was possible when I was at work upon them some forty or fifty years ago.

It was De Bary who impelled botanists to abandon older methods, and he who laid the foundation of modern mycology. Later on he pointed out that when the infecting germinal tube of a fungus enters a plant-cell, two phenomena must be taken into account, the penetration of the cell-walls and tissues, and the attraction which causes the tips of the growing hypha to face and penetrate these obstacles, instead of gliding over them in the lines of apparent least resistance. The further development of these two factors shows that in the successful attack of a parasitic plant on its victim or host these fungi can excrete cellulose-dissolving enzymes, and that they have the power of destroying lignine. Zopf has also furnished examples of fungi which can consume fats. There is, however, one other connection in which these observations on enzymes in the plant-cell promise to be of considerable importance, viz., the remarkable action of certain rays of the solar light on bacteria. It has been known for some time past that if bacteria in a nutrient liquid are exposed to sunlight they quickly die. The further researches of Professor Marshall Ward and other workers in the same direction have brought out the fact that it is really the light rays, and not high temperatures, that it is especially the blue-violet and ultra-violet rays, which exert the most effective bactericidal action. This proof depended upon the production of actual photographs in bacteria of the spectrum itself. Apart from this, the Professor demonstrated that just such spores as those of anthrax, at the same time pathogenic and highly resistant to heat, succumb soonest to the action of these cold light-rays, and that under conditions which preclude their being poisoned by a liquid bath. It is in all probability the action of these rays of light upon the enzymes, which abound in the bacterial cells, that bring about their death.

The sun, then, is seen to be our most powerful scavenger, and this apparently receives confirmation in connection with Martinaud’s observations, that the yeasts necessary for wine-making are deficient in numbers and power on grapes exposed to intense light, and to this is due that better results are obtained in central France as contrasted with those in the south. “When we reflect, then, that the nature of parasitic fungi, the actual demonstration of infection by a fungus spore, the transmission of germs by water and air, the meaning and significance of polymorphism, heterÆcism, symbiosis, had already been rendered clear in the case of fungi, and that it was by these studies in fermentation, and in the life-history of the fungus Saccharomyces, that the way was prepared for the Ætiology of bacterial diseases in animals, there should be no doubt as to the mutual bearings of these matters.”

Industrial uses of Fungi and Saccharomycetes.

There are many industrial processes which are more or less dependent for success on bacterial fermentations. The subject is young, but the results already obtained are seen to be of immense importance from a scientific point of view, and to open up vistas of practical application already being taken advantage of in commerce, while problems are continually being raised by the forester, the agriculturist, the gardener, the dairyman, the brewer, dyer, tanner, and with regard to various industries, which will eventually confer great advantages in their economic application.

The remarkable discovery made by Alvarez of the bacillus, which converts a sterilised decoction of the indigo plant into indigo sugar and indigo white, the latter then oxidising to form the valuable blue dye, whereas the sterile decoction itself, even in presence of oxygen, forms no indigo, plainly proves how these minute organisms may be turned to a good account. There are, however, important points to be determined as to the action of the fermentation brought about by these enzymes, and the appearance of certain mysterious diseases in the indigo vats. Again, certain stages in the preparation of tea and tobacco leaves are found to depend upon very carefully regulated fermentations, which must be stopped at the right moment, or the product will be spoilt. Regarding the possible rÔle of bacteria, the West Indian tobacco has a special bacterium, which has been isolated and found to play a very important part in its flavour. Every botanist knows that flax and hemp are the best fibres of Linum and Cannabis respectively, separated by steeping in water until the middle lamella is destroyed and the fibres isolated; but it is not so well known that not every water is suitable for this “retting” or steeping process; and for a long time this was as much a mystery as why some waters are so much better than others for brewing. Quite recently Fribes has succeeded in isolating the bacillus upon which the dissolution of the middle lamella depends. This investigation brought out other interesting details as to the reaction produced by living micro-organisms, and which can be utilised in deciding questions of plant chemistry too subtile for testing with ordinary re-agents. One other important fact connected with these researches is that botanists have now discarded the view that the middle lamella of the plants referred to is composed of cellulose, and know that it consists of pectin compounds. Fribes’ anÆrobic bacillus is found to dissolve and destroy pectins and pectinates, but does not touch cellulose or gum. It is well known that the steeping of skins in water in preparation for tanning involves bacterial action, owing to which the hair and epidermal coverings are removed, but it also appears that in the process of swelling the limed skins, the gases evolved in the substance of the tissues, and the evolution of which causes the swelling and loosens the fibre so that the tanning solutions may penetrate, are due to a particular fermentation caused by a bacterium, which, according to some investigators, is identical with a lactic ferment introduced by the pine bark, and which is responsible for the advantageous acidification of the tanning solutions.

Hay is made in different ways, and in those where a “spontaneous” heating process is resorted to the fermentation is no doubt dependent upon the presence of thermogenic bacteria. But probably no other subject has attained to so much importance as the bacteriology of the dairy: the study of the bacteria found in milk, butter, and cheese in their various forms.

Of milk, especially, much has been written and said as a disease-transmitting medium, and with every good reason, and, if the statement of a Continental authority may be accepted that each time we eat a slice of bread and butter we devour a number of bacteria equal to the population of Europe, we have sure grounds for seeking for further information as to what these bacteria are and what they are doing. And similarly so with cheese, which teems with millions of these minute organisms.

“Some few years ago it was found that the peculiar aroma of butter was due to a bacterium. There are two species of bacteria, one of which develops an exquisite flavour and aroma, but the butter keeps badly, while the other develops less aroma, but the butter keeps better. In America, however, they have isolated and distributed pure cultures of a particular butter bacillus which develops the famous ‘June’ flavour, hitherto only met with in the butter made in a certain district during a short season of the year. This fine-flavoured butter is now constantly manufactured in a hundred American dairies; and the manufacture of pure butter with a constant flavour has become a matter of certainty.

“Properly considered, the manufacture of cheese is a form of ‘microscopic gardening’ even more complex and more horticultural in nature than the brewing of beer. From the first moment, when the cheesemaker guards and cools his milk, till his stock is ready he is doing his best to keep down the growth of micro-organisms rushing about to take possession of his milk. He therefore coagulates it with rennet—an enzyme of animals, but also, as we have seen, common in plants—and the curd thus prepared is simply treated as a medium, on which he grows certain fungi and bacteria, with every needed precaution for favouring their development, and protecting them against the inroads of other pests and against unsuitable temperature, moisture, and access of light. Having succeeded in growing the right kind of plants on his curd, his art then demands that he shall stop their growth at the critical moment, and his cheese is ready for market.

“Furthermore, the particular flavour and peculiar odours of cheeses, as Camembert, Stilton, and Roquefort, have to be obtained, and this is secured, for instance, by cultivating a certain fungus, Penicillium, on bread, and purposely adding it to Roquefort. This is found to destroy the lactic and other acids, and so enables certain bacteria in the cheese to set to work and further change the medium; whereas in another kind of cheese the object is to prevent this fungus paving the way for these bacteria. Another kind of bacillus has been discovered which gives a peculiar clover aroma to certain cheeses.

“It is thought that more definite results will be obtained by the investigation of the manufacture of the vegetable cheeses of China and Japan, which are made by exposing the beans of the leguminous plant, Glycine—termed soja-beans—to bacterial fermentations in warm cellars with or without certain mould-fungi. Several kinds of bean-cheeses are made in this way, known by special names. They all depend upon the peculiar decompositions of the tissues of the cotyledons of the soja-beans, which contain 35 to 40 per cent. of proteids and quantities of fatty matter. The softened beans are first rendered mouldy, and the interpenetrating hyphÆ render the contents accessible to certain bacteria, which peptonise and otherwise alter them. There is the further question of the manufacture of vinegar by fermentation, of the preparation of soy from a brine extract of mouldy and fermented soja-beans, of bread-making, and other equally interesting manufactures.”

Results of De Bary’s Investigations in Parasitism.

“When the idea of parasitism was rendered definite by the fundamental distinction drawn by De Bary between a parasite and a saprophyte, it soon became evident that some further distinction must be made between obligate facultative parasites and saprophytes respectively. De Bary, when he proposed these terms for adoption, was clearly alive to the existence of transitions which we now know to be numerous and so gradual in character that we can no longer define any such physiological groups. Twenty years ago penicillium and mucor would have been regarded as saprophytes of the most obligate type, but we now know that under certain circumstances these fungi can become parasites, and the borderland between facultative parasites and saprophytes on the one hand, and between the former and true parasites on the other, can no longer be recognised.”

In 1866 the germ of an idea was sown which has taken root and extended. De Bary pointed out that in the case of lichens we have either a fungus parasite on an algÆ, or else certain organisms hitherto accepted as algÆ are merely incomplete forms.

“In 1879 the same observer definitely launched the new hypothesis of symbiosis. The word itself is due to Frank, who, in a valuable paper on the biology of the thallus of certain lichens, very clearly set forth the existence of various stages of life in common among all the lower forms of plants. The details of these matters are now principally of historical interest. We now know that lichens are dual organisms, composed of various algÆ, symbiotic with Ascomycetes, with Basidiomycetes, and, as Massee has shown, even with Gastromycetes. The soil contains also bacterio-lichens. Hence arose a new biological idea—that a fungus may be in such nicely-balanced relationship with the host from which it derives its sustenance, that it may be attended with nearly equal advantage to both.

“In the humus of forests we find the roots of beeches and other CupuliferÆ (willows, pines, and so forth) clothed with a dense mantle of hyphÆ, and swollen into fleshlike masses of mycorhiza. In similar soils, and in moorlands, which abound in the slowly decomposing root-fibres and other vegetable remains so characteristic of these soils, the roots of orchids, heaths, gentians, &c., are similarly provided with fungi, the hyphÆ of which penetrate further into the tissues, and even send haustoria into the living cells, but without injuring them. As observations multiplied it became clear that the mycorhiza, or fungus-root, was not to be dismissed as a mere case of roots affected by parasites, but that a symbiotic union, comparable to that of the lichens, exists, and we must assume that both tree and fungus derive benefit from the connection.

Fig. 280.—Fine Section through Truffle.

a. Asci filled with spores; b. Mycelia, × 250.

“Frank stated, as the result of his experimental research, that seedling forest-trees cannot be grown in sterilised soil, where their roots are prevented from forming mycorhiza; and he concluded that the fungus conveys organic materials to the roots, which it obtains by breaking down the leaf-mould and decaying plant remains, together with water and minerals from the soil, and plays the especial part of a nitrogen-catching apparatus. In return for this import service the root pays a tax to the fungus by sparing it certain of its tissue contents. It is a curious fact then that the mycorhiza is only formed where humus or vegetable mould abounds.”

These instructive investigations offer an intelligible explanation of the growth of that well-known subterranean fungus, the truffle (Tuber cibarium), the microscopic appearances of a section of which formed the subject of a paper I contributed to “The Popular Science Review” some years ago (1862). The fungus, as will be seen by the fine section cut through a truffle, Fig. 280, consists of flocculent filaments, which in the first instance cover the ground at the fall of the leaf in autumn, under oak or beech trees, the hyphÆ of which penetrate the ground, through the humid soil to the root-hairs of the tree. Filaments (mycelia) are again given off which terminate in asci or sacs filled with minute spores of about ¹/₂₅₀₀th of an inch in size, while the interspaces are filled up by mycelia, that become consolidated into a firm nut-like body.

What happens, then, is this: Trees and plants with normal roots and root-hairs, when growing in ordinary soil, can adapt their roots to life in a soil heavily charged with humus only by contracting symbiotic association with the fungus and paying the tax demanded by the latter in return for its supplies and services. If this adaptation is impossible, and no other suitable variation is evolved, such trees cannot grow in such soils. The physiological relations of the root to the fungus must be different in details in the case of non-green, purely saprophytic, plants, Neottia, Monotropa, &c., and in that of green plants like Erica, Fagus, and Pinus. It is, however, a well-known fact that ordinary green plants cannot utilize vegetable dÉbris directly, and forest trees do so in appearance only, for the fungi, yeasts and bacteria there are actively decomposing the leaves and other remains. A class of pseudo-symbiotic organisms are, however, being brought into the foreground, where the combined action of two symbionts results in the death of or injury to a third plant, each symbiont alone proving harmless. Some time ago Vuillemin showed that a disease in olives results from the invasion of a bacillus (B. oleÆ), which can, however, only obtain its way into the tissues through the passages driven by the hyphÆ of a fungus (ChÆtophoma). The resulting injury is a sort of burr. This observer also observed the same bacillus and fungus in the canker burrs of the ash.

Among many similar cases well worth further attention are the invasion of potato-tubers by bacteria, these making their way down the decaying hyphÆ of pioneer fungi. Professor Marshall Ward has seen tomatoes infected by similar means, and other facts show that many bacteria which quicken the rotting of wood are thus led into the tissues by fungi.

Probably no subject in the whole domain of cryptogamic botany has wider bearings on agricultural science than the study of the flora and changes on and in manure and soil. Nitrifying bacteria play a very important part by providing plant life with a most necessary food. They occur in the soil, and two kinds have been described—the one kind converting ammonia into nitrous acid, and the other changing nitrous into nitric acid. We are principally indebted to Winogradsky for our knowledge of these bacteria; he furnishes instances of the bearing of bacteriological work on this department of science, and explains, not only the origin of nitre-beds and deposits, but also the way the ammonia compounds fixed by the soil in the neighbourhood of the root-hairs are nitrified, and so rendered directly available to plant life. The investigations of other observers show that the nitrifying organism is a much more highly-developed and complex form than had been suspected; that it can be grown on various media, and that it exhibits considerable polymorphism—i.e., it can be made to branch out and show other characteristics of a true fungus. “I have,” writes Professor Ward, “for some time insisted on the fact that river water contains reduced forms of bacteria—i.e., forms so altered by exposure to light, changes of temperature, and the low nutritive value of the water, that it is only after prolonged culture in richer food media that their true nature becomes apparent.” Strutzer and Hartleb show that the morphological form of the nitrifying organism can be profoundly altered by just such variations of the conditions described by Ward, and that it occurs as a branched mycelial form; as bacilli or bacteria; or as cocci of various dimensions, according to the conditions.

“These observations, and others made on variations in form (polymorphism) in other fungi and bacteria, open out a vast field for further work, and must lead to advancement in our knowledge of these puzzling organisms; they also help us to explain many inconsistencies in the existing systems of classification of the so-called ‘species’ of bacteria as determined by test-tube culture.”

AlgÆ.—The algals have a special charm for microscopists. I am free to confess my interest in these organisms, and for several reasons. In this humid climate of ours they are accessible during the greater part of the year; they can be found in any damp soil, in bog, moss, and in water—indeed, wherever the conditions for their existence seem to be at all favourable for development. Should the soil dry up for a time, when the rain returns algÆ are seen to spring into life and give forth their dormant spores, which once more resume the circle of formation and propagation. In the earliest stage of development the spore or spore cell is so very small when in a desiccated state, that any number may be carried about by the slightest breath of air and borne away to a great distance. To all such organisms I originally gave the name of Ærozoa; now recognised as Ærobic and anÆrobic organisms (Fig. 281).

Fig. 281.—Ærobic Spores × 200.

1. Ærobic fungi caught over a sewer; 2. Fragments of Penicillium spores; 3. Ærobic fungi taken in the time of the cholera visitation, 1854.

With reference to the Ærobic bacteria I have only to add that in addition to the simple mode of taking them on glass slides smeared over with glycerine, special forms of Æroscopes are now in use for the purpose, consisting of a small cylinder in which a current of air is produced by an aspirator and diffused through a glass vessel containing a sterilised fluid. These are in constant use in all bacteriological laboratories. The results obtained are transferred to sterilised flasks or tubes as those shown in a former chapter.

Miquel, who has given considerable attention to the subject of Ærobic and anÆrobic bacteria, reckons that the number of spores that find their way into the human system by respiration, even should health be perfectly sound, may be estimated at 300,000 a day.

One of the most commonly met with forms of micro-organisms is Leptothrix buccalis. It chiefly finds its nutritive material in the interstices of the teeth, and is composed of short rods and tufted stems of vigorous growth, to which the name of Bacillus subtilis has been given (Fig. 282). Among numerous other fungoid bodies discovered in the mouth, SarcinÆ have been found. See Plate IX., No. 7.

Fig. 282.—Section of the Mucous Membrane of the Mouth, × 350.

Showing: a. The denser connective tissue; b. Teased out tissue; c. Muscular fibre; d. Leptothrix buccalis, together with minute forms of bacteria and micrococci; e. Ascomycetes and starch granules.

The Beggiatoa, a sewage fungus, found by me in the river Lea water of 1884 growing in great profusion, consists chiefly of mycelial threads and a number of globular, highly refractive bodies, and may be regarded as evidence of the presence in the water of an abnormal amount of sulphates which set free a gas, sulphuretted hydrogen, of a dangerous and offensive character. Another curious body closely allied to Beggiatoa alba is Cladothrix; this assumes a whitish pellicle on the surface of putrefying liquids.

These saprophytes obtain nourishment from organic matter; nevertheless they are not true parasites in the first stage of their existence, during which they live freely in the water or in damp soil; they, however, become pathogenic parasites when they penetrate into the tissues of animals, and necessarily live at the expense of their host.

Fungi, AlgÆ, Lichens, etc.

Tuffen West, del. Edmund Evans.

Plate I.

Bacteria, as I have said, were for a long time classed with fungi under the name of Schizomycetes. But the more recent researches into their organisation, and more especially into their mode of reproduction, show that they rather more resemble a group of algÆ devoid of chlorophyll. Zopf asserts that the same species of algals may at one time be presented in the form of a plant living freely in water, or in damp ground, in association with chlorophyllaceous protoplasm, and at another in the form of a bacterium devoid of green colouring matter, and receiving nourishment from organic substances previously elaborated by plants or animals, thus accommodating itself, according to circumstances, to two very different modes of existence.

That widely-distributed single-cell plant, the Palmogloea macrococca of KÜtzing, that spreads itself as a green slime over damp stones, walls, and other bodies, affords an example. If a small portion be scraped off and placed on a slip of glass, and examined with a half or a quarter-inch power, it will be seen to consist of a number of ovoid cells, having a transparent structureless envelope, nearly filled by granular matter of a greenish colour. At certain periods this mass divides into two parts, and ultimately the cell becomes two. Sometimes the cells are united end to end, just as we see them united in the actively-growing yeast plant; but in this case the growth is accelerated, apparently, by cold and damp. Another plant belonging to the same species, the Protococcus pluvialis, is found in every pool of water, the spores of which must be always floating in the air, since they appear after every shower of rain.

Protococcus pluvialis is furnished with motile organs—two or more vibratile flagella passing through perforations in the cell-wall—whereby, at certain stages, they move rapidly about. The flagella are distinctly seen on the application of the smallest drop of iodine. The more remarkable of the several forms presented by the plant is that of naked spores, termed by Flotow HÆmatococcus porphyrocephalus. These minute bodies are usually seen to consist of green, red, and colourless granules in equal proportions, and occupying different portions of the cell. They seem to have some share in the after subdivision of the cell (Fig. 283). There are also still-cells, which sub-divide into two, while the motile cells divide into four or eight. It is not quite clear what becomes of the motile zoospores, B, but as they have been seen to become encysted, they doubtless have a special function, or become still-cells under certain circumstances.

It appears that both longitudinal and transverse division of the primordial cell takes place; and that the vibratile flagella of the parent cell retain to the last their function and their motion after the primordial cell has become detached and transformed into an independent secondary cell (Fig. 283, G).

Fig. 283.—Cell Development. (Protococcus pluvialis.)

Protococcus pluvialis, KÜtzing. HÆmatococcus pluvialis, Flotow. Chlamidococcus versatilis, A. Braun. Chlamidococcus pluvialis, Flotow and Braun.

A. Division of a simple cell into two, each primordial vesicle having developed a cellulose envelope; B. Zoospores, having escaped from a cell; C. Division of an encysted cell into segments; D. Division of another cell, with vibratile flagella projecting through cell-wall; E. An encysted flagellate cell; F. Division of an encysted nucleated cell into four parts, with vibratile filaments projecting; G. Fission of a young cell.

The most striking of the vital phenomena presented by Protococcus is that of periodicity. Certain forms—for instance, encysted zoospores, of a certain colour, appear in a given infusion, at first exclusively, then they gradually diminish, become more and more rare, and finally disappear altogether. After some time their number again increases, and this may be repeated. Thus, a cell which at one time presented only still forms at another contained only motile ones. The same may be said with respect to segmentation. If a number of motile cells be transferred from a larger vessel into a smaller one, in the course of a few hours most of them will have subsided to the bottom, and in the course of the day observed to be on the point of sub-division. On the following morning division will have become completed; on the next day the bottom of the vessel will be found covered with a new generation of self-dividing cells, which, again, will produce another generation. This regularity, however, is not always observed. The influence of every change in the external conditions of life upon the plant is very remarkable. It is only necessary to pour water from a smaller into a larger or shallower vessel to at once induce segmentation of cells. The same phenomenon occurs in other algals; thus Vaucheria almost always develops zoospores at whatever time of year they may be brought from their natural habitat into a warm room. Light is conducive to the manifestation of vital action in the motile spores; they usually collect in great numbers on the surface of the water, and at that part exposed to the strongest light.

But in the act of propagation, on the contrary, and when about to pass into the still condition, the motile Protococcus cell seems to shun light, and falls to the bottom of the vessel. Too strong sunlight, as when concentrated by a lens, quickly kills the young zoospores. A temperature of undue elevation is injurious to the development of their vital activity and the formation of the zoospores. Frost destroys motile, but not still zoospores.55

StephanosphÆra pluvialis is a conspicuous variety of the fresh-water algals, described by Cohn. It consists of a cell containing eight primordial cells filled with chlorophyll, uniformly arranged (see Plate I., No. 24 d). The globular mother-cell rotates, somewhat in the same way as the volvox, by vibratile flagella, two of which are seen projecting from each cell and piercing the transparent outer cell wall. Every cell divides first into two, then four, and lastly eight cells, each one of which again divides into a number of micro-gonidia, which have a motion within the mother-cell, and ultimately escape from it. Under certain circumstances each of the eight young cells is observed to change places in the interior of the cell; eventually they escape, lose their flagella, form a thicker membrane as at b, and for a time remain motionless, and sink to the bottom of the vessel in which they are contained. If the vessel is permitted to become thoroughly dry, and then again has water poured into it, motile cells reappear; from which circumstance it is probable that these represent the resting spores of the plant. When in the condition of greatest activity its division into eight is perfected during the night, and early in the morning light the young cells escape and pass through similar changes. It is calculated that in eight days, under favourable circumstances, 16,777,216 families may be formed from one resting-cell of StephanosphÆra. In certain of the cells, and at particular periods, remarkable amoeboid bodies (Plate I., No. 24 c) make their appearance. There is a marked difference between StephanosphÆra and Chlamydococcus, for while in the latter the individual portions of a primordial cell separate entirely from one another, each developing its own enveloping membrane, and ultimately escaping as a unicellular individual; in the former, on the other hand, the eight portions remain for a time living in companionship.

VolvocineÆ.—A fresh-water unicellular plant of singular beauty and interest to the microscopist is the Volvox globator (Plate I., No. 15). No. 16 represents a portion of another cell, with brownish amoeboid bodies enclosed in the protoplasmic web. It is common to our fresh-water pools, and attains a diameter of about ¹/₂₀th or ¹/₃₀th of an inch. Its movement is peculiar, a continued roll onwards, or a rotation like that of a top; at other times it glides along smoothly. When examined under a sufficiently high power, it is seen to be a hollow sphere, studded with green spots, and traversed by green threads connecting each of the spots or spores with the maternal cell. From each of the spores proceed two long flagella, lashing filaments, which keep the globular body on the move. After a time the sphere bursts, and the contained sporules issue forth and speedily pass through a similar stage of development. These interesting cells were long taken to be animal bodies. Ehrenberg described them as Monads, possessing a mouth, stomach, and an eye.

The setting free of the young volvox is essentially a process of cell division, occurring during the warmer periods of the year, and, as Professor Cohn shows, is a considerable advance upon the simpler conjugation of two smaller cells in desmids; it more closely resembles that which prevails among the higher algÆ and a large number of cryptogams. As autumn advances the volvox spherules usually cease to multiply by the formation of zoosporanges, and certain of their ordinary cells begin to undergo changes by which they are converted, some into male or sperm-cells, others into germ-cells, but the greater number appear to remain sterile. Both kinds of cells at first so nearly resemble each other that it is only when the sperm cells begin to undergo sub-division that they are seen to be about three times the size of the sterile cells. Then the primary cell resolves itself into a cluster of peculiar secondary cells, each consisting of an elongated body containing an orange-coloured endochrome and a pair of long flagella, as seen in the antherozoids of the higher cryptogams. As the sperm-cells approach maturity the clusters may be seen to move within them; the bundles then separate and show an independent active movement while still within the cavity of the primary cell, and finally escape through a rupture in the cell-wall, rapidly diffusing themselves as they pass through the cavity. The germ-cells continue to increase in size without undergoing sub-division, at first showing large vacuoles in their protoplasm, but subsequently becoming filled with a darker coloured endochrome. The form of the cell also changes from its flask-like shape to the globular, and at the same time seems to acquire a firmer envelope. Over this the swarming antherozoids diffuse themselves and penetrate the substance to the interior, and are then lost to view. The product of this fusion, Cohn tells us, is a reproductive cell, or “oospore,” which speedily becomes enveloped in another membrane with a thicker external coat, beset with conical-pointed processes; and now the chlorophyll of the young cell gives place, as in PalmoglÆ, to starch and reddish or orange-coloured, and a more highly refractive, fluid. As many as forty of such oospores have been counted in a single sphere of volvox, which then acquires the peculiar appearance observed by Ehrenberg, and described by him under the name of Volvox stellatus. The further history of this wonderful spheroid unicellular plant has been traced out by Kirchner, who found that their germination commences in the early months of the year—in February—with the liberation of the spherical endospore from its envelope and its division into four cells. A remarkable phenomenon has been observed by Dr. Braxton Hicks—the conversion of an ordinary volvox cell into a moving mass of protoplasm that bears a striking resemblance to the well-known amoeba. “Towards the end of the autumn the endochrome mass of the volvox increases to nearly double its ordinary size, but instead of undergoing the usual sub-division so as to produce a macrogonidium, it loses its colour and regularity of form, and becomes an irregular mass of colourless protoplasm, containing a number of brownish granules.” The final change and the ultimate destination of these curious amoeboid bodies have not been satisfactorily made out, but from other observations on the protoplasmic contents of the cells of the roots of mosses, which in the course of two hours become changed into ciliated bodies, it is believed that this is the mode in which these fragile structures are enabled to retain life and to resist all the external conditions, such as damp, dryness, and the alternations of heat and cold.

It would be quite impossible to deny the great similarity there is between the structure of volvox and that of the motile cell of Protococcus pluvialis. The influence of reagents will sometimes cause the connecting processes of the young cells, as in Protococcus, to be drawn back into the central mass, and the connecting threads are sometimes seen as double lines, or tubular prolongations of the membrane. At other times they appear to be connected by star-like prolongations to the parent cell (Plate I., No. 15), presenting an almost identical appearance with Pediastrum pertusum. Another body designated by Ehrenberg SphÆrosira volvox is an ordinary volvox in a different stage of development; its only features of dissimilarity being that a large proportion of the green cells, instead of being single, are double or quadruple, and that the groups of flagellate cells form by their aggregation discoid bodies, each furnished with a single flagellum. These clusters separate themselves from the parent cell, and swim off freely under the forms which have been designated Uvella and Syncrypta by Ehrenberg. Mr. Henry Carter, F.R.S., who made a careful investigation of unicellular plants, described SphÆrosira as the male, or spermatic form of volvox.

Among other organisms closely allied to volvox and included in VolvocineÆ, affording the microscopist many interesting transitional forms in their various modes of fructification, are the Eudorina, still-water organisms that pass through a similar process of reproduction as the volvox. In the Pandorina morum, its reproduction is curiously intermediate between the lower and the higher types; as within each cell is a mulberry-like mass, composed of cells possessing a definite number of swarm spores, sixteen usually, which rupture the mother cell, and swim off furnished with a pair of flagella. A similar process takes place in some of the ConfervaceÆ and other fresh-water algÆ. The Palmella, again, consist of (Plate I., No. 21) minute organisms of very simple structure, which grow either on damp surfaces or in fresh water. The stonework of some of our churches is often seen to be covered with a species of Palmella, that take the form of an indefinite slimy film. The “red snow” of Arctic or Alpine regions, considered to be a species of Protococcus, is frequently placed among Palmella. A more characteristic form of the P. cruenta is the HÆmatococcus sanguinis, the whole mass of which is sub-divided by partitions enclosing a larger or smaller number of cells, which diffuse their granular contents through the gelatinous mass in which their several changes take place. The albuminoid envelope of these masses is seen to contain parasitic growths, which have given rise to some discussion, especially when their filaments are observed to radiate in various directions.

The OscillariaceÆ constitute a genus of ConfervaceÆ which have always had very great interest for the microscopist in consequence of their very remarkable animal-like movements, and from which they derive their generic name. For more than a century these Bacillaria have excited the curiosity of all observers without any one having derived more than an approximate idea of their remarkable rhythmical movements. The frustule consists of a number of very fine short threads attached together by a gelatinous sheath, in one species all of equal length. Their backward and forward movement is of a most singular character; the only other conferva in which I have seen a motion of a similar kind is the Schizonema. In this species the frustules are packed together in regular series, the front and side views being always in the same direction. These several bodies move along within the filamentous sheath without leaving their respective places. On carefully following the movement, it is seen at first much extended, and then more compressed, while the frustules become more linear in their arrangement, and present a closer resemblance to Bacillaria paradoxa, augmented by the circumstance that the frustules are seen to move in both directions. A frustule of Schizonema can move independently of the sheath, and so will a detached frustule of bacillaria. This peculiar and exceptionally anomalous phenomenon as that of the movements of bacillaria can hardly be confined to a solitary species. The movements of the frustules are much accelerated by warmth and light. The longer filaments of other minute species only slightly exhibit any motion of the kind, but have peculiar undulating motions.

Fig. 284.—ConfervaceÆ.

1. Volvox globator; 2. A section of volvox, showing the flagellate margin of the cell; 3. A portion more highly magnified, to show the young volvocina, with their nuclei and thread-like attachments; 4. Spirogyra, near which are spores in different stages of development; 5. Conferva floccosa; 6. Stigeoclonium protensum, jointed filaments and single zoospores; 7. Staurocarpus gracilis, conjugating filaments and spores.

ConfervaceÆ are a genus of algals. The species consist of unbranched filaments composed of cylindrical or moniliform cells, with starch granules. Many are vesicular, and all multiply by zoospores generated in the interior of the plant at the expense of the granular matter. They are, for the most part, found in fresh water attached or floating, some in salt water, and a few in both, in colour usually green, but occasionally olive, violet, and red. The ConfervaceÆ proper are often divided into four families: 1. HydrodictidÆ; 2. ZygnemidÆ; 3. ConfervidÆ; 4. ChÆtophoridÆ. To the microscopist all the plants of this genera are extremely interesting as subjects for the study of cell multiplication. The process usually takes place in the terminal cell, the first step in which is the division of the endochrome, and then follows a sort of hour-glass contraction across the cavity of the parent cell, whereby it is divided into two equal parts. This is better seen in some of the desmids than in Fig. 284, Nos. 4, 5, and 6. Some species are characterised by a different mode of reproduction; these possess a number of nuclei, and multiply by zoospores of two kinds, the largest of which have either two or four cilia, which germinate directly the smaller are biciliated; conjugation has been seen to take place in a few instances.

Allied to the ConfervaceÆ is an interesting plant, SphÆroplea annulina, which has received careful attention from Cohn. The oospores of this plant are the product of a process partaking of a sexual nature, and when mature are filled with reddish fat vesicles which divide by segmentation.

The ÆdogoniaceÆ also closely resemble ConfervaceÆ in habits of life, but differ in some particulars, especially so in the mode of reproduction (only a single large zoospore being set free from each cell) and by the almost complete fission of the cell-wall or one of the rings which serve as a hinge. The zoospores are the largest known among algals, and each is described as having a red eye-spot. The ChÆtophoraceÆ form an interesting group of confervoid plants, and are usually found in running streams, as they prefer pure water. One of the characteristics of the group is that the extremities of the branches are prolonged into an acute-shaped termination, as represented in Fig. 284, No. 6. A very pretty object under the microscope is Draparnaldia glomerata, belonging to this species. It consists of an axis composed of a row of cells, and at regular intervals whorls of slender prolongations, containing chlorophyll or endochrome of a deeper green; these attain to an extraordinary length.

The BatrachospermÆ bear a strong resemblance to frog-spawn, from which they derive their name, and are chiefly a marine group of algals allied to the RhodespermeÆ or red seaweeds. The late Dr. A. Hassall first described them; they have since received more careful attention from M. Sirodot. They are reddish-green, extremely flexible, and nothing can surpass the grace of their movements in water; but when removed from their element they lose all form, and resemble a jelly-like substance without a trace of organisation; but if allowed to remain quiet they regain their original shape.

The presence of the cell-membrane will be best demonstrated by breaking up the filaments, either by moving the thin glass cover, or by cutting through a mass of them in all directions with a fine dissecting knife. On now examining the slide, in most instances many detached empty pieces of the cell-membrane, with its striÆ, will be seen, as well as filaments partly deprived of protoplasm. On the application of iodine all these appearances become more distinguishable in consequence of the filament turning red or brown, while the empty cells remain either unaffected, or present a slight yellowish tint, as is frequently the case with cellulose when old.

With regard to the contents of the cell, the endochrome is coloured in the OscillatoriÆ, and is distinguishable by circular bands or rings around the axis of the cylindrical filament. Iodine stains them brown or red, and syrup and dilute sulphuric acid produce a beautiful rose colour. As to their mode of propagation, nothing positive is known. If kept for some time they gradually lose their green colour; a portion of the mass, becoming brown, sinks to the bottom of the vessel, and presents a granular layer.

Mesoglia vermicularis (Fig. 285) consists of strings of cells cohering and held together by their membranous covering. In the lowly organised plant Vaucheria (Plate I., No. 23, V. sessilis)—so named after its discoverer Vaucher, a German botanist—a genus of SiphonaceÆ, we have an example of true processes of sexual generation. The branching filaments are often seen to bear at their sides peculiar globular bodies or oval protuberances, nipple-shaped buddings-out of the cell-wall, filled with a dark-coloured endochrome and distributed in pairs, one of which curves round to meet the other, when conjugation is seen to take place. Near these bodies others are found with pointed projections, which have been described as “horns,” but these, Pringshelm says, are “antherids which produce antherozoids in their interior,” while the capsule-like bodies constituting the oospores become, when fertilised, a new generation, which swarm out through a cavity or aperture in the parent cell-wall.

The fruit of fresh-water and most olive-green algals is enclosed in spherical cavities under the epidermis of the frond, termed conceptacles, and may be either male or female. The zoids are bottle-shaped and have flagella; the transparent vesicle in which they are contained is itself enclosed in a second of similar form. In monoecious and dioecious algals the female conceptacles are distinguished from the male by their olive colour. The spores, when developed, are borne on a pedicle emanating from the inner wall of the conceptacle. They rupture the outer wall at its apex; at first the spore appears simple, but soon after a series of changes takes place, consisting in a splitting up of the endochrome into six or eight masses of spheroidal bodies. A budding-out occurs in a few hours’ time, and ultimately elongates into a cylindrical thread. The Vaucheria present a double mode of reproduction, and their fronds consist of branching tubes resembling in their general character that of the Bryophyta, from which indeed they differ only in respect of the arrangement of their green contents. In that most remarkable plant Saprolegnia ferox, which is structurally so closely allied to Vaucheria, though separated from them by the absence of green colouring matter, a corresponding analogy in the processes of development takes place. In the formation of its zoospores, an intermediate step is presented between that of the algÆ and a class of plants formally placed among fungi.

The UlvaceÆ.—The typical form of seaweeds is the Ulva lactuca, well known from its fronds of dark-green “laver” on every coast throughout the world. UlvÆ are seen to differ but little from the preceding group of fresh-water algals. The specific difference is that the cells, when multiplied by binary subdivision, not only remain in firm connection with each other but possess a more regular arrangement. The frond plane of the algal is either more simple or lobed, and is formed of a double layer of cells closely packed together and producing zoospores. The whole group is chiefly distinguished from Porphyra by their green colour, the latter being roseate or purple. UlvÆ are mostly marine, with one or two exceptions. One species (U. thermalis) grows in the hot springs of Gastein, in a temperature of about 117° Fahr. The development of UlvÆ is seen in Fig. 286. The isolated cells, A, resemble in some points those of the Protococcus; these give rise to successive subdivisions determining the clusters seen at B and C, and by their aggregation to the confervoid filament shown at D. These filaments increase in length and breadth by successive additions, and finally take the form of fronds, or rows of cells.

The marine greenish-olive algÆ present a general appearance which might at first sight be mistaken for plants of a higher order of cryptogams. Their fronds have no longer the form of a filament, but assume that of a membranous expansion of cells. The cells in which zoospores are found have an increased quantity of coloured protoplasm accumulated towards one point of the cell-wall; while the zoospores are observed to converge with their apices towards the same point. In some algÆ, which seem to be closely related in form and structure to the Bryophyta, we notice this important difference, that the zoospores are developed in an organ specially destined for the purpose, presenting peculiarities of form and distinguishing it from other parts of the branching tubular frond. In the genus Derbesia distinct spore cases develop, a young branch of which, when destined to become a spore case, instead of elongating indefinitely, begins, after having arrived at a certain length, to swell out into an ovoid vesicle, in the cavity of which a considerable accumulation of protoplasm takes place. This is separated from the rest of the plant, and becomes an opaque mass, surrounded by a distinct membrane. After a time a division of the mass takes place, and a number of pyriform zoospores, each of which is furnished with flagella, are set free.

In Cutleria (Fig. 288) we have a special feature of interest with two kinds of organs, seemingly opposed to each other with regard to their reproductive functions. The sporangia not only differ from those of other species, but the frond consists of olive-coloured irregularly-divided flagella, on each side of which tufts (sori) consisting of the reproductive organs, intermixed with hair-like bodies, are scattered. The zoospores are divided by transverse partitions into four cavities, each of which is again bisected by a longitudinal median septum. When first thrown off they are in appearance so much like the spores of Puccinia that they may be mistaken for them, although so very much larger than those of other olive-coloured algÆ.

FlorideÆ, the red algÆ (Plate II.), present many varieties of structure, although less appears to be known of their reproductive processes than of lower forms of cryptogamic plants. These are, however, of three kinds. The first, to which the term polyspore has been applied, is that of a gelatinous or membranous pericarp or conceptacle, in which an indefinite number of zoospores are contained. This organ may be either at the summit or base of a branch, or it may be concealed in or below the cortical layer of the stem. In some cases a number of spore-bearing filaments emanate from a kind of membrane at the base of a spheroidal cellular perisporangium, by the rupture of which the zoospores formed from the endochrome of the filaments make their escape. Other changes have been observed; however, they all agree in one particular, namely, that the zoospore is developed in the interior of a cell, the wall of which forms its perispore, and the internal protoplasmic membrane endochrome, the zoospore itself, for the escape of which the perispore opens out at its apex.

The second form is more simple, and consists of a globular or ovoid cell, containing a central granular mass; this ultimately divides into four quadrate-shaped spores; these, on attaining maturity, escape by rupture of the cell-wall. Another organ, called a tetraspore, takes its origin in the cortical layer. The tetraspores are arranged either in an isolated manner along the branches, or in numbers together; in some instances the branches that contain them are so modified in form they look like special organs, and have been called stichidia; as, for example, in Dasya (Fig. 289). Of the third kind of reproductive organ a difference of opinion exists as to the signification of their antheridia; although always produced in precisely the same situations as the tetraspores and polyspores, they are agglomerations of little colourless cells, either united in a bunch, as in Griffithsia, or enclosed in a transparent cylinder, as in Polysiphonia, or covering a kind of expanded disc of peculiar form, as in Laurencia. According to competent observers, the cells contain spermatozoids. NÄgeli describes the spermatozoid as a spiral fibre, which, as it escapes, lengthens itself in the form of a screw. Thuret, on the contrary, says the contents are granular, and offer no trace of a spiral filament, but are expelled from the cells by a slow motion. The antheridia appear in their most simple form in Callithamnion (Plate II., Nos. 32 and 34), being reduced to a small mass of cells composed by numerous little bunches which are sessile on the bifurcations of the terminal branches. The spores are simpler structures than the tetraspores, and mostly occupy a more important position. They are not scattered through the frond, but grouped in definite masses, and generally enclosed in a special capsule or conceptacle, which may be mistaken for a tetraspore case. The simplest form of the spore fruit consists of spherical masses of spores attached to the wall of the frond, or imbedded in its substance, without a proper conceptacle; such a fruit is called a favellidium, and occurs in Halymenia; the same name is applied to the fruits of similar structures not perfectly immersed, as those of Gigartina, Gelidium, &c., where they form tubercular swellings on the lobes. In some, the tubercles present a pore at the summit, through which the spores emerge forth. In other cases, as in Ceramium (Plate II., Nos. 27 and 37), the spores occupy a more conspicuous place; a characteristic species is Delessaria (Plate II., No. 39), the coccidium either occurring on lateral branches, or is sessile on the face of the frond, when it consists of a case filled with angular-shaped spores attached to the wall of the case. The general external appearance of the red algÆ is very varied, but it seems to attain to its deepest colouring in the Red Sea, which, it is said, is entirely due to the peculiarly vivid red seaweed. They are all exquisite objects for the microscope, as may be surmised from the few varieties presented in Plate II. The FlorideÆ of the warmer seas exhibit most elegantly formed fronds, as will be seen on reference to the “Phycologia Australica” of the late Dr. William Harvey, F.R.S.

The CharaceÆ may be placed among the highest of the algals, if only for the complexity of their reproductive organs, which certainly offer a contrast in their simplicity of structure. Chara vulgaris, stonewort, is a simple fresh-water plant, preferring still freshwater ponds or slow-moving rivers running over a chalky soil. It thus derives the calcareous matter found in the axis of the plant, together with a small portion of silica. Its filaments (or branches, as some botanists prefer to call them) are given off in whorls. The CharaceÆ are a small family of acrogens, consisting of only two or three at most. They are monoecious and dioecious, the two kinds of fruit being often placed close together. They may easily be grown in a tall glass jar for observation. All that is necessary is to put the jar occasionally under the house tap and let the water run slowly over the top for a short time, thus renewing the contents without disturbing the plant. The hard water supplied to London suits chara better than softer water. Both chara and nitella are objects of great interest to microscopists, since in the former the important fact of vegetable circulation was first observed. A portion of the plant of the natural size is shown in Fig. 290, No. 1.

Each plant is composed of an assemblage of long tubiform cells placed end to end, with fixed intervals, around which the branchlets are disposed with great regularity. In nitella the stem and branches are composed of simple cells, which sometimes attain to several inches in length. Each node, or zone, from which the branches spring, consists of a single plate, or layer, of small cells, which are a continuation of the cortical layer of the internode (Fig. 290, No. 3) as an outgrowth. Each cell is partially filled with chlorophyll granules, and it is these that are seen under the microscope taking the course shown by the arrows (Fig. 290, No. 2). The rate of movement of the granules is accelerated by moderate warmth and retarded by cold. It is in viewing the circulation in water plants that the warm stage of the microscope is brought into use. Borne along with the protoplasmic stream are a number of solid particles consisting of starch granules and other matters. The method of viewing the circulation is by cutting sections off a portion of the plant with a very sharp knife, and arranging them in a growing cell with a few drops of water, and covering over with a thin cover-glass.

The reproductive process of Chara is effected by two sets of bodies, both of which are placed at the base of the branches (Fig. 291, E and D) either on the same or different plants, one set known as globules or antherids, and the other as nucules, containing the oospores or archegones. These are often of a bright red colour, and have covering plates, or shields (B and E), curiously marked, and the central portion is composed of a number of filaments rolled up (as in E) or free (as seen at B), projecting out from the centre of the sphere. The antherid is supported on a short flask-shaped pedicle, which projects into the interior. At the apex of each of the eight manubria is a roundish hyaline cell, termed a capitulum, and at its apex again six smaller or secondary capitula. The long whip-shaped filaments are divided by transverse septa into a hundred or more compartments, every one of which is filled with an antherozoid (as at A), consisting of a spiral thread of protoplasm packed into two or three coils; these escape and become free (as seen at C), each having two long fine flagella. The young antherozoid swims off with a lashing action, and the whole field appears for a time filled with life. They swim about freely, but their motion gradually ceases, and soon they arrive at a state of inaction.

Nitella appears to have a somewhat different mode of fructification to that of its congener. It puts forth a long filamentous branch from one of its joints, which, on reaching the surface of the water, terminates in a whitish fruit-like cluster. It is even a more delicate and less robust algal than chara, and every care should be taken to imitate the still water in which it grows. It delights in shady woods and in calcareous open pools.

Similar care is requisite with regard to Vallisneria; and a more equal temperature is better suited to the growth of this aquatic plant. It should be planted in the middle of the jar or aquarium, about two inches deep in mould, closely pressed down, then gently fill the jar with water. When the water requires changing, a small portion only should be run off at a time. It appears to thrive in proportion to the frequency of changing the water, and taking care that the water added rather increases the temperature than lowers it.

The natural habitat of the Frog-bit, another water-plant of much interest, is found on the surface of ponds and ditches; in the autumn its seeds fall, and become buried in the mud at the bottom during the winter; in the spring these plants rise to the surface, produce flowers, and grow throughout the summer. Chara may be found in many places around London, and in the upper reaches of the Thames.

Anacharis alsinastrum.—This remarkable plant is so unlike any other water-plant that it may be at once recognised by its leaves growing in threes round a slender stem. It is also known as “Waterthyme,” from a resemblance it bears to that plant.

The colour of the plant is deep green; the leaves are nearly half an inch long, by an eighth wide, egg-shaped at the point, with serrated edges. Its powers of increase are prodigious, as every fragment is capable of becoming an independent plant, producing roots and stems, and extending itself indefinitely in every direction. The specific gravity of it is so nearly that of water, that it is more disposed to sink than float. A small branch of the plant is represented, with a hydra attached to it, in a subsequent chapter.

The special cells in which the circulation is most readily seen are the elongated cells around the margin of the leaf and those of the midrib. On examining the leaf with polarised light, the cells are observed to contain a large proportion of silica, and present a very interesting appearance. A bright band of light encircles the leaf, and traverses its centre. In fact, the leaf is set, as it were, in a framework of silica. By boiling the leaf for a short time in equal parts of nitric acid and water, a portion of the vegetable tissue is destroyed, and the silica rendered more distinct, without changing the form of the leaf.

It is necessary to make a thin section or strip from the leaf of Vallisneria for the purpose of exhibiting the circulation in the cells, as shown in Fig. 290, No. 4. Among the cell granules, a few of a more transparent character than the rest, are seen to have a nucleolus within.

The phenomenon of cell cyclosis occurs in other plants beside those growing in water. The leaf of the common plantain or dock, Plantago, furnishes a good example, the movement being seen both in the cells of the plant and hairs of the cuticle torn from the midrib.

Cell-division.—In order to study the process of cell-division the hairs on the stamens of Tradescantia should be taken. Remove one from a bud on a warm day and let a drop of a one per cent. sugar solution fall upon it, and cover it with a thin glass cover. Place it for a short time in a moist-chamber (Fig. 256), and then examine it with a magnifying power of 500 diameters. The nucleus of the cell will be seen, near its terminal position, to gradually elongate in the direction of the longer axis of the cell and become more granular, while the protoplasm moves towards the extreme end; the nucleus at the same time will present a striated appearance, with the fibrilla arranged parallel to the longer axis of the nucleus, and at length approach each other at the poles. A nuclear spindle will now be produced, and the fibres ruptured in the equatorial plane, so that two nuclei will be found in place of the one. The best preparations of nuclei are obtained by making thin longitudinal sections of actively-growing plants (young rootlets of Pinus, for example), and staining them with hÆmatoxylin in the manner described in a former chapter.

DesmidiaceÆ and DiatomaceÆ.

The two groups of DesmidiaceÆ and DiatomaceÆ differ so little in their general characters that they may be spoken of as members or representative families of microscopic and unicellular algÆ alike in their remarkable beauty and bilateral symmetry, and of such peculiar interest as to call for special notice. Desmids differ from diatoms chiefly in colour, in lacking a non-silicious skeleton, and in their generative process, which for the most part consists in the conjugation of two similar cells. Diatoms, on the other hand, have dense silicious skeletons and a general absence of green colouring matter. Ralfs, in his systematic monograph, enumerates twenty genera of desmids. The limiting membrane is alike firm and flexible, since it exhibits some elasticity and resistance to pressure, and is not readily decomposable. Traces of silica are found in only a few of the desmids, while the frustule of the diatom is chiefly composed of this substance; both have an external membranous covering, so transparent and homogeneous in structure as to be in danger of being entirely overlooked, unless some staining material is used, together with a high-power objective possessing considerable penetration. In some species, however, the mucous covering is more clearly defined, as in Staurastrum and Didymoprium Grevelli. Openings occur in the outer membrane of other species, as the Closterium.

Many species of desmids have a power of motion, the cause of which must be due either to cilia or a flagellate organ. This, however, is denied by some observers, who regard their movements as due to an exudation of the mucilaginous contents of the cell, to exosmose, or diffusion, neither of which hypotheses will at all help us to understand the gliding movements of the OscillariÆ or the sharp jerky movement of the Schizonema. The movements of desmids are especially exerted when in the act of dividing, and by sunlight, towards which they are always observed to move. The force with which some diatoms move about is very great, and this can only be satisfactorily explained by admitting a specialised organ.

The appearance of the DesmidiaceÆ (Plate X.) is much modified by their eminences, depressions, and processes, as well as that of the surface, the margin of the fronds, and the depth and width of the central constriction. The surfaces may be dotted over irregularly, the dots themselves being elevated or depressed points in their structural character. The margins of some have a dentate appearance, as in Cosmarium. In the elongated forms, such as Penium, the puncta are disposed in lines parallel to the length. In several these lines are either elevations or furrows, it is not always easy to say which; they are peculiar, however, to the elongated forms of Closterium. When the lines are fine they produce a striation of the surface, but in order to discover this the fronds should be viewed when empty and by a fairly good power. The modification of surface in several genera seems to be due, not to mere simple appendages, but to expansion of the limiting membrance into thickened processes, and which may terminate in spines, as in Xanthidium and Staurastrum (Plate X., Nos. 8-19 and 22). A general distribution over the surface is characteristic of the former, but in Euastrum the surfaces are very irregular, and therefore described as “swellings or inflations.” Micrasterias has its margin deeply incised into lobes, which in some have a radiating arrangement; when the lobes on the margin are small they constitute crenations or dentations. The fronds of Euastrum binatum are bicrenate on the sides, as are those of Desmidium and Hyalotheca and other species. Another variety of margin exists, known as undulating or wavy, while the general concavity or convexity of the margins furnish other specific characteristics.

PediastreÆ (Plate X., Nos. 24-29).—The members of this family formerly included the Micrasterias and Arthrodesmius of Ehrenberg. From their arrangement of cells in determinate numbers and definite forms, it has been thought by some observers that they should be removed from the desmids to a special or sub-family. The points of difference consist in the firmness of the outer covering, in the frequent interruptions on the margin of the cells, and in the protrusion of “horns,” or rather a notch more or less deep. It is true that the cells are not made up of two symmetrical halves, and that they are in aggregation, which is not (except in the Scenedesmus, a genus that distinctly connects this group with desmids) in linear series, but in the form of discoidal fronds. They, however, divide into 8, 16, or 32 gonidia, and these move about for some time before the formation of a new frond. It was NÄgeli who first instituted a sub-genus of Pediastrum, under the designation of Anomopedium, the chief characteristic of which is the absence of bilobed peripheral cells. In Coelastrum the cells are hexangular, the central ones very regularly so; in Sorastrum they are wedge-shaped, or triangular, with rounded-off angles. Viewed laterally the cells appear oblong. The cells of Pediastrum are considerably compressed, so that when aggregated they form a flattened tubular structure; in figure they are polygonal, frequently hexagonal, a shape owing, in all probability, to mutual lateral pressure during growth. There is a pervading uniformity in the contents of the cells of the different genera, which consist of protoplasmic endochrome. At first the colour is pale green, but it becomes deeper with full maturity, while the decaying cells are seen to change to a deep reddish-yellow or brown. The protoplasm is also clear and homogeneous, but in time granules appear, enlarge, and multiply in number; moreover, each cell presents a single bright green vesicle, around which are collected clear circular spaces or globules, recalling those of Closterium, and varying in number from two to six or more, their position not being regulated by the partition wall as in PalmellÆ, but by the centre of the entire frond. Oil globules are also contained in the cells; their presence is indicated by the addition of a drop of tincture of iodine. On one occasion NÄgeli saw in Pediastrum boryanum the endochrome disposed in a radiating manner, an arrangement which often obtains in algals and in other vegetable cells with a central nucleus. The cells of PediastreÆ are always united together in compound fronds, as represented in Plate X., Nos. 24 and 29.56

The differences pointed out in no way constitute a claim to remove PediastreÆ from among DesmidiaceÆ, certainly not to rank as a distinct species.

Reproduction of DesmidiaceÆ.—A true reproductive act is presented by the conjugation or coupling of two fronds, and by the resulting development of a sporangium and subsequent interchange of the contents of the two cells. At another time self-division is frequently seen to take place in all respects as in the cells of other algÆ. The proceeding is varied in some essential particulars by the form of the fronds and by other circumstances; as in fission of Euastrum, for instance (seen in Plate X., Nos. 1, 2, and 12), when the narrow connecting bands between the two segments of the fronds are rapidly pushed aside by growth and finally divide. Two modes of conjugation of fronds are represented in Plate X., Nos. 25 and 33, in Closterium and Penium. The act of conjugation admits of variations in character, as shown in Staurastrum and Microsterias; the contents of both fronds are discharged into a delicate intermediate sac; this gradually thickens and produces spines (Plate X., Nos. 8 and 19). In Didymoprium the separate joints unite by a narrow process pushed out from each other, often of considerable length, through which the endochrome of one cell is transferred to the other, and thus a sporangium is produced within one of two cells, just as in the conjugatÆ (No. 5). In Penium Jennereri the conjugation takes a varied form; the fronds do not open and gape at the suture, but couple by small but distinct cylindrical tubes (No. 27).

Among those enumerated, the compressed and deeply constricted cells of Euastrum offer the more favourable opportunities for studying the manner of their division; for although the frond is really a single cell, in all its stages it appears like two, the segments being always distinct, from the earliest stage. The segments, however, are separated by a connecting link, which is subsequently converted into two somewhat round hyaline bodies. These bodies gradually increase and acquire colour, and as they grow the original segments are further divided, and at length become disconnected, each taking a new segment to supply the place of that from which it is separated. It is curious to trace the progressive development of the newer portions, which at first are devoid of all colour; but as they become larger a faint green tint is observed, which gradually darkens, and then assumes a granular appearance. Soon the new segments attain their normal size, while the covering in some species shows the presence of puncta. In Xanthidium, Plate X., Nos. 9, 10, and Staurastrum, Nos. 15-18, the spines and processes make their appearance last, beginning as mere tubercles, and then lengthening until they attain their perfect form and size, armed with setÆ; but complete separation frequently occurs before growth is fully completed. This singular process is repeated again and again, so that the older segments are united successively, as it were, with many generations. When the cells approach maturity, molecular movements may be at times noticed in their contents, precisely similar to what Agardh and others aptly term “swarming.” Meyen describes this granular matter as starch.57 Closterium, early in the spring, when freshly secured and exposed to light, presents a wonderful appearance, these bodies being kept continually in motion at both ends of the frustule by the ciliary action within the cell, and the whole frond is seen brilliantly glittering with active cilia. When a gleam of stronger light is allowed for a moment to fall on the frond, the rapid undulations of the cilia produce a series of most delicate prismatic Newton’s rings. The action and distribution of the cilia, together with the cyclosis of the granular bodies in the frond, are better seen by the aid of Wenham’s parabola or a good condenser with a central stop. One of the wide angular objectives shows the circulation around the marginal portions of the whole frond. The stream is seen to be running up the more external portion, internal to which is another stream following a contrary direction; this action, confined to the space between the mass of endochrome and the outer portion of the cell-wall, is seen to pass above or around the space in which cyclosis of the spores is taking place.

During the summer of 1854, the late Rev. Lord Sidney Godolphin Osborne and myself became much interested in the remarkable family of Closteria. Fig. 292 is a highly magnified view of Closterium lunula which I drew by the aid of the camera-lucida at the time. There could be no doubt about the ciliary action within the frond: it was in every way similar to that of the branchiÆ of the muscle, the same wavy motion, which gradually became slower as the death of the desmid drew near. This was brought about earlier when the cell was not kept supplied with fresh water.

In diagram A, line b points to a cluster of ovoid bodies; these are seen at intervals throughout the endochrome within the investing membrane. These bodies are attached to the membrane by small pedicles, and are occasionally seen in motion about the spot, from which they eventually break away, and are carried off, by the circulating fluid, to the chambers at the extremities of the frond; there they join a crowd of similar bodies, in constant motion within the chambers, when the specimen is quite fresh. That the action of these free granules or spores is “Brownian,” as surmised by some writers, is in my opinion entirely fallacious. It is doubtless in a measure due to the current brought about by the ciliary motion of the more fluid contents of the cell.

The circulation, when made out over the centre of the frond, for instance at a, is in appearance of a wholly different nature from that seen at the edges. In the latter the matter circulated is that of granules, passing each other in distinct lines, but in opposite directions; in the circulation as seen at a, the streams are broad, tortuous, of far greater body, and passing with much less rapidity. To see the centre circulation, use a Gillett’s illuminator and a ¹/₈th or a ¹/₁₀th immersion; work the fine adjustment so as to bring the centre of the frond into focus, then almost lose it by raising the objective; after this, with great care, work the milled head until the darker body of the endochrome is clearly brought out.

At B is an enlarged sketch of one extremity of the frond. The arrows within the chamber pointing to b denote the direction of a strong current of fluid, which can be occasionally followed throughout. It is acted upon by cilia at the edges of the chamber, the greater impetus appearing to come from the centre of the endochrome. The fluid is here acting in positive jets, that is, with an almost arterial action; and according to the strength with which it is propelled at the time, the loose floating bodies are sent to a greater or less distance from the end of the frustule; the fluid is thus impelled from a centre, and kept in activity by the lateral cilia, that create a rapid current and give a turning motion to the free bodies. The line—a, in this diagram, denotes the outline of the membrane which encloses the endochrome; on both sides cilia can be seen. The circulation exterior to it passes and repasses in opposite directions, in three or four distinct courses; these, when they arrive at—c, seem to encounter a stream making its way towards an aperture at the apex of the chamber; then they appear to be driven back again by a stronger force. Some, however, do occasionally enter the chamber, but very rarely will one of the bodies escape into the outer current, and should it do so, is carried about until it becomes adherent to the side wall of the frond.

With regard to the propagation of the C. lunula, I have never seen anything like conjugation; but I have repeatedly seen self-division (shown at D a a). This act is chiefly the work of one half of the frond. Having watched for some time, one half is seen to remain passive, while the other has a lateral motion from side to side, as if moving on an axis at the point of juncture; the motion increases, is more active, until at last with a jerk one segment separates itself from the other, as seen at E. It will be noticed that each end of the segment is perfectly closed before separation finally takes place; there is, however, only one perfect chamber, that belonging to the extremity of the original entire frond. The circulation continues for some time previous to and after subdivision, in both fronds, and by almost imperceptible degrees increases in volume. From the end of the endochrome symptoms of elongation of the frond take place, the semi-lunar form gradually changes, elongates, and is more defined, until it takes the form and outline of the fully-formed frustule at the extremity. The obtuse end—b of the other portion of frond is at the same time elongating and contracting, and in a few hours from the division of the one segment from the other the appearance of each half is that of a nearly perfect frustule, the chamber at the new end is complete, the globular circulation exterior to it becomes affected by the circulation from within the said chamber, and, shortly afterwards, some of the free bodies descend, and become exposed to the current already going on in the chamber. E is a diagram of one end of a C. didymotocum, in which the same process was well marked, and completed while it was under observation.

It will appear to most observers that if the continuation of the widely-spread family of DesmidiaceÆ was wholly dependent upon conjugation and subdivision of their frustules, a process requiring several hours for its completion, the whole species must have long ago disappeared. It may be presumed then that some other mode of reproduction must prevail. In the fresh-water algÆ the two more general methods of multiplication are clearly governed by the conditions of the seasons; the resting-spores securing continuity of life during the winter, the swarm-spores spreading the plant profusely during the warmer portion of the year, when rapid growth is possible. I therefore regard the actively swarming bodies seen in continuous motion at the two extreme portions of the frustule of Closterium lunula as being either oospores or zoospores, by means of which reproduction takes place.

DiatomaceÆ, commonly called brittleworts, Plate XI., are chiefly composed of two symmetrical valves, narrow and wand-like, navicular, miniature boat-shaped, hence their name Navicula (little ship). Hitherto they have excited the deepest interest among microscopists because of their wonderfully minute structure, and the difficulty involved in determining their exact nature and formation. Each individual diatom has a silicious skeleton, spoken of as a frustule, frond, or cell, having a rectangular or prismatic form, which mostly obtains in the whole family, the angles of the junction of the two united valves being, as a rule, acute, and enclosing a yellowish-brown endochrome. Deeply-notched frustules, like those of the DesmidiaceÆ, do not occur, and the production of spines and tubercles so common in that family is rare in the DiatomaceÆ. Great variety of outline prevails, so much so that no rule in this respect can be formulated.

The frustules, however, are usually composed of two equal and similar halves, but exceptions to this are found in the ActinomtheÆ, CocconcidÆ, and one or two other families. The extremities of some species, e.g., Nitzshia and Pleurosigma, are extremely elongated, forming long, filiform, tubular processes; in Biddulphia and Rizoselenia, short tubular processes from their margins. Great variety of outline may prevail in a genus, so considerable indeed that no accurate definition can be given, the characteristics shading off through several species until the similarity to an assumed typical form is much diminished, which may again be modified by accidental circumstances that surround the development of the silicious frustule. It must not be forgotten that the figure is greatly modified or entirely changed by the position of the valves, whether seen in one position or another, as already explained in connection with “Errors of Interpretation.” Again, in the genera Navicula, Pinnularia (Plate II., Nos. 33, 38, and 40), and others, the frustules are in one aspect boat-shaped, but in the other either oblong with truncated ends, or prismatic. In the genus Triceratium (Plate XI., No. 10), the difference of figure is very remarkable as the front or side view is examined.

The sudden change in appearance presented to the eye as the frustule is seen to roll over is rather peculiar. As a rule, therefore, we must examine all specimens in every aspect, to accomplish which very shallow cells should be selected, say of ¹/₁₀₀th of an inch deep, and covered with glass ¹/₂₅₀th of an inch thick. A good penetrating objective should be used, and careful illumination obtained. The DiatomaceÆ are perhaps more widely distributed than any other class of infusorial life; they are found in fresh, salt, and brackish water; many grow attached to other bodies by a stalk (Plate II., No. 33, Licmophora and Achnanthidium); while others, as Pleurosigma, No. 40, swim about freely.

PLATE XI.

DIATOMACEÆ, RECENT AND FOSSIL.

There are a considerable number of DiatomaceÆ which, when in the young state, are enclosed in a muco-gelatinous sheath; while others are attached by stipes or stalk to algÆ. It would be vain, in a limited space, to attempt a description of this numerous and extensive family. NÄgeli and other observers describe a “mucilaginous pellicle on the inner layer of the valves,” while, as Menghine observes, “an organic membrane ought to exist both inside and outside, for the silica could not become solid except by crystallizing or depositing itself on some pre-existing substance.” The surface of the frustules is generally very beautifully sculptured, and the markings assume the appearance of dots (puncta), stripes (striÆ), ribs (costÆ), pinnules (pinnÆ), of furrows and fine lines; longitudinal, transverse, and radiating bands; canals or canaliculi; and of cells or areolÆ; whilst all present striking varieties and modifications in their form, character, and degree of development. Again, the fine lines or striÆ of many frustules are resolvable into rows of minute dots or perforations, as occur in Pleurosigma angulatum, delineated in the accompanying microphotograph (Fig. 294), taken for the author purposely to show the markings on this especially selected test diatom.

The nature of the markings on the diatom valves is one of considerable interest, and attempts have been made to produce them artificially, but without success.

Professor Max Schultze devoted a great amount of time to the investigation of the subject, and has recorded in a voluminous paper58 the results of his observations. He says, “Most of the species of the DiatomaceÆ are characterised by the presence on their outer surface of certain differences of relief, referable either to elevations or to depressions disposed in rows. The opinions of microscopists with respect to the nature of these markings are still somewhat divided. Whilst in the larger forms, and those distinguished by their coarser dots, the appearance is manifestly due to the existence of thinner spots in the valve, we cannot so easily explain the cause of the striation or punctation in Pleurosigma angulatum and similar finely-marked forms.”

Dr. R. Zeiss some time ago furnished me with a microphotograph of a frustule magnified 4500 diameters that seemed to confirm Mr. T. F. Smith’s view of the structure of these valves. Dr. Van Heurck has also made a study of this diatom, and concludes that the valves consist of two membranes of thin films, and of an intermediate layer, the outer being pierced with openings. The outer membrane is, he believes, “so delicate that it is easily destroyed by acid or by friction, and the several processes employed in cleaning and preparing it for microscopical examination. When the openings or apertures of the internal portion are arranged in alternate rows they assume the hexagonal form; when in straight rows, the openings are seen to be square or oblong.” A description hardly in accord with Fig. 294.

Movements of Diatoms.

The late Professor Smith, in his “Synopsis of Diatoms,” refers to their movements in the following terms: “I am constrained to believe that the movements observed in the DiatomaceÆ are due to forces operating within the frustule, and are probably connected with the endosmotic and exosmotic action of the cells. The fluids which are concerned in these actions must enter, and be emitted through the minute foramina at the extremities of the silicious valves.” Schultze’s researches, which were made at a later date, carried this debatable question somewhat further. He is of opinion “that a sarcode (protoplasmic) substance envelops the external surface of the diatoms, and its movements are due to this agent exclusively.” His investigations were mainly confined to P. angulatum, and to the larger P. attenuatum (Fig. 293, 1 and 2), as the transverse markings on the frustule do not impede to so great an extent the observation of what is going on within. The living specimen of P. angulatum under the microscope usually has its broad side turned to view, with one long curved “raphe” uppermost, and the other in contact with the glass cover (Fig. 293). Within the frustule the yellow colouring matter, “endochrome,” fills the cavity more or less completely. In the broader part of the frustule these bands of endochrome describe one or two complicated windings. It is only possible in those specimens in which the bands are narrow to properly trace their foldings, and determine their number. The next objects which strike the eye on examining a freshly-gathered Pleurosigma are numerous highly refractive oil-globules. These are not, however, all in the same place, and one globule appears nearer the observer than the other; their relative position is best seen when a view of the narrow side of the frustule can be obtained, so that one raphe is to the left and the other to the right. The blue-black colour which is assumed by these globules after treating with acid demonstrates their oleaginous nature. The middle of the cavity of the frustule is occupied, in the larger navicula, by two large oil-globules (seen in the diagrammatic Fig. 295), and by a colourless finely granular mass, whose position in the body is not so clearly seen in the flat view as in the side view. Besides the central mass, the conical cavities at either end of the frustule are seen to enclose granular substance, and two linear extensions from each of three masses are developed, closely underlying the raphÆ. In the side view, therefore, they appear attached to the right and left edges of the interior of the frustule. This colourless granular substance carries in its centre, near the middle part of the diatom, an imperfectly developed nucleus which is not very easy to see, but may be demonstrated by the application of an acid. The colourless substance is protoplasm, and encloses numerous small refractive particles; this, on adding a drop of a one per cent. solution of osmic acid, is coloured blue-black, and proves to be fat. It is, however, exceedingly difficult to determine the exact limitations of the protoplasm, on account of the highly refractive character of the silicious skeleton, and the obstruction to the light presented by the endochrome.

At a short distance the protoplasm reappears, contracted into a considerable mass, within the terminal ends of the frustule. Schultze observed in this part of the protoplasm a rapid molecular movement, “cyclosis,” such as occurs in Closterium, and also a current of the granules of the protoplasm along the raphe. “Pleurosigma angulatum ‘crawls,’ as do all diatoms possessing a raphe, along this line of suture. To crawl along, it must have a fixed support.” “There is obviously,” adds Schultze, “but one explanation; it is clear that there must be a band of protoplasm lying along the raphe, which causes the particles of colouring matter to adhere, and gives rise to a gliding movement. For there is but one phenomenon which can be compared with the gliding motion of foreign bodies on the DiatomaceÆ, and that is, the clinging to and casting off of particles by the pseudopodia of the rhizopod, as observed, for instance, on placing a living Gromia or Miliolina in still water with finely-powdered carmine. The nature of the adhesion and of the motion is in both cases the same. And since, with diatoms as unicellular organisms, protoplasm forms a large part of the cell (in many cases two distinctly moving protoplasms), this implies that the external movements are referable to the movements of the protoplasm.” It is quite evident to those who have studied the movements of diatoms that they are surrounded by a sarcode structure of a more pellucid character than that of Amoeba. Six years before Schultze’s observations were published, I wrote in a third edition of my book, page 307, “The act of progression favours the notion of contractile tentacular filaments—pseudopodia—as the organs of locomotion and prehension.”

Since my former observations on the movements of diatoms, I have given much attention to two forms, P. angulatum and Pinnularia. The powers used were Hartnack’s No. 8, and Gunlack’s ¹/₁₆-inch immersion; Gillett’s condenser illumination, with lamp flame edge turned to mirror and bull’s-eye lens; a perforated slide with a square of thin glass ·006 cemented to it, and a cover-glass of ·005. So far as I could satisfy myself, no terminal space, as in the Closteria, could be seen, otherwise the course of the gemmules is as freely traced as in that form. They are more minute than the Closterium lunula granules, more steadily or slowly seen to pass up and down one half the frustule towards the extremity, one half of the current seeming to turn round upon its axis and descending towards the other. The granules were thickly scattered at the apex, but gradually became fewer, and the ascending and descending current tapered away towards the central nodule, which became more filled up or closed in.

This beautiful sight was not confined to one frustule, but was exhibited in all that were in a healthy condition. I examined several, and watched them for a long time. The phenomenon described depends much upon the healthy condition of the frustule at the time; as the movements of the diatoms became sluggish, the circulation gradually slackens and then ceases altogether. I also saw a somewhat similar action in the more active specimens of P. hippocampus and Navicula cuspidata, but the coarser markings and thickness of the wall of these diatoms seemed to place greater difficulties in the way of observation than the finer valves of the P. angulatum. One thing I believe is certain, that the circulation described is precisely similar to that seen in the Closteria, or, on a much larger scale, in Chara and the leaf of the Anacharis, bearing in mind also that in the Closterium the cell is divided by a transverse suture, and in P. angulatum by a longitudinal one (Plate II., Nos. 38-40). About the same time some very lively specimens of the PinnulariÆ were sent to me, and the movements of these frustules were more closely observed. One or two of the more active would attack a body relatively larger than itself, it would also force its way into a mass of granular matter, and then recede from it with a jerky motion. In more than one instance a cell of PalmoglÆa was seized and carried away by the Pinnularia, the former at the time being actively engaged in the process of cell division. Other diatoms present among my specimens were also in an active condition, and the circulation of granular matter in all was distinctly visible. In the PinnulariÆ two large colourless vesicles were seen on either side of the median nodule, each having a central nucleus, as represented in the accompanying sketch, made while under observation in two positions. The central portion of each frustule was closely packed with a rich yellowish-brown coloured endochrome, interspersed with a few fat globules. The phenomenon of cyclosis was not seen in any of these diatoms, but I have satisfied myself, by staining, of the presence of a delicately fine external protoplasmic covering in many diatoms. That their movements resemble the gliding movements exhibited by the Amoeba can scarcely be doubted. Numerous forms of DiatomaceÆ are found growing on or attached to water-plants or pieces of detached stalks, which, although generally simple, are sometimes compound, dividing and subdividing in a beautiful ramous manner. PinnulariÆ, Nitzschia, &c., are seen adherent by one extremity, about which they turn or bend themselves as on a hinge. By the process of cell-division, groups of SynedrÆ become attached by a point, in a fan-like form. The fan-like collection of frustules is said to be flabellate, or radiate. In Licmophora, Achnanthes and other species (Plate II., Nos. 29-33) the double condition of union of frustules and of attachment by a pedicle are illustrated. When a stipe branches it does so normally in a dichotomous manner, each new individual being produced by a secondary pedicle. This regular dichotomy is seen in several genera: Cocconema and Gomphonema, the latter more perfectly in Fig. 296, from a microphotograph, in which a branching, or rather longitudinal, rupture of the pedicle takes place at intervals, and the entire organism presents a more or less complete flabella, or fan-like cluster, on the summit of the branches, and imperfect or single frustules irregularly scattered throughout the whole length of the pedicle.

Isthmia enervis (Fig. 297).—The unicellular frustule of this species is extremely difficult to define, owing to the large areolations of the valves; it has a remarkable internal structure. The olive-brown cell contents are found collected, for the most part, into a central mass, from which radiating, branched, granular threads extend to and unite with the periphery. When viewed by a magnifying power of 600 or 700 diameters, these prolongations are seen to be composed of aggregations of ovate or spindle-shaped corpuscles, held together by protoplasmic matter. These bodies are sometimes quiescent, but more often travel slowly to and fro from the central mass. The general aspect under these conditions so nearly corresponds to the characteristic circulation in the frustules of unicellular plants and of certain rhizopoda, that it is difficult to realise that the object when under examination is an elegant marine diatom.

There is a large section of diatoms in which the frustules are diffused throughout a muco-gelatinous envelope in a definite manner. Histologically this is homologous with the pedicles and connecting nodules thrown out during the act of self-division, and in some species (Cocconeis, Fragillaria, &c.) it often persists after that act is complete.

DiatomaceÆ, Recent and Fossil.

Fossilised DiatomaceÆ.—Dr. Gregory was of opinion that a large number of diatoms separated into species are only transition forms, and more extended observations have proved that form and outline are not always to be trusted in this matter. Species-making is a modern invention, and can hardly apply to the indestructible fossilised forms of the frustules of DiatomaceÆ, with their beautiful sculpturings and geometrical constructions, which have not been materially changed since they were first deposited. Startling and almost incredible as the assertion may appear to some, it is none the less a fact established beyond all question, that some of the most gigantic mountain-ranges, as the mighty Andes, towering into space 25,250 feet above the level of the sea, their base occupying vast areas of land; as also massive limestone rocks; the sand that covers boundless deserts; and the soil of many wide-extended plains, are each and all principally composed of DiatomaceÆ. And, as Dr. Buckland once observed: “The remains of such minute animals have added much more to the mass of materials which compose the exterior crust of the globe than the bones of elephants, hippopotami, and whales.”

In 1841 the late Mr. Sollitt, of Hull, discovered the beautiful longitudinal and transverse striÆ (markings) on the Pleurosigma hippocampus. A curved graceful line runs down the shell, in the centre of which is an expanded oval opening. Near to the central opening the dots elongate crossways, presenting the appearance of small short bands.

In the vicinity of this town many interesting varieties of DiatomaceÆ have been found, the beauty of the varied forms of which are constantly under investigation; at the same time some of them are highly useful, as forming that class of test objects which are better calculated than many others for determining the excellence and powers of certain objectives. Mr. Sollitt carefully measured the markings on some of the frustules and found they ranged between the ¹/₃₀₀₀₀th and ¹/₁₃₀₀₀₀th of an inch; the Pleurosigma strigilis having the strongest markings, and the Pleurosigma acus the finest.

Mr. J. D. Sollitt not only first proposed their use, but he also furnished the measurements of the lines of the several members of this family, as follows:—

Amphipleura pellucida, or Acus, 130,000 in the inch, cross lines.
" sigmoidea, 70,000 in the inch.
Navicula rhomboides, 111,000 in the inch, cross lines.
Pleurosigma fasciola, fine shell, 86,000 in the inch, cross lines.
"" strong shell, 64,000 in the inch, cross lines.
" strigosum, 72,000 in the inch, diagonal lines.
" angulatum, 51,000 in the inch, diagonal lines.
" quadratum, 50,000 in the inch, diagonal lines.
" Spencerii, 50,000 in the inch, cross lines.
" attenuatum, 42,000 in the inch, cross lines.
" Balticum, 40,000 in the inch, cross lines.
" formosum, 32,000 in the inch, diagonal lines.
" strigilis, 30,000 in the inch, cross lines.

LichenaceÆ.

The lichens are a family of autonomous plants, an intermediary group of algals or cellular cryptogams, drawing their nourishment from the air through their whole surface medium, and propagating by spores usually enclosed in asci, and always having green gonidia in their thallus. Their gonidia, bright coloured globular cells, form layers under the cortical covering of the thallus, and generally develop in the form of incrustations, which cover stones, wood, and the bark of trees, or penetrate into the lamellÆ of the epidermis of woody plants. The gonidia of lichens partake of both the character of vegetative and reproductive cells.

The thallus in the fructicose group attaches itself by a narrow base, growing in the form of a miniature shrub. Another group is met with in a slimy condition—the gelatinous lichens. These species, for the most part, furnished dyes before the discovery of the coal-tar dyes. In many of the Palmella cruenta, commonly found growing on the walls and roofs of houses, a colourless acid liquid is found, which, on being treated with alkali, produces a bright yellow colour; and another, Avernia vulpina, furnishes a brown dye; the Rocella fuciformis and R. tinctoria yield the purple dye substance known as orchil, or archil, from which the useful blue paper of the chemist for testing acidity is manufactured. Usnic acid, combined with green and yellow resins, seems to be more or less a constituent of many lichens.

A vertical section of Palmella stellata is given in Plate I., No. 26, in which the emission of the ripe spores of the lichens is seen to be not unlike that which takes place in some of the fungi, PezizÆ, SphÆriÆ, &c. If a portion of the thallus be moistened and placed in a common phial, with the apotheca turned toward one side, in a few hours the opposite surface of the glass will be found covered with patches of spores, easily perceptible by their colour; or if placed on a moistened surface, and one of the usual glass slips laid over it, the latter will be covered in a short time. As to the powers of dissemination of these lowly organised plants, an observation led to the conclusion that the gonidia of lichens have greater powers in this direction than was formerly supposed. It is found that by placing a clean sheet of glass in the open air during a fall of snow, and receiving the melting water in a tube or bottle, quantities of what has been looked upon as a “unicellular plant” can be taken, the cells of which may be kept in a dormant condition for a long time during cold weather, but upon the return of spring warmth and moisture they begin to increase, by a process of subdivision, into two, four or eight portions; these soon assume a rounded form, and burst the parent cell-wall open; these secondary cells then begin to divide and subdivide again, and the process may go on without much variation for a long time. The phenomena described may be watched by taking a portion of the bark of a tree on which Chlorococcus has been deposited, and placing it under a glass to keep it in a moderately moist atmosphere; the only difference being a change in colour, caused by the growth of the fibres, as may be seen on microscopical examination. “And this,” says Dr. Hicks, who first observed this phenomenon in plant life, “is an instructive point, because it will be found that the colour varies notably according to the lichen prevalent in its neighbourhood.”59 He believes there can be no doubt that what has been called Chlorococcus is nothing more than the gonidia of a lichen; and that under suitable conditions, chiefly drought and warmth, the gonidium often throws out from its external envelope a small fibre, which, adhering and branching, forms a “soridium.” “The soridia remain dormant for a very long time, and do not develop into thalli unless in a favourable situation, in some cases it may be for years. It will be perceived that the soridium contains all the elements of a thallus in miniature; in fact, a thallus does frequently arise from one alone, and the fibres of neighbouring soridia interlace; thus a thallus is matured very rapidly. This is one of the causes of the variation of appearance so common in many species of lichens, more readily seen towards the centre of the parent thallus. When the gonidia remain attached to the parent thallus, the circumstances are, of course, more favourable, and they develop into secondary thalli, attached more or less to the older one, which, in many instances, decays beneath them. This process being continued year after year gives an apparent thickness and spongy appearance to the lichen, and is the principal cause of the various modifications in the external aspect of the lichens which caused them formerly to be misunderstood and wrongly classified.”60

The erratic lichens are found among the genus Palmella, some of which grow among boulders of the primary and metamorphic formations, curled up into a ball, and only fixed to their matrix by a slender thread. The globular Lecanora esculenta will at times suddenly cover large tracts of country in Persia and Tartary, where it is eaten by the cattle. During a scarcity of food a shower of these lichens, Mr. Berkeley tells us, fell at Erzeroum, and saved the cattle from starvation.61

Another group of the Palmella, or Peltigeri, so named from the target-like discs on their surface, spread their foliaceous fronds over the ground, and as the fruit is marginal, it gives the thallus a digitate appearance. These are often spotted over by a little red fungus. The Lecidinei contains numerous species of the most varied habits, and always crustaceous, and so closely adherent to the hard rocks and stones on which they grow, that at length they disintegrate them. From this low species a higher form arises, with erect branching stems, and clothed with foliaceous, brightly-coloured scales.

The Coccocarpei is mainly distinguished by having orbicular discs entirely deprived of the cortical envelope called an excipulum. The discs spring at once from the medullary stratum, and contain asci and sporidia similar to those of the minute fungi SphÆriÆ. Some of the lichens are themselves parasitic, and begin existence under the thick skin of the leaves of tropical plants, and spread encrusting thallus over their surface, the excipulum and perithecia being black; but in most cases these are beautifully sculptured, and are interesting objects for the microscope. Indeed, the sphere-bearing lichens, with upright stems bearing globular fruit at the extremity of their branches, are at first indicated by a swelling, but in time the outer layer bursts and exposes sporidia, which are beautiful objects under the microscope on account of their spherical form and more or less deep blue tint. Humble and lowly as lichens may appear to be, they have been divided into fifty-eight or more genera and 2,500 species. The brothers Tulasne, De Bary, the Rev. Mr. Berkeley, and others, devoted great attention to the peculiarities of their structure and natural history.

HepaticÆ.—An intermediary group of much interest to the microscopist are the HepaticÆ (liverworts). These are found growing on damp rocks in the neighbourhood of springs and dripping banks. The scale-moss, the Marchantia polymorphia (Fig. 299), may be taken as typical of this little group, with its gemmiparous conceptacles and lobed receptacles, bearing archegones on transparent glass-like fruit stalks, carrying on their summits either round shield-like discs or radiating bodies with a striking resemblance to a wheel without its tyre.

The liverworts are closely allied to the mosses, and as much difficulty was experienced in dividing the two, Hooker placed the whole under one genus, the Jungermannia. More recently, however, they have been divided into those with a stem and leaves confluent in a frond, Marchantia; those with stem and leaves distinct, Jungermannia; and those with a solitary capsule, filiform, bivalved, stalked, with a free central placentation, AnthocotaceÆ. Some botanists have further divided them, but they are all extensively propagated by gemmÆ.

The fronds carry the male organs, or antherids, and the disc, in the first instance, bears the female organs, or archegones, and after a time gives place to the sporanges, or spore cases. It is these bodies which are of so much interest to microscopists; if the plant is brought into a warm room, they suddenly burst open with some violence the moment a drop of water is applied to them, and the sporanges are dispersed in a small cloud of brownish dust. If this dust is examined under a medium power, it is seen to consist of a number of chain-like bodies, somewhat like the spring of a small watch; and if the process of bursting be closely watched, these minute springs will be found twisting and curling about in every direction. The structure of the frond itself will be seen to be interesting when cut in the vertical direction and placed under the microscope.

The gemmÆ of Marchantia polymorphia are produced in elegant membranous cups, with a toothed margin growing on the upper surface of the frond, especially in very damp courtyards between the stones, or near running water, where its lobed fronds are found covering extensive tracts of moist soil. At the period of fructification the fronds send up stalks, which carry at their summit round shield-like radiating discs, which bear upon their surface a number of little open basket-shaped “conceptacles.” These again expand into singularly graceful cups (as in Fig. 300), and are found in all stages of development. When mature, the basket contains a number of little green round or oblong discs, each composed of two or more layers of cells; the wall itself being surmounted by a glistening fringe of teeth, whose edges are themselves regularly fringed with minute outgrowths. The cup seems to be formed by a development of the superior epidermis, which is raised up, and finally bursts and spreads out, laying bare the seeds.

The archegones of Marchantia are very curious bodies, while the elater and spores are even still more so. These are elongated cells, each containing a double spiral fibre coiled up in the interior. It is the elasticity of this which tears apart the cell-membrane, and sends forth the spores with a jerk, and thus assists in their dispersion. Marchantia is the type of the malloid HepaticÆ.

Musci, Bryophyta.

Mosses are a beautiful class of non-vascular cryptogams. LinnÆus called them servi, servants or workmen, as they seem to labour to produce vegetation in places where soil is not already formed. The Bryophyta form three natural divisions: the BryinÆ, or true mosses; the SphagnaceÆ, or peat-mosses; and the HepaticÆ, or liverworts. The two first are commonly united. In these the sexual organs consist of antheridia and archegonia, but they are of simpler structure than will be found in ferns; and the first generation from the spore is asexual.

The common or wall screw-moss (Fig. 301) grows almost everywhere, and if examined closely, is seen to have springing from its base numerous very slender stems, each terminating in a dark brown case, which encloses antheroids. If a patch of the moss is gathered when in this state, and the green part of the base is put into water, the threads of the fringe will uncoil and disentangle themselves in a most curious and beautiful manner; from this circumstance the plant takes its popular name of screw-moss. The leaf usually consists of either a single or a double layer of cells, having flattened sides, by which they adhere one to another. The leaf-cells (Fig. 302) of the Sphagnum or bog-moss exhibit a curious departure from the ordinary type; they are large, polygonal, and elongated, and contain spiral fibres loosely coiled in their interior. The young leaf does not differ from the older; both are evolved by a gradual process of differentiation.

Mosses, like liverworts, possess both antheridia and pistillida, which are engaged in the process of fructification. The fertilized cell becomes gradually developed into a conical body elevated upon a footstalk, the walls of the flask-shaped body carrying the higher part upwards as a calyptra or hood upon its summit, while the lower part remains to form a kind of collar round the base. These spore-capsules are closed on their summit by opercula or lids, and their mouths when laid open are surrounded by a beautiful toothed fringe, termed the peristome. This fringe is shown in Fig. 303, in the centre of a capsule of Funaria, with its peristome in situ. The fringes of teeth are variously constructed, and are of great service in discriminating the genera. In Neckera antipyretica the peristome is double, the inner being composed of teeth united by cross bars, forming a very pretty trellis. The seed spores are contained in the upper part of the capsule, where they are clustered round the central pillar, termed the columella; and at the time of maturity, the interior of the capsule is almost entirely occupied by spores.

The undulating hair-moss, Polytrichum undulatum (Fig. 304), is found on moist, shady banks of pools and rivulets. The seed-vessel has a curious shaggy cap; but in its construction it is very similar to that of the screw-moss, except that the fringe around its opening is not twisted. The reproductive organs of mosses are of two kinds; the capsule containing minute spores, archegonia, and the antheridia, or male efflorescence. The capsule, theca, or sporangium, is lateral or terminal, sessile, or on a fruit stalk (seta) of various shapes, indehiscent, or bursting by four valves at the sides, or more commonly by a deciduous cup, operculum. When this falls the mouth of the capsule becomes exposed. The rim is crowned with tooth-like or cilia-like appendages in sets of four or multiples of that number—peristome. These are often brightly coloured and hydroscopic. By simply breathing upon them they suddenly fly open, and are endowed with motion, that is, if they contain spores. The spores on germination produce a green confervoid-like mass of threads, from which the young plant arises.

The SphagnaceÆ, or “bog mosses,” have been separated from true mosses from the marked differences they present. The stem is more widely differentiated, and throughout its structure a rapid passage of fluid takes place. It has the power of absorbing moisture from the atmosphere, so that if a plant be placed dry in a glass of water with its rosette of leaves hanging over the edge, it acts like a syphon, and the water will drop from it until the glass is emptied. As may be supposed, the leaf is composed of large open cells, and it absorbs more water than the root. The antherids or male organs of SphagnaceÆ resemble those of liverworts, rather than those of mosses, both in form and arrangement; they are grouped in “catkins” at the tips of the lateral branches, each of the imbricated perigonal leaves enclosing a single globose antherid on a slender foot-stalk, and surrounded by long branched paraphyses of cobweb-like tenuity. The female organs, or archegones, do not differ materially in structure from those of mosses; they are grouped together in a sheath of deep green leaves at the end of the shorter lateral branchlets at the side of the rosette or terminal crown of leaves. The sporange is very uniform in all the species, and the spores are in groups of fours, as in mosses, around a hemispherical columella. These plants grow so rapidly that they soon cover a pool with thin matted bundles of branches, and as they decay they fall to the bottom, and become the foundation of the future bog or peat moss.

Felices.—Of all the spore-bearing families the ferns are the more universally known. They constitute an exceedingly numerous genera and species, and vary from low herbaceous plants of an inch in height to that of tree ferns, which attain a height of fifty or more feet, terminating in a graceful coronet of fronds or leaves. Of whatever size a fern may be, its spores are, for the most part, microscopic, produced within the sporangium by cell division, and are therefore free and variously shaped.

The true mode of development of ferns from their spores was that furnished by NÄgeli, who announced the existence of antheridia. On the spore starting into life it sends out from the cell-wall of its outer coat a white tubular projection, or root fibre (Fig. 305, A, B, and C), which passes through the cell-wall of its outer coat. This attracts sufficient moisture to burst open the outer, and then it begins to increase by the subdivision of its cells, until the primary green prothallus D is formed. This falls to the ground, and, being furnished on its under side with thread-like fibres, fixes itself to the earth, and thus is developed the rhizome, or root of the future plant. In each of the antheridia, which are numerous, a cell is formed, chiefly filled with albuminous matter and free spores, each having attached a flat ribbon-like filament, or stermatoid, curled in a spiral manner. These are ultimately set free by the rupture of the cell-wall, and commence revolving rapidly by the agency of the whip-like appendage at the larger end.

The sporangia, or spore-cases, are, for the most part, globular in form, and are nearly or quite surrounded by a strong elastic ring, which in some cases is continued to form a stalk. When the spores are ripe, this ring, by its elastic force, tears open the sporangia and gives exit to a quantity of microscopic filaments, curled in corkscrew-like fashion (Figs. 305 and 307). The ring assumes various forms; in one group it passes vertically up the back of the sporangium, and is continued to a point termed the stomata, where the horizontal bursting takes place. This form is seen in Fig. 306, a, b. In other groups it is vertical, as in c, c; in others transverse, as in d; or apical, as at e; and in a few instances it is obsolete, as in f. These are the true ferns, and their systematic arrangement is chiefly founded on the peculiarity of the sori and sporangia, characters which become quite intelligible by the aid of the microscope.

The beautiful ringed sporangium of the fern (Fig. 307) when ruptured gives exit to the dust-like spores; these, examined under a moderate power, are seen to be sub-globose and pyramidal, the outer coat or exospore being a coloured hyaline cell with nuclei similar to the spores of mosses, but in which chlorophyll soon begins to form, and from this little green embryonic growth the organs of reproduction are formed.

In all ferns the pistillidia or archegonia are analogous to the ovules or nascent seeds of flowering plants, and contain, like them, a germinal vesicle, which becomes fertilized through the agency of the spiral filaments, and then gradually develops into an embryo plant possessing a terminal bud. This bud begins at once to unfold and push out leaves with a circinate vernation, of a very simple form at first, and growing up beneath the prothallium, coming out at the notch; single fibrous roots are at the same time sent down into the earth, the delicate expanded prothallium withers away, and the foundation of the perfect fern plant is laid. When a fern acquires a considerable stem, as in a tree fern, it consists of cellular tissue and an external cortical portion forming fibro-vascular bundles, scalariform ducts, and woody fibre. Fig. 308, b, shows an oblique section of the footstalk of a fern leaf with its bundle of scalariform ducts.

These observations on ferns have acquired increased interest from subsequent investigations made on the allied Cryptogams, and on the processes occurring in the impregnation of the Conifers. Not only have later researches furnished a satisfactory interpretation of the archegonia and antheridia of the mosses and liverworts, but they have made known and co-ordinated the existence of analogous phenomena in the EquisetaceÆ, LycopodiaceÆ, and RhizocarpeÆ, and prove, moreover, that the bodies described by Dr. Brown in the Conifers under the name of “corpuscles” are analogous to the archegonia of the Cryptogams; so that a link is hereby formed between these groups and the higher flowering plants.

EquisetaceÆ.—The development of Horse-tails (Fig. 309), the name by which they are commonly known, corresponds in some respects with that of ferns. They comprise a little group, and the whole of their structure is composed in an extraordinary degree by silex, so that even when the organic portion has been destroyed by prolonged maceration in strong acid, a consistent skeleton still remains. It is this flinty material that constitutes their chief interest for microscopists. A portion of their silicious particles is distributed in two lines, arranged parallel to the axis of the plant, others are grouped into oval forms, and connected by a chain as in a necklace. The form and arrangement of the crystals are better seen under polarised light. Plate VIII., No. 170, a portion of the epidermis, forms an extremely beautiful object. Sir David Brewster pointed out that each silicious particle has a regular axis of double refraction. What is usually said to be the fructification of the EquisetaceÆ forms a cone or spike-like extremity to the top of the stem (Fig. 309), the whole resembling a series of spike-like branches (the real stem being a horizontal rhizome), and a cluster of shield-like discs, each of which carries a circle of sporanges that open by longitudinal slits to set free the spores which are attached to it in two pairs of elastic filaments (shown in Fig. 291, F, G), elaters; these are at first coiled up around the spore in the manner represented at G, but on their liberation they extend themselves as shown at F. The slightest moisture will close them up again, and their purpose having been served in the distribution of the spores, they are no longer required. If a number of spores be spread out on a glass-slip under the microscope and, while watching, a bystander breathes upon them, they immediately respond, are set in motion, presenting a curious appearance, but as soon as the hydroscopic effect has passed off they return to their previous condition. These spores can be mounted in a cell with a movable cover, and made to exhibit the same effect over and over again.

Fig. 309.—Equisetum giganticum.

a. Fragment of stem showing mode of branching out; b. Cone or spike of fructification; c. Scale detached from cone; d. Spore with elastic filaments; e. Vertical section of stem; f. Transverse section showing hexagonal cells.

The vascular tissue of the EquisetaceÆ (Fig. 309, e, f) shows them to be of a higher grade than the ferns. More recently discovered Horse-tails, of Brazil, grow to a gigantic size, but even these are comparatively small when compared with the Calamites, and other fossil EquisetaceÆ of the coal measures and new red sandstone. They all require a calcareous flinty soil for growth. A spring water-course making its way to the sea, as in the Chines of the Isle of Wight, is very favourable, the author having gathered them more than once in Bramble Chine.

Nearly allied to ferns is a little group of small aquatic plants, the RhizocarpeÆ (pepperworts), which either float on the water or creep along shallow bottoms. These are chiefly curious from having two kinds of spores produced from separate sporanges; smaller and larger “microspores” undergoing progressive sub-division without the formation of a distinct prothallium; each cell giving origin to an antherozoid, a generative process said to belong exclusively to flowering plants, corresponding indeed to the pollen grains of higher plants.

Structure of PhanerogamiÆ or Flowering Plants.

The two great divisions of the vegetable kingdom are known as Cryptogamia and Phanerogamia. It does not follow, however, that there is any abrupt break between the two, as will appear from the context. Although it is customary to speak of the flowering plants as a higher grade of life, yet there is an intermediary class of PhanerogamiÆ in which the conspicuous parts of the generative system partake of a condition closely resembling those of the higher CryptogamiÆ, observed in Gymnosperms, ConiferÆ, and CycadÆ. So it may be said the distinctive character of the former is that of reproduction by seeds rather than flowers. The progress of botanical science during the latter half of the Victorian reign has been quite as remarkable as that of histology; while the comparative physiology and morphology of plants have perhaps advanced even more rapidly because the ground was newer. The consequence is that the specialisation of botanical science has been brought about con-currently with a more comprehensive nomenclature. The chief cause in this instance of modern specialisation is utility. “If we look at the great groups of plants from a broad point of view, it will be seen that the fungi and the phanerogams occupy public attention on other grounds than do the algÆ, mosses and ferns. AlgÆ are especially a physiologist’s group, employed in questions on nutrition, reproduction, and cell division and growth. The Bryophyta and Pteridophyta, are, on the other hand, the domain of the morphologist concerned with such questions as the alternations of generations and the evolution of the higher plants.

“Fungi and phanerogams, while equally or even more employed by specialists in morphology and physiology, appeal widely to general interest, and evidently so on the ground of utility. Without saying that this enhances the importance of either group, it certainly attracts scientific attention to them. However, the histology of the minute cell, in addition to its importance from an academical point of view, has a special interest for the microscopist.”

It would be impossible to find anything more remarkable in histology than the detailed agreement in the structure and behaviour of the nucleus in the higher plants and the higher animals, an agreement which is conspicuously manifest in those special divisions which take place during the maturation of the sexual cells.

So with regard to the question of “alternation of generations.” We have known since the important discoveries of Hofmeister that the development of a large part of the vegetable kingdom involves a regular alternation of two distinct generations, the one which is sexual being constantly succeeded, so far as the normal cycle is concerned, by the other which is asexual. This alternation is most marked in the mosses and ferns. In the Bryophyta the ordinary moss or liverwort plant is the sexual generation of the ovum, which, when fertilised, gives rise to the moss-fruit, and represents the asexual stage. The latter is once more seen to form spores from which the sexual plant is again developed.

In the Pteridophyta the alternation is equally regular, but the relative development of the two generations is totally different, the sexual form being the insignificant prothallus, while the whole fern-plant, as we ordinarily know it, is the asexual generation.

The thallus of some of the lower Bryophyta is quite comparable with the prothallus of a fern, so as regards the sexual generation there is no difficulty in seeing the relation of the two classes; but when we come to the asexual generation or sporophyte the case is totally different. There is no appreciable resemblance between the fruit of any of the Bryophyta and the plant of any vascular Cryptogam.

“It is now known that in the higher plants a remarkable numerical change takes place in the constituents of the nucleus of the cell shortly before fertilisation. In angiospermous plants a reduction of the chromosomes occurs shortly before differentiation of the sexual cells. Thus, in the case of the lily, fertilisation is not the simple fusion of nuclear bodies. These spheres are seen to fuse in pairs, and then by position to determine the plane of first cleavage of the ovum; agreeing, in fact, closely with what is observed to take place in the animal kingdom.”

In the higher grades of plants it will be evident that the several tissues that compose their bodies are not found in the root, stem, and leaf without definite order and purpose, but that they are grouped into systems for the performance of different kinds of work. In all flowering plants at least three different systems may be clearly distinguished. These are the epidermal or boundary tissue system, the fundamental or ground tissue system, and the fibro-vascular or conducting system. All three systems of tissue originate from meristem cells, located at the growing point of the stem and root.

Although these systems characterise the higher types of plants, the elementary tissues (represented in Plate XIII. and in other figures) enter alike into the several component parts of nearly all plants. The stem, the branch, and the root, are alike constituted of an outer coating which affords a mechanical support, and once formed takes no further share in the economy of the plant, excepting that of assisting to convey fluid from the roots to the branches and leaves, an action more of a capillary nature than vital. The nourishment of the plant is brought about by other material structures, as the pith, the cortex, the cambium, and so forth, all of which greatly assist in the formative process. The woody portion of the plants is especially concerned in furnishing support to the softer pulpy textures, while the tissues of leaves and flowers are chiefly composed of cells compactly held together by protoplasmic or albuminoid matter. Water, of course, enters largely into the constituents of all plants. Beneath the epidermis is another layer of importance, the parenchymatous, which becomes more or less solid with the growth of the pith and cellular wall. In the pulpy substance of some leaves the epidermis presents a thin lamina of palisade-tissue, the bulk of the mesophyll consisting of spongy parenchyma or sclerenchymatous fibres (seen in Fig. 310), which also serve to show the disposition of the several layers about to be brought under notice.

Development of the Tissue Systems.—In the growing plant the embryonic cells soon become differentiated into three primary meristem layers, known as dermatogen, periblem, and phloem, from which are developed respectively the primary cortex, epidermis, and the stele or vascular cylinder. The dermatogen forms the outermost layer of cells at the growing point, and when present always develops into true epidermal tissue. In stems the dermatogen is always single-layered, while in roots it consists of several layers and develops a many-layered epidermis.

The periblem occurs immediately beneath the dermatogen, forming a hollow cylinder of tissue, which surrounds the phloem. From the periblem is developed the fundamental tissue of the primary cortex. When no dermatogen is present in the growing-point (stems of vascular cryptogams) the external layer of the periblem develops cells which perform epidermal functions. The phloem occupies the centre of the growing-point, and consists of a solid mass of somewhat elongated cells. From the phloem are developed the fibro-vascular and fundamental tissues of the vascular-cylinder or stele.

Epidermal or Boundary Tissue System.—This system constitutes the external covering of the plant, and is commonly called the epidermis. It includes, besides the ordinary epidermal cells, the guard-cells of the stomata and water pores, the plant hairs or trichomes, and the epidermal or external glands. The epidermal tissues are chiefly protective in function, serving to prevent excessive evaporation from the interior tissues of the plant.

In stems the external layer of cells, whatever its origin, is known as the epidermis, while in roots it is called the epiblema. The epidermis usually consists of a single layer of cells, but in some cases it is two or three-layered, as in the leaves of figs and begonias.

In land plants the epidermis is usually strongly cutinised, while in submerged plants it is never cutinised. The epidermis of land plants is also often waxy, the wax occurring on the surface as minute grains, rods or flakes, constituting the so-called bloom of leaves and fruits, and giving to them their glaucous appearance. Chlorophyll bodies are usually absent from the ordinary epidermal cells of land plants, while they commonly occur in the epidermal cells of aquatic plants.

Ordinary epidermal cells are usually thin-walled and transparent, and contain a nucleus and colourless watery protoplasm, but are destitute of both chlorophyll-bodies and starch-grains.

The external layers of the outer walls constitute the cuticle of the plant, while the internal layers and the radial and inner walls are composed of cellulose. The cells of the epidermis are always very compactly arranged, having their walls so closely adherent that the intercellular spaces are entirely obliterated except at the stomata and water-pores.

There are exceptions to this rule, as, for example, in Cinchona calisaya, which shows no trace of epidermis, this being replaced by a corky layer of tubular cells. Where this occurs in a plant to any extent, the whole of the outer tissues are displaced, and the bark consists exclusively of phloem tissues. This, although of constant occurrence in C. calisaya, is not so common in other species, as C. succirubia, the middle structure of which consists of parenchyma in which appear more or less numerous isolated store-cells, and when these are absent there is a formation of rhytidoma and displacement of the tissues containing the store-cells and ducts. The chlorophyll of C. succirubia is very marked, and its spectrum presents seven distinct absorption bands.

The epidermal system of plants in general includes other tissues than those already named, as the guard-cells of the stomata, the water-pores, plant-hairs or trichomes, and the external or epidermal glands, all of which are but modifications of ordinary epidermal tissue.

The Stomata or Breathing Pores are apertures in the epidermal which lie over large intercellular spaces (Fig 312, 2, b). These are usually bordered by two modified epidermal cells, called guard-cells. Stomata are formed in the following manner: A young epidermal cell divides into two equal portions by the formation of a septum across its middle, each half developing into a guard-cell; the septum now splits lengthwise and separates the guard-cells, leaving an aperture or stoma between them.

In the higher plants the guard-cells of the stomata are crescent-shaped and occur in pairs, the concave sides of the cells facing each other with the aperture between, while in mosses the stomata possesses but a single annular guard-cell which surrounds the aperture. The guard-cells of stomata usually contain chlorophyll-bodies in addition to the ordinary protoplasm. They have the power of increasing or diminishing the size of the aperture under the influence of light and moisture, thus regulating the amount of evaporation from the internal tissues of the plant.

Water Pores or Water Stomata are apertures in the epidermis, similar in structure to ordinary stomata, but differ from them both in function and distribution. Water-pores excrete water in the form of drops, and have their guard-cells fixed and immovable. They always occur at the ends of vasal bundles, and are found on the margin and at the apex of leaves.

Plant Hairs or Trichomes are modified epidermal cells prolonged externally, and may be either unicellular or multicellular. Each hair consists of a basal portion, or foot, which is embedded among the ordinary epidermal cells, and an apical portion or body, which is prolonged externally. Ordinary epidermal hairs are usually thin-walled, the inner layers of the wall being composed of cellulose, while the outer layer is more or less strongly cutinised. The walls may become hardened by deposits of lime-salts or silica. Sometimes the cells become glandular and secrete oily, resinous, or irritating matters, as in stinging-nettle hairs (Plate XIII., No. 19), when they are known as glandular hairs. The development of resin-passages may be observed in transverse sections of the stem of the ivy (Hedera helix) cut from a young succulent stem, and mounted in glycerine. The resin is seen scattered through the cortex and pith, and in the soft bast which lies outside the cambium in various stages of development, starting from a group of four cells without intercellular spaces.

Root hairs spring from the epiblema and are never cutinised, but are frequently more or less mucilaginous. The root-hairs are the principal absorbing organs of the plant, and are confined to the younger roots, occurring just above their tips. Root-hairs are never present in aquatic plants, and are absent from the roots of certain of the ConiferÆ. It is a curious fact with regard to bell-heather growing in higher latitudes, that the plants possess a peculiar root structure as a protection against droughts. In most of them the sustentation of life depends upon the formation of a number of long thin filaments on their roots resembling root-hairs, which penetrate the root, forming nodular masses within it. These filaments belong to a fungus entirely parasitic to the root, and yet different from a common parasite, inasmuch as the plant in this way obtains so much of its nourishment, and when the fungus is not present, or is removed, the plant can no longer live on a peaty soil. The leaf-blade of the coarse moorland grass Nardus is likewise endowed with a singular property—that of rolling up cylindrically and spreading out again to adapt itself to the dry and wet weather of the moorlands of Scotland.

Fundamental or Ground Tissue System.—This system constitutes the groundwork of plants, and is the system through which the vasal bundles are distributed. The fundamental tissues are composed largely, though not wholly, of parenchyma, and are chiefly concerned in the metabolic work of plant life.

Ground tissue includes, besides ordinary parenchyma, collenchyma, selerenchymatous parenchyma, fibrous tissue, cork, laticiferous and glandular tissues. To the fundamental system also belongs the chlorophyll cells of leaves, the thin-walled cells of the pith and medullary rays, the cells of the cortex of stems and roots, and most of the soft cellular tissues in all plants.

The lower plants consist almost entirely of fundamental tissue. In the herbaceous forms of the higher plants the ground tissues largely predominate, while in woody plants they are present in much smaller proportion, the vascular tissues being the most abundant. In aquatic plants generally, the fundamental tissues constitute the principal system.

The hypoderma occurs immediately beneath the epidermis, and consists of several layers of cells. A collenchymatous hypoderma is found in the stems and petioles of most herbaceous dicotyls, and frequently occurs next the mid-rib of leaves, where it forms a strengthening tissue. A sclerenchymatous hypoderma occurs either as a continuous layer beneath the epidermis, as in the stems of some ferns, Pteris aquilina, and in leaves of the pine; or it may form numerous isolated strands beneath the epidermis, as in the stems of horsetails and in certain UmbelliferÆ.

The endodermis is the innermost layer of the extra-stelar fundamental tissues, and always abuts on the stele or steles. In monocotyls it marks the boundary between the cortex and the central cylinder, and it is sometimes spoken of as the nucleus sheath.

In stems the endodermal cells are usually thin-walled and unlignified, having a suberous thickening band extending round the upper, lower and lateral surfaces, which in cross-section appears as a black dot on the radial wall (Fig. 314, c.)

According to its position in the stele, the conjunctive tissue is divided into three principal portions, viz., that portion which invests the vasal bundles, the pericycle; that portion which lies between the bundles of the stele, the interfascicular conjunctive tissue; and that which occupies the centre of the stele, the medullary conjunctive tissue. The pericycle, formerly called the pericambium, is the outermost layer of the conjunctive tissue of the stele.

The bundle-sheath of the young stem is more easily recognised than in the older stem. It is, in fact, a continuous layer of cells, whose radial walls have a characteristic dark spot on each radial wall. The bundle-sheath lies immediately outside the vascular bundles, curving slightly towards the centre of the stem in the spaces between the bundles. It is more prominent in the stem when very young, as the cells are then filled with starch granules. This layer of cells will be readily seen in sections treated with iodine.

In dicotyls and gymnosperms the medullary rays consist essentially of interfascicular ground tissue. The medullary conjunctive tissue occupies the centre of the stele, constituting the so-called pith, and usually consists of parenchymatous cells, but may contain, in addition, either stone cells, sclerenchyma fibres, laticiferous or glandular tissues.

The Fibro-vascular or Conducting Tissue System.—This system constitutes the fibrous framework of the plant, and is the system by means of which fluids are conducted from one part of the plant to another. Its function is partly to give strength and support, but principally to conduct the crude and elaborated juices to and from the leaves. It is found only in the higher plants, constituting the tough and stringy tissues in stems and roots, and the system of veins in leaves. The fibro-vascular system consists essentially of vascular tissue (ducts, tracheids, and sieve-tubes), and forms long strands—the fibro-vascular bundles—which extend vertically through the fundamental tissues of the plant. The term “fibro-vascular,” as applied to the conducting system, is not strictly correct, since fibres do not always accompany the vascular elements, hence this system is often spoken of as the vascular system, and the bundles as vascular, or more briefly as vasal bundles.

That the arrangement, and course of the vascular bundles in dicotyledous stems are connected with those of the leaves is an obvious fact. It may be seen in sections of Helianthus, but is more markedly shown in plants with regularly decussate leaves, as Cerastium, Clematis, &c. Still, the arrangement of the bundles may differ radically from that of the leaves, and is, to a certain extent, independent of them. This will be noticed in sections of Iberis amara, where the bundles do not run longitudinally, but in tangential spirals. These, as NÄgeli pointed out, have no direct relation with the leaves; and he recommends a series of types for investigation, in which it will be seen how closely the arrangement of the bundles is connected with the arrangement of the leaves, and the number of bundles entering the stem from each leaf: Iberis amara, leaves alternate, leaf-trace with one bundle; Lupinus, leaves alternate, leaf-trace with three bundles; Cerastium, leaves opposite, leaf-trace with one bundle; Clematis, leaves opposite, leaf-trace with three bundles; Stachys, leaves opposite, leaf-trace with two bundles.

The connection of the leaf and stem will be best seen by cutting longitudinal sections through a young node of Helianthus, so as to include the median plane of the leaf, or of both leaves if opposite to each other, as they often are; steep them in dilute potash for twenty-four hours and mount in glycerine. A medium power will serve for their examination. The course of the vascular bundles will appear dark through the more transparent parenchyma. The continuity of the tissues of the stem and petiole if followed will be seen to have no definite boundary between the two parts; the bundles from the petiole pass into the stem, and no bundle of the upper internode lies in the same vertical plane as that which enters from the petiole between two successive bundles of the vascular ring.

Every complete vasal bundle consists of xylem or wood and phloem or bast.

The former consists essentially of trachery tissue (ducts and tracheids), and may contain in addition both wood fibres and wood parenchyma. The phloem or bast consists essentially of sieve tissue, and usually contains some ordinary parenchyma. In angiosperms companion-cells always accompany the sieve-tubes in the phloem, while in gymnosperms they are absent.

According to the relative positions of the xylem and phloem elements, there are two principal kinds of conjoint bundles—the collateral and the concentric. Of these again there are three varieties, but the experiments with leaves bring out parallel facts; that in ordinary stems the staining of the wood by an ascending coloured liquid is due, not to the passage of the coloured liquid up the substance of the wood, but to the permeability of its ducts and such of its pitted cells as are united into regular canals; and the facts showing this at the same time indicate with tolerable clearness the process by which wood is formed, for what in these cases is seen to take place with dye may be fairly presumed to take place with sap.

Taking it, then, as a fact that the vessels and ducts are the channels through which the sap is distributed, the varying permeability of their walls, and consequent formation of wood, is due to the exposure of the plant to intermittent mechanical strains, actual or potential, or both, in this way. If a trunk, a bough, shoot, or a petiole is bent by a gust of wind, the substance of its convex side is subject to longitudinal tension, the substance of its concave side being at the same time compressed. This is the primary mechanical effect. The secondary is when the tissues of the convex side are stretched, and also produce lateral compression. In short, the formation of wood is dependent upon transverse strains, such as are produced in the aerial parts of upright plants by the action of the wind.

In concentric bundles one of the elements, either the xylem or the phloem, occupies the centre, and is more or less surrounded by the other, as seen in Fig. 310. Meristem tissue is never present, hence concentric bundles are always closed. They, however, occur in the stems of most ferns, and are always surrounded by a pericycle and endodermis, and should be regarded as steles. Concentric bundles with a central phloem occur in the rhizomes of some monocotyles, as Calamus, Iris, Convallaria, &c.

The Stele, or Vascular Cylinder, is developed from the phloem of the growing plant, and consists of one or more vasal bundles imbedded in fundamental tissue, the whole being enclosed by a pericycle and an endoderm. The typical stele includes all the tissues evolved by the endodermis, which, however, forms no part of the vascular cylinder itself, but merely surrounds it. The pericycle is always the outermost layer of the tissues of the stele, while the endodermis is the innermost layer of the extra-stelar tissues.

The arboreus type of stem can be best followed by making sections of a twig of the elm (Ulmus campestris), which will be found to be cylindrical hirsute, green or brown according to age, the latter colour being due to the formation of cork. Small brown excrescences are scattered over its surface; these are termed lenticels. The cork will be seen to lie immediately below the epidermis, and to consist of cubical cells, with little or no cell contents; they are arranged in radial rows, without intercellular spaces. The walls of these cork cells will stain yellowish-brown with Schultze’s solution. Treat a thin section with sulphuric acid and the walls will swell out and gradually lose their sharpness of outline, with the exception of the cuticularised outer wall of the epidermis and the cork. This material is occasionally found developed in the twigs of the elm, so that it can be separated as thick radial plates of tissue.

“By comparing sections of twigs cut of various ages, the following information may be gleaned: That cork cambium, or phellogen, appears as a layer of cortical cells below the epidermis, and that these divide parallel to the surface of the stem. The result of successive divisions in this direction is the formation of secondary tissue, which develops externally as cork, internally as phelloderm. The true cork cambium consists of only a single cell in each radial row, from which, by successive division, all these secondary tissues are derived—i.e., cambium of vascular bundles. As stems grow older, layers of cork appear successively further and further from the external surface; not only the cortex, but also the outer portions of the phloem are thus cut off from physiological connection with the inner tissue. The term bark is applied to tissues thus cut off, together with the cork which forms the physiological boundary. The stem of Vitis affords a good example of such successive layers of cork.”

For the study of sieve-tubes take the vegetable marrow, in which they are of extraordinary size. Cut transverse sections of the stem and stain with eosin, and mount them in glycerine. The general arrangement will be seen to differ from that of most other herbaceous plants. Below the epidermis a thick walled band of sclerenchyma with lignified walls will be seen distinct from the vascular bundles, which readily take a stain. The vascular bundles are separate and distinct, and the structure of the bundle is abnormal, there being in each a separate central mass of xylem, with the phloem masses lying, the one central, the other in the peripheral side. Between the xylem and the phloem masses is the cambium layer. The structure being the same in both will serve for the study of the punctate sieve-plates; these are readily stained with eosin, as shown in Sach’s text-book.

Laticiferous Tissues (Fig. 318).—In cutting sections of latex care must be taken to at once transfer them to alcohol so as to prevent the flow of the latex from the cells, otherwise the laticiferous vessels will be much less easily traced. The better method is to plunge the root of the dandelion (Leontodon taraxacum), after cleaning, into alcohol, and there let it remain until it has become hardened; then cut thin tangential sections from the phloem, and longitudinal sections through the cambium, and mount them in potash and glycerine. The laticiferous vessels appear circular in the transverse sections with brown contents; these are distributed in groups round the central xylem. Observe in such sections the presence of sphere crystals of inulin. These are formed quite irrespective of the cell-walls.

Laticiferous cells are readily seen in the cortex of Euphorbia splendens, cut just outside the vascular ring. Long tubes will be seen to run through the cortical parenchyma, with thick cellulose walls and granular contents. These are the laticiferous cells, the branching of which distinguishes them from the preceding structure. Included in the granular contents are starch grains of a peculiar dumb-bell form.

Leaf or Petiole.—The general morphology of leaf tissue is essentially the same as that of the stem from which it proceeds. In the typical monostotic stem of PhanerogamÆ each leaf receives a portion of the stele or central cylinder of the stem. Such portion is termed a meristele, and may be either entire or split up into a number of schizosteles.

The microscopical structure of leaves should be studied in the whole organ, and by the aid of isolating elements. The whole or portion of a leaf should be soaked in chloral hydrate solution; this will render it transparent, whereby the internal structure can be studied as a whole. Sections should be prepared from fresh leaves, or dried ones softened by soaking in water. Cut them transversely, both in the direction of the mid-rib and at right angles to it. This is best effected by placing the material between two pieces of elder pith or fresh carrot. Sections of the whole are made and transferred to a dish of water. Leaf sections are easily made for examination by macerating the leaves in solution of caustic potash varying in strength from one to five per cent. The epidermis on both sides may be detached, and the elements of the mesophyll and vascular bundles isolated for separate examination.

Potassium permanganate proves to be a useful reagent. A weak solution causes the protoplasmic structures to swell up, thus assisting in the observation of the structure of the chromatophores. This solution may also be employed as a macerating fluid. Beautiful preparations are obtained in this way of the sieve-tubes of Vitis.

Special structural peculiarities are to be observed in the leaves of various plants in which the epidermis consists of more than a single layer of cells (e.g., the leaves of Ficus, PeperaceÆ, BegoniaceÆ, &c.), cystoleths in the cells of the epidermis of Urtica; glandular structure in Ruta, Psorales; the coriaceous leaves of the Cherry Laurel, and the cylindrical leaves of Stonecrop (Sedum acre).

Reproductive Organs.—The development of the rudiments of flowers is of an extremely interesting nature, and the complete flower should be carefully studied. Median sections are best suited for the purpose. In the large majority of plants the calyx is developed first, then the corolla, and next the stamens. Preparations should be made from materials hardened in alcohol, or first fixed with a strong solution of picric acid and then hardened in alcohol.

Pollen-grains.—Microspores are found lying free in sections made of the reproductive organs; these may be transferred to a glycerine fluid and examined under a high power. They are mostly spherical, with granular protoplasmic contents, in which with much difficulty two nuclei can be made out. Mount and examine, as types of the various forms of granules, the pollen of Helianthus, Althoea, Cucurbita, Ænothera, Orchis, Mimosa, Tulipa, &c. Mount any of these pollen-grains in a weak solution of cane-sugar (about five per cent.), examine with a high power, and note the configuration of their walls with a medium power under polarised light. If transverse sections be made from very young buds, the development of the anther and the pollen may be traced. The material should be preserved in strong alcohol, and the sections treated with equal parts of alcohol and glycerine, and exposed in a watch-glass that the alcohol may evaporate. By this method sections may be prepared for illustrating the formation of the tapetum, special mother-cells, and division of the nucleus.

Starch Granules.—One of the most universally distributed materials found in plants is starch composed of two substances, granulose, which constitutes by far the largest part, and a skeleton of farinose. It is only the former of these that stains blue with iodine solutions; the latter partially assumes a brownish colour. The structure of starch granules is not of equal density throughout; the hilum or nuclear portion is most conspicuous, around which the rest of the material is deposited in layers, indicative of stratification. The several layers next to the hilum are less dense than those farthest from it. The position of the hilum determines the form of the grain, a few being rounded, others oval or elongated. The grain also contains different proportions of water; this conveys the appearance of concentric lines or curves about the nucleus. The latter is more conspicuous in the potato starches, as seen in Plate XIII., Nos. 6-15. Starch grains, in nearly all cases, are formed by the agency of proteid bodies, either chloroplasts or amyloplasts, and under the action of sunlight are gradually broken up and employed in the process of growth. There are some plants, however, notably the CompositeÆ, in which another carbohydrate, inulin, takes the place of starch from the first, and is used as a reserve food material. For this reason we look in vain for starch in the cells of Inula, Taraxacum, &c. From the whole group of fungi starch is absent; this seems to explain the fact that chlorophyll, or colouring matter, is rarely met with in the fungi, hence their inability to utilize, like green plants, carbon-dioxide as food.

The tissues which most commonly contain starch, or which contain it in largest quantity, are those of the parenchymatous series, though it sometimes occurs in the latex of laticiferous tissues, and even in ducts and tracheids. In the stems of Dicotyledons it occurs chiefly in the parenchyma of the middle and inner bark, in the medullary ray cells, and in the cells of the pith. In the roots of these plants it has a similar distribution, being for the most part confined to the middle or inner bark and the medullary rays, pith not being present in these organs. In succulent stems and roots, of course, it also commonly occurs in the xylem tissues of the fibro-vascular bundles.

A study of the various kinds of starches is important, since this material is very largely used as an adulterant. Other than microscopical means of detecting frauds are practically useless; assaying is tedious and expensive, while the microscope is always available and at hand. The limits of variation should be studied in starches from the same species of plants; the variations are not very wide, but in most cases characteristic, so that the discrimination is at all times an easy task. The reagents required are simply iodine and dilute potassium hydrate, aided by polarised light.

The starch grains of the potato are the best to study in the first instance on account of their large size (Fig. 320).

In arrowroot starch (Fig. 321) the stratification is almost as distinct as in that of the potato; the grains much resemble each other. Although somewhat smaller, the grains of arrowroot are more uniform in size. The starches are much used as an adulterant of drugs and various articles sold as cocoas.

Wheat-starch (Fig. 322) consists of circular flattened grains varying much in size, the central nucleus and stratification of which are very difficult to distinguish.

In the smaller starches the hilum becomes more indistinct, and without stratification, as in rice-starch, the latter being angular in shape. The hilum in other leguminous plants forms a longitudinal cleft; white rye-starch exhibits distinct cracks. Compound grains are occasionally met with, as in the oat. In Plate XIII. will be found small groups of starches taken under the same medium power for the sake of comparison. In the microscopical examination of starches first use a ²/₃-inch or a ½-inch and then a ¹/₆-inch objective.

The bran of the husk of wheat when broken by grinding is seen to be composed of two coats of hexagonal cells, the outer of which is detached by the roasting process. The hexagonal cell layer is, however, so little altered as to be perfectly distinguishable under the microscope. Thus even a small admixture of roasted corn with coffee or chicory can be detected without much difficulty. As to whether starch granules should be regarded as crystalline or colloid bodies, a difference of opinion still prevails. There are, however, reasons for believing that the polarisation effects produced by starch grains are not due to crystalline structure but to stress or strain, of the same nature as the polarisation of glass when it is subject to strain. The polarising phenomena are precisely such as would be induced in any transparent solid composed of layers, the inner of which being kept in a state of stress by the compression exerted by the outer layers. Moreover, when by use of a swelling reagent, such as caustic potash solution, the outer wall of the starch is made to expand by the imbibition of water, the polarisation effects immediately disappear. Were the solid particles of crystal thus forced apart by water each particle would still exhibit polarisation phenomena.

Want of space will not permit me to further enlarge upon other micro-chemical substances that enter into the composition of plants; as, for example, the oil secreting glands. These when present take the place of starch. There is, however, one product among the cell contents of plants of some interest to the microscopist—those extremely fine crystals known as raphides, composed of calcium-phosphate and oxalate. Mr. Gulliver insisted upon the value of raphides as characteristic of several families of plants. Schleiden states that “needle-formed crystals, in bundles of from twenty to thirty in a cell, are present in almost all plants,” and that so really practical is the presence or absence of raphides, that by studying them he has been able to pick out pots of seedling OnagraceÆ, which had been accidentally mixed with pots of other seedlings of the same age, and at that period of growth when no other botanical character would have been so readily sufficient.

If we examine a portion of the layers of an onion (Plate XIV., No. 3), or a thin section of the stem or root of the garden rhubarb (No. 4), we shall find many cells in which either bundles of needle-shaped crystals or masses of a stellate form occur, not strictly raphides.

Raphides were first noticed by Malpighi in Opuntia, and subsequently described by Jurine and Raspail. According to the latter observer, the needle-shaped or acicular are composed of phosphate, and the stellate of oxalate of lime. There are others having lime as a basis, in combination with tartaric, malic, and citric acids, all of which are destroyed by acetic acid; others are soluble in many of the fluids employed in mounting. These crystals vary in size from the ¹/₄₀th of an inch, while others are as small as the ¹/₁₀₀₀th. They occur in all parts of the plant; in the stem, bark, leaf, petals, fruit, root, and even in the pollen, and occasionally in the interior of cells. In certain species of aloe, as Aloe verrucosa, we are able to discern small silky filaments; these are bundles of the acicular form of raphides, and probably, as in sponges, act as a skeleton support to the internal soft pulpy mass.

PLATE XIV.

STELLATE AND CRYSTALLINE TISSUE OF PLANTS.

In portions of the cuticle of the medicinal squill (Scilla maritima) large cells are found full of needle-shaped crystals. These cells, however, do not lie in the same plane as the smaller cells of the cuticle. In the cuticle of an onion every cell is occupied either by an octahedral or a prismatic crystal of calcium oxalate. In some specimens the octahedral form predominates; in others, even from the same plant, the crystals are prismatic and arranged in a stellate form, as in that of the grass (Pharus cristatus). (Plate XIV., No. 6.)

Raphides of peculiar figure are found in the bark of certain trees. In the hickory (Carya alba) may be observed masses of flattened prisms having both extremities pointed. In vertical sections from the stem of ElÆagnus angustifolia, numerous raphides of large size are embedded in the pith, and also found in the bark of the apple-tree, and in elm seeds, every cell containing two or more minute crystals.

In the GraminaceÆ, especially the canes; in the Equisetum hyemale, or Dutch rush; in the husk of rice, wheat, and other grains, silica in some form or other is abundant. Some have beautifully-arranged masses of silica with raphides. The leaves of Deutzia scabia, No. 7, are remarkable for their stellate hairs, developed from the cuticle of both their upper and under surfaces; forming most interesting and attractive objects examined under polarised light. (Plate VIII., No. 173.)

Silica is found in the structure of RubiaceÆ both in the stem and leaves, and, if present in sufficient thickness, depolarises light. This is especially the case in the glandular hairs on the margins of the leaves. One of the order CompositÆ, a plant popularly known as the “sneezewort” (ArchillÆ ptarmica), has a large amount of silica in the hairs found about the serratures of its leaves.

All plants are provided with hairs; some few with hairs of a defensive character. Those in the Urtica dioica, commonly called the Stinging-nettle, are glandular hairs, developed from the cuticle, and contain an irritating fluid; in other hairs a circulation is visible: examined under a power of 100 diameters, they present the appearance seen at Plate XIII., No. 19.

The fibrous tissue of plants is of great value in many manufactures. It supplies material for our linens, cordage, paper, and other industries. This tissue is remarkable for toughness of fibre, and exhibits an approach to indestructibility, in the use it is put to in connection with the electric light. It is of importance, then, to be able to distinguish it from other fibres with which it is often mixed in various manufactures. Here the use of the microscope is found of considerable importance. In flax and hemp, in which the fibres are of great length, there are traces of transverse markings at short intervals. In the rough condition in which flax is imported into this country, the fibres have been separated, to a certain extent, by a process termed hackling, and further subjected to hackling, maceration, and bleaching, before it can be reduced to the white silky condition required by the spinner and weaver, and finally assumes the appearance of structureless tubes, Fig. 323 B. China-grass, New Zealand flax, and some other plants produce a similar material, but are not so strong, in consequence of the outer membrane containing more lignine. It is important to the manufacturer that he should be able to determine the true character of some of the textures employed in articles of clothing; this he may do by the aid of the microscope. In linen we find each component thread made up of the longitudinal, unmarked fibres of flax; but if cotton has been mixed, we recognise a flattened, more or less rounded band, as in Fig. 323 A, having a very striking resemblance to hair, which, in reality, it is; since, in the condition of elongated cells, it lines the inner surface of the pod. These, again, should he contrasted with the filaments of silk, Fig. 323 C, and also of wool, Fig. 323 D. The latter may be at once recognised by the zigzag transverse markings on its fibres. The surface of wool is covered with furrowed and twisted fine cross lines, of which there are from 2,000 to 4,000 in an inch. On this structure depends its felting property, in judging of fleeces, attention should be paid to the fineness and elasticity of the fibre—the furrowed and scaly surface, as shown by the microscope, the quantity of fibre in a given surface, the purity of the fleece, upon which depend the success of the scouring and subsequent operations.

In the mummy-cloths of the Egyptians flax only was used, whereas the Peruvians used cotton alone. By the many improvements introduced into manufacturing processes, flax has been reduced to the fineness and texture of silk, and even made to resemble other materials.

Fossil Plants.—It is well known that the primordial forests furnish a number of families of plants familiar to the modern algÆologist. The cord-like plant, Chorda filium, known as “dead men’s ropes,” from its proving fatal at times to the too adventurous swimmer who gets entangled in its thick wreaths, had a Lower Silurian representative, known to palÆontologists as PalÆochorda, or ancient chorda, which existed, apparently, in two species,—a larger and a smaller. The still better known Chondrus crispus, the Irish moss, or Carrageen moss, has likewise its apparent, though more distant representative, in chondritis, a Lower Silurian algal, of which there seems to exist at least three species. The fucoids, or kelpweeds, appear to have also their representatives in such plants as Fucoides gracilis, of the Lower Silurians of the Malverns; in short, the Thallogens of the first ages of vegetable life seem to have resembled in the group, and in at least their more prominent features, the algÆ of the existing time. And with the first indications of land we pass from the thallogens to the acrogens—from the seaweeds to the fern-allies. The LycopodiaceÆ, or club-mosses, bear in the axils of their leaves minute circular cases, which form the receptacles of their spore-like seeds. And when high in the Upper Silurian system, and just when preparing to quit it for the Lower Old Red Sandstone, we detect our earliest terrestrial organisms, we find that they are composed exclusively of those little spore-receptacles.

The existing plants whence we derive our analogies in dealing with the vegetation of this early period contribute but little, if at all, to the support of animal life. The ferns and their allies remain untouched by the grazing animals. Our native club-mosses, though once used in medicine, are positively deleterious; horsetails (EquisetaceÆ), though harmless, so abound in silex, which wrap them round with a cuticle of stone, that they are rarely cropped by cattle; while the thickets of fern which cover our hill and dell, and seem so temptingly rich and green in their season, scarce support the existence of a single creature, and remain untouched, in stem and leaf, from their first appearance in spring until they droop and wither under the frosts of early winter.

The flora of the coal measures was the richest and most luxuriant, in at least individual productions, with which the fossil botanist has formed an acquaintance. Never before or since did our planet bear so rank a vegetation as that of which the numerous coal seams and inflammable shales of the carboniferous period form but a portion of the remains—the portion spared, in the first instance, by dissipation and decay, and in the second by denuding agencies. Nevertheless almost all our coal—the stored-up fuel of a world—is not, as it is often said to be, the product of destroyed forests of conifers and flora of the profuse vegetation of the earliest periods in the history of our globe. Later investigations show that our coal measures are the compressed accumulations of peat-bogs which, layer by layer, have sunken down under the superimposed weight of the next. The vertical stems of coniferous trees became imbedded by a natural process of decay, and were subsequently overwhelmed in the erect position in which they are found. The true grasses scarcely appear in the fossil state at all. For the first time, amid the remains of a flora that seems to have had but few flowers—the Oolitic ages—do we detect, in a few broken fragments of the wings of butterflies, decided traces of the flower-sucking insects. Not, however, until we enter into the great Tertiary division do these become numerous. The first bee makes its appearance in the amber of the Eocene, locked up hermetically in its gem-like tomb—an embalmed corpse in a crystal coffin—along with fragments of flower-bearing herbs and trees. Her tomb remains to testify to the gradual fitting up of our earth as a place of habitation for creatures destined to seek delight for the mind and eye, as certainly as for the proper senses, and in especial marks the introduction of the stately forest trees, and the arrival of the charmingly beautiful flowers that now deck the earth.62