Mary Somerville

The principal objects in the study of plant-life are the organs by means of which they obtain and assimilate substances that are essential for their nourishment and growth, and those by which the perpetuity of their race is maintained and their type transmitted from age to age. In the lowest group of plants, represented by the AlgÆ, which come first into consideration, the two properties are combined; in the highest they are distinctly different, but the progress from one to the other may be traced through an ascending series of vegetable structure. In the simple grades of vegetables, the primordial cell frequently constitutes the whole plant; it appears first, and then envelopes itself with a coat either of cellulose or of a gelatinous substance.

Many instances of this are to be found amongst the AlgÆ, which are all aquatic plants, and are found growing either attached to other bodies, or floating independently, and live, some species in fresh water, and others in the sea and its estuaries. The AlgÆ absorb carbonic acid and give out oxygen, under the influence of sun-light, exactly as do the flowering plants; and the quantity of oxygen disengaged by them is said to be enormous.

Before proceeding to trace the structure and development of the AlgÆ, it may be desirable to indicate something of the classification of this curious group of plants. As already stated, they are without exception aquatic plants. They comprise three distinct orders, the ChlorospermeÆ, having green spores; the RhodospermeÆ, having red spores; and the MelanospermeÆ, having olive-coloured spores. These groups embrace all the varied plants known as sea-weeds, as well as the cellular plants which are developed in fresh water.

The ChlorospermeÆ are separable into three groups, namely, those which are simply cellular, including the PalmelleÆ, the green DesmidiaceÆ, and the yellow-brown silicious-coated DiatomaceÆ; those which are filamentous, called generally confervas, and including the true ConfervaceÆ, in which the threads have no compound axis, the BatrachospermeÆ, in which the threads are partially incorporated with an axis, the NostochineÆ, in which the slender moniliform threads are invested with a mucous or gelatinous mass, the OscillatoriÆ, and some others; and those which are foliaceous, comprising the UlvaceÆ. All these are monoecious plants, whose reproductive bodies are zoospores provided with ciliary appendages, or motionless cysts filled with endochrome, true spermatozoids being rarely present.

The RhodospermeÆ divide primarily into two groups defined by the nature and position of their spores: one having the spores indefinite, produced within mother cells; the other having the spores single in the upper joints of the threads of the nucleus. The first group includes the CeramiaceÆ, which are filiform articulate plants, with the nucleus naked, and the RhodymeniaceÆ, which are compound inarticulate plants, with the spores generated within the cells of moniliform threads. The second group includes, amongst others, the RhodomeliaceÆ and the LaurenciaceÆ, the former articulate, the latter inarticulate, and both bearing terminal spores, and having the nucleus conceptacular. To this group also belong the calcareous CorallinaceÆ and the cartilaginous or membranaceous SphÆrococcoideÆ. The plants of this group are dioecious, with two kinds of fruit, spores and tetraspores, and they bear antheridia filled with active spermatozoids.

The MelanospermeÆ divide into two series, the articulate and inarticulate. The former comprise the EctocarpeÆ, which are filiform plants with external cysts, and the ChordariÆ, which are interlaced cylindrical plants with immersed cysts. The latter include the LaminariÆ, flat, often strap-shaped, sometimes gigantic plants, having the spores superficial and indefinite, and the FucaceÆ, which constitute a large proportion of the shore-weeds of our seas and estuaries, and which bear their spores in elliptic or spherical conceptacles sunk in the frond. The MelanospermeÆ are either monoecious or dioecious, and spermatozoids are general amongst them, though occasionally propagation is effected by means of zoospores resembling the spermatozoids.

Having thus indicated the several groups of the great Algal family, their structure and development will now be traced, commencing with the most simple forms, which occur among the ChlorospermeÆ.

Spring water absorbs oxygen, nitrogen, and a large proportion of carbonic acid gas from the earth and the atmosphere, without losing its limpidity, but notwithstanding this apparent purity, if exposed for a time to the sun, green slime appears, and this the microscope shows to be full of globules or vesicles filled with green matter—the primordial cell in its earliest form. No green slime is formed in spring water if kept in darkness, so solar light is the principal agent in this growth, which is by no means a spontaneous birth; it is merely the development of one or more of the many kinds of germs, invisible to the naked eye, that exist in the earth, air, and water in myriads, waiting till favourable circumstances enable them to germinate.

Fig. 6. Palmogloea macrococca:—A, full grown cell; B-E, successive stages of binary division; F, row of cells produced by a succession of subdivisions; G-I, cells treated by iodine; K-M, cells in conjugation.

The slime that covers damp walls or stones, and moist cliffs or rocks in the sea, also the slime or mucus that sometimes swims on the surface of water, are said by M. Bory de St. Vincent to be provisional creations waiting to be organized. Of this the conferva, Palmogloea macrococca (fig. 6), is an example. It is a green slime covering damp places, consisting of microscopic primordial cells, each of which is surrounded by a gelatinous envelope, and filled with green granular matter occasionally concentrated into a nucleus. This singular plant is propagated in two different ways. The endochrome or green matter within the cell spontaneously divides into two equal parts, the thin coat of the cell bends round the two ends, separates them, then each half takes a globular or ovoid form, and secretes a gelatinous substance round itself which completes the separation, so that they form two distinct and independent plants, in every respect similar to that from whence they were derived. After a little time, each of these plants undergoes a similar bisection, so that four new plants are formed with their gelatinous envelopes; by the same process eight are produced and so on indefinitely, the organ of nutrition being the same with that of reproduction. Again, the membrane or film that covers each of these primordial cells is so thin and soft, that occasionally two adjacent cells of the series unite into one mass by a fusion of their sides and internal matter, which is then coated by a membrane, and after various internal changes becomes a spore which terminates a generation. By and by the spore germinates, produces a green primordial cell which secretes a gelatinous coat, and becomes by the process of bisection the parent of a new generation, which terminates by the union of two adjacent cells to produce a spore, a cycle of alternate modes of reproduction that may be continued till ended by some external circumstance, as the cold of winter.

When the matter in two adjacent cells joins to form a spore, it becomes granular, and mixed with minute particles of oil, which unite in a drop; and the spore, which is at first green, gradually assumes a yellow brown colour; conversely when the spore begins to grow, the oil disappears, and the green matter takes its place. This is a frequent occurrence during the formation of spores in this class of plants, for the endochrome or internal matter,—which consists of a small variety of elements probably in a state of unstable equilibrium or change,—is easily decomposed and recombined into new substances by chemical action, but the bisection of the cells of the Palmogloea so as to form new individuals is probably owing to heat alone. There is no apparent difference between the cells selected to produce spores by their union, and the others. It seems that in every plant certain cells are reserved for certain purposes. Professor Karsten conceives the nucleated cells to be reserved for reproduction, while those destitute of nuclei are designed for secretion.

The Protococcus pluvialis (fig. 7), one of the unicellular ConfervÆ, is frequently met with in rain-water cisterns. The spore of the plant (fig. 7 A) is a globular primordial cell invested with a double coat of cellulose, sometimes separated by an aqueous fluid, sometimes not. The cell is filled with protoplasm, a colourless watery liquid in which red and green particles are scattered. When this spore begins to grow, the endochrome, or solid matter in the primordial cell, divides spontaneously into two similar and equal parts, round at one end, and tapering to a point or beak at the other, each being coated by a very thin film of the transparent colourless protoplasm.

Fig. 7. Protococcus pluvialis:—A, encysted cell; B, C, cells divided into two; D, cell divided into four; E, cell divided into eight; F, cell divided into thirty-two; G, escaped motile gonidia; H-L, primordial utricles furnished with cilia.

After various changes, the green matter with several red spots is condensed into the thick round half, while the tapering beaked part is left transparent, being only filled with the watery liquid. Both bodies are then coated with cellulose, and two vibratile filaments called cilia, from their resemblance to eye-lashes, proceed from a point near the beak. The whole of these changes take place while the two bodies are still within the common cellulose covering; the moment they come out of it, by a rupture in the cell-wall, they swim about with the greatest velocity by means of their cilia, which lash the water so rapidly that they are invisible even with a microscope. The activity of these zoospores, as they are called, continues for about an hour and a half; the motion then becomes gradually less rapid; the cilia may now be seen, and soon fall off; then the bodies acquire a firmer coat of cellulose, and sink to the bottom of the water, where they remain at rest as still, or winter spores. There is great variety in the Protococcus, for the matter in the primordial cell sometimes divides not only into two equal and similar parts, but into 4, 8, 16, 32 equal and similar parts consecutively; each brood is developed into zoospores, which ultimately become resting spores.

When a spore is to be formed in a primordial cell, the starch and green matter condense into a nucleus in its centre, and a membrane envelopes the liquid and the nucleus within it, so that a spore in its first stage is a free and independent cell containing azotized matter swimming in a formative liquid. If the spore is to be motile it remains of a green colour, and gets cilia; but if it is to be a winter spore, the internal matter forms into granules, mixed with particles of red oil, which coalesce into a drop, and it generally undergoes the same transformations as those which take place after the conjugation or union of two adjacent cells into one as already described. The zoospores may lose their cilia, fall to the bottom of the water as green spores, and reproduce a facsimile of the parent plant as buds do, or they may acquire a cellulose coat, undergo the transformations and change of colour mentioned, and sink to the bottom of the water as red winter spores.

Under certain circumstances which do not seem to be perfectly known, it happens that during the formation of some of the zoospores the green matter is gradually changed into a red oily substance; they lose their cilia, acquire by secretion their cell-walls and a mucous envelope, and float on the water as winter spores. Should they be left dry, they may remain in that state for an indefinite length of time without losing their vitality, and as they are extremely small, they are carried by currents of air into the atmosphere, from whence they are brought down in the rain, and having fallen occasionally in places where they were never seen before, have given rise to the idea of spontaneous generation.

Many cycles may be accomplished from the still cell to the zoospores, and back again, producing numerous generations from the same plant before it returns to the red thick-walled cell, which may again be dormant for an unlimited time. These cycles, however, do not finish the history of the plant, for there can be little doubt that, in some stage of its existence, a conjugation of two cells occurs, as in the Palmogloea.

Sometimes when the division of the endochrome of the spore of the Protococcus is successively divided into sixteen parts, or even sooner, the new cells thus produced get two long cilia, as in fig. 7 H, and are liberated before they acquire their cellulose coat. This motile primordial cell soon acquires a bag-like investment (fig. 7 I, K, L,) of cellulose, through which the cilia pass, and thread-like extensions of the protoplasm are not unfrequently seen to radiate from the primordial cell to the surrounding bag, as in fig. 7 I, showing that the transparent space is only occupied by a watery liquid. The varieties of this plant are very numerous, and all related to one another. Sometimes the whole of the matter within the primordial cell of the spore divides at once into 4, 8, or 16 parts, giving rise to as many minute primordial cells.

The cilia are extensions of the colourless transparent film which covers the zoospores, and their vibrations are generally believed to be a consequence of the vital contractibility of that film, and intimately related to the changes taking place in the cell on which they are borne. The persistence of their motions after a cell is detached from a compound body covered with them, being like the persistence of the contractibility of muscle fibre after being detached from a living animal, proves that we must look to a contractile energy in the film of protoplasm for the maintenance of these curious operations.

It appears that a cell cannot perform two functions at the same time, and that one must either precede or follow the other. Thus, the zoospores have two distinct periods of action; the first is that of mechanical motion alone, which is followed by one of growth and multiplication, manifestations which, though very dissimilar, are really modes of action of the same vital energy that formed these bodies while they were yet in their parent cell. In fact, it seems to be a general law, that each cell is endowed, altogether or for a time, with its own mode of action, and is incapable of any other.

Of the VolvocineÆ, by some regarded as fresh-water microscopic plants, the StephanosphÆra pluvialis may be taken as a type. This plant consists of a colourless transparent globe not more than the ⁴⁸/₁₀₀₀th of an inch in diameter, containing eight green primordial cells arranged in a circle in its equator. Each primordial cell is furnished with a pair of cilia; these 16 cilia pierce through the hyaline globe, and by their vibrations, they make it rotate about an axis perpendicular to the plane of its equator, and move actively through the water. Each of the primordial cells, which are green with a spot of red in the centre, secretes a cellular covering, and they swim about in the interior of the globe as free cells. Eventually they escape either by fissure of the globe, or by its gradual dissolution. After swimming about for a short time they become motionless, lose their cilia, and sink to the bottom as green still spores.

If, after being dried, water be poured on one of these green still spores, it takes up the water, its contents become closely granular, and fill the whole membrane of the spore. Then it divides, first into halves, then into quadrants or heart-shaped segments, meeting in a point in the centre of the membrane. These quadrants are ultimately divided into 8 wedge-shaped segments, whose contour lines, like the spokes of a wheel, meet in the centre, and each gets a pair of cilia. The coloured matter is driven back in each individual towards the thick end of the wedge as if by centrifugal force, and a colourless plasm remains in the points or beak. These disappear, a cavity is formed in the centre of the disc, the eight bodies assume the form of a wreath in close contact, and the original cilia, which continue to vibrate, cause the rotatory and progressive motion of the whole organism.

Sometimes the eight globular bodies have been seen to divide into a number of extremely minute motile cells, while yet within the parent globe. These gonidia, as they are called, are, with a few exceptions which may reproduce the plant, believed to perish when they come into the water.

The division of the primordial cell of this plant is confined to a certain time of day; it begins towards evening, and is completed the following morning, and according to Mr. F. Currey, the exact time is the same in Lapland, where there is no night, and at Berlin in spring when the day and night are almost equal. The fertility is enormous. It is calculated that in eight days, under favourable circumstances, 16,777,216 families of the StephanosphÆra pluvialis may be formed from one resting spore.

The transmutation of chlorophyll in the Protococcus and VolvocineÆ, from green to red and vice versÂ, which so frequently occurs in the lowest class of plants, shows that its molecules must be united by very feeble affinities, and easily converted into new combinations either by direct chemical action, or by other substances also in a state of change.

Fig. 8. Volvox globator.

The VolvocineÆ consist of various species according as the internal matter of the primordial cell divides into 2, 4, 8, 32, or a greater number of equal parts, forming respectively as many free cells which ultimately become ciliated spores, by means of which the globe either rotates on the spot, or in straight lines. The Volvox globator found in fresh-water pools is one of the most remarkable of these, both for its peculiarity and beauty of structure and for its comparatively large size, since in some lights it is visible to the naked eye while swimming in a drop of water. When viewed with a microscope, it is a pellucid sphere whose surface is studded with green spots, often connected by green threads; each of the spots has two cilia, so that the surface is bristled with these filaments, whose vibrations give the sphere either a rolling or smooth motion, or make it spin like a top in the same place.

In the interior of the sphere there are from two to twenty dark green globes of different sizes; the smaller are attached to the internal surface, while the larger rotate freely by their cilia in the internal cavity. After a time the sphere bursts open and its inhabitants swim forth, and soon assume the form and character of that which gave them birth.

The growth and development of the Volvox globator are peculiar, for in the primordial cell the red and green endochrome breaks up into numerous angular masses, and a central globe rather larger than the rest. The angular masses are connected by green threads, the interstices between all the bodies are filled with a hyaline substance secreted from their surfaces, and the whole is enclosed in a distinctly membranous globular envelope.

As this young Volvox increases gradually in size, the hyaline matter is increased, the green threads lengthen, and the angular masses assume the form of a flask the ¹/₃₀₀₀th of an inch in diameter exactly as in the Protococcus; for the green matter with a few red spots is collected in the thick end, while the hyaline beak is turned towards the circumference of the sphere, which is pierced by their long cilia. Each of them is invested with a pellucid envelope of considerable thickness, the borders of which are flattened against those of similar envelopes. While these ciliated bodies are approaching maturity their endochrome exhibits vacuoles or apparently empty cavities of a spherical form about one-third of its own diameter. Mr. G. Busk discovered that these vacuoles expand and contract at regular intervals of about forty seconds. The contraction, which almost obliterates the cavity of the vacuole, is rapid and sudden; the dilatation is slow and gradual. This action ceases when the body comes to maturity.

When this mass of zoospores connected by green threads is immature and begins to expand into a hollow sphere, then the central globe is continually bisected so as to form 4, 8, 16, 32, 64, or a greater number of equal and similar parts, each of which is ultimately developed into a zoospore exactly the same with the matured green zoospores on the surface of the primary sphere, so that the ‘Volvox globator is a composite fabric made up of a repetition of organisms in all respects similar to each other,’ which Professor Ehrenberg the first to discover, though he did not investigate the development of the plant.

It appears that certain spheres of the Volvox are monoecious, that is, each sphere contains male and female cells, though the greater number of cells are neutral. The germ or female cells are larger and of a deeper green than the others; the male cells resemble them, but the endochrome within them breaks up symmetrically into a multitude of linear particles aggregated into discoid bundles beset with vibratile cilia, which move about within their cells and soon become decomposed into their component corpuscles. Each of these corpuscles has a linear body, thicker at its posterior end, and furnished with two long cilia. The female cell, when fertilized, gets a smooth envelope, and then a thicker one, beset with conical-pointed processes, and the contained chlorophyll gives place, as in Palmogloea, to starch and a red or orange coloured oil. It appears that the Volvox stellatus and V. aureus are only phases of the Volvox globator.

The DesmidiaceÆ are minute green algÆ inhabiting fresh-water pools or slow running streams, never those that are muddy. They are free unicellular plants, sometimes triangular, sometimes cylindrical, crescent or bow-shaped, smooth or spined. So varied are their microscopic forms that a description would be tedious. In plants of such extreme minuteness, the only means of ascertaining the nature of their component materials is by chemical tests. A solution of iodine turns starch blue, and cellulose brown, and thus it is found that the interior of the DesmidiaceÆ is occupied by a mass of starch granules, covered with chlorophyll, and mixed with a formative fluid. This mass, enclosed in a delicate membrane, constitutes the primordial cell; it has an exterior coat of firm cellulose, and the whole is more or less enveloped in a gelatinous substance. Like other plants, when in bright sunshine, the DesmidiaceÆ decompose carbonic acid gas, give off the oxygen, and assimilate the carbon into chlorophyll.

Fig. 9. Various species of Staurastrum:—A, vestitum; B, aculeatum; C, paradoxum; D, E, brachiatum.

These plants are frequently distinguished by projections from their cellulose coat above their surface, these being sometimes short and conspicuous, but often projected in spines, which form a beautiful symmetrical hyaline border round the green internal cell, as shown in fig. 9. Another peculiarity of the DesmidiaceÆ is the appearance of their being divided into two symmetrical parts by a satural line, as the name implies, though there is no real division.

Many of the DesmidiaceÆ, but more especially the genus Closterium, are remarkable for having a double circulation of the internal fluid in opposite directions, maintained by a vital contractile energy. One current flows between the cellulose horny coat, and the thin film covering the chlorophyll, while the other spreads in a broad stream in the contrary direction between the thin film and the chlorophyll mass, carrying from the latter some of its coloured particles to the extremities of the frond, where there seems to be a connection between the two streams.

Fig. 10. Economy of Closterium Lunula:—A, frond showing central separation; D, frond in a state of self-division.

The type of the DesmidiaceÆ is continued by various modes of bisection, depending upon the genus and species of the plant. In the Closterium Lunula, which has an elongated crescent shape, as in fig. 10 A, the endochrome or internal matter divides into two equal parts, which retreat from one another at the middle line; and a constriction of the cellulose coat takes place between them, which increases till it closes entirely round the extremities, as in fig. 10 D; then one of the halves remains at rest while the other moves from side to side, and finally detaches itself from the other with a jerk. In each of these halves a constriction of the endochrome may be seen, dividing it into an obtuse and an elongated part, and for some time the circulating fluid flows round the obtuse end, but the latter gradually assumes the form of the elongated end, the regular circulation of the fluid is established, and in five or six hours after the separation, two young desmids are formed precisely similar to their parent, the Closterium Lunula.

The Cosmarium, another Desmid, consists of a cell of two lobes united by a narrow isthmus. When about to multiply, the isthmus swells into two globular expansions, separated from each other and from the two lobes of the cell, by a narrow neck. These enlargements increase and assume the appearance of half segments of the original cell. In this state the plant consists of four segments lying end to end, the two old ones forming the extremes, with the two new ones in the middle. At last, each of the middle segments gets a new half, which soon acquires the full size and characteristics of the old one. This process, which is accomplished in twenty-four hours, is repeated ere long, and being continued indefinitely, the extreme lobes of the row are thrust farther and farther asunder, and the whole constricted thread or chain of Cosmaria is enclosed in a gelatinous sheath. The last two central lobes contain no portion of the original frond or plant, and may thus be considered to be entirely new individuals.

Many of the DesmidiaceÆ multiply by the subdivision of their endochrome into a multitude of granular particles called gonidia, which are set free by the rupture of the cell wall, and of which every one may develop itself into a new cell. The gonidia may be zoospores with cilia and active locomotion, or they may be enclosed in a firm envelope, and become resting spores. The movement of the zoospores at first within the cavity of the cell which gave them their origin, and afterwards externally to it, has frequently been observed in the varied species of the genus Cosmarium, and has been described under the name of the ‘swarming of the granules,’ from the resemblance of the moving mass to a swarm of bees. Their subsequent history is unknown.

In the Pediastrum, a plant consisting of a cluster of cells, the zoospores are not emitted separately, but those formed by the subdivision of the endochrome of one cell into 4, 8, 16, 32, or 64 parts, escape from the parent plant still enclosed in the inner tunic of the cell, and it is within this that they develop themselves into a cluster resembling that in which they originated.

Mr. Thwaites discovered that the DesmidiaceÆ are also propagated by conjugation, which would be impossible if the hard coat of the adjacent cells about to unite did not split open; then the whole endochrome in one cell passes into and blends with that in the other cell, so as to form one mass, which soon acquires a delicate membranaceous envelope. At first the mass consists of granular green matter, but when the membrane becomes thicker, it changes to brown or red. This body, which is called a sporangium, is sometimes smooth, sometimes granular, covered with tubercles or rough with spines, according to the nature of the original plants. The filamental species are propagated by conjugation, but the subsequent history of the produce is still obscure, though there is reason to believe that they give rise to plants of different forms, while all the other modes of increase only reproduce a facsimile of the parent.

DesmidiaceÆ exist in America, but their distribution is little known. In Europe, their maximum seems to be in the south of England. They abound in small shallow pools that do not dry up in summer, and also on boggy moors. The larger kinds are spread out as a thin gelatinous stratum at the bottom of water, or collected in little tufts; others form a dirty cloud upon the stems and leaves of aquatic plants. They have been found in a fossil state in flint, their spores have been discovered in the grey chalk at Folkestone, and the cells of various species of Closterium and Euastrum are imbedded in the marls of the United States of North America.

The DiatomaceÆ, or Brittleworts, are unicellular microscopic plants so numerous that there is hardly a spot on the face of the earth, from Spitzbergen to Victoria Land, where they may not be found. They abound in the ocean, in still and running fresh water, and even on the surface of the bare ground. They extend in latitude beyond the limits of all other plants, and can endure extremes of temperature, being able to exist in thermal springs, and in the pancake ice in the south polar latitudes. Though much too small to be visible to the naked eye, they occur in such countless myriads as to stain the berg and pancake ice wherever they are washed by the swell of the sea; and when enclosed in the congealing surface of the water they impart to the brash and pancake ice a pale ochreous colour.

Although the diatoms have a vast variety of forms, they all consist of a simple primordial cell whose external coat of cellulose is so deeply interpenetrated with silex that it is indestructible, a structure which constitutes the peculiar characteristic of the tribe. This primordial cell, as in other plants, contains organizable liquid or protoplasm, through which golden-brown granules are pretty regularly distributed, except in the centre, where they are collected into a nucleus. Round this nucleus they commonly form a ring from which radiating lines of granules diverge to the interior wall of the cell. In each of these there is a double current of granules, similar to the circulation in the DesmidiaceÆ; it was discovered by Professor Smith in some of the comparatively large diatoms. At times oil globules are seen in the protoplasm. The golden-brown matter is supposed to be chlorophyll, whose green tint has been changed by the presence of iron, which is assimilated in this group. Such is the internal structure of a race of plants altogether invisible to the naked eye. Their external forms, reproduction and movements, are no less wonderful.

The silicious envelope of the simple cell of a Diatom or frustule, as a single plant is usually called, consists of two valves or plates, commonly of the most perfect symmetry, closely applied to each other along a line of junction like the two valves of a bivalve shell, and each valve being more or less concavo-convex, a cavity is left between the two which is occupied by the golden-brown cell described above. The form of the cavity differs greatly, for sometimes each valve is hemispherical, so that the cavity is globular; sometimes it is a small segment of a sphere, resembling a watch-glass, so that the cavity is lenticular; in short, the form of the cavity depends upon that of the valves, which may be heart-shaped, or much elongated, square, triangular, boat-shaped, or furnished with outgrowths, which, however, is rare.

Fig. 11.—A, Diatoma vulgare:—a, side view of frustule; b, frustule undergoing self-division.
B. Grammatophora serpentina:—a, front and side view of single frustule; b, front and end view of divided frustule; c, frustule about to undergo division; d, frustule completely divided.

The diatom or frustule is considered to present its front view when the joint or suture of the valves is turned to the eye, as in fig. 11 B, b, whilst the side view is seen when the centre of either valve is directly beneath the eye, as in fig. 11 A, a. When the diatoms are young the valves are in close contact, but as they increase in size by a secretion round their edges, the valves separate from one another, and the cell membrane which is left exposed is immediately consolidated by silex, and forms a kind of hoop between the valves, as in fig. 12. This hoop increases in breadth as the cell increases in length. When the two valves are circular discs, they are separated by a circular hoop, round the edges of which water is admitted to nourish the plant; but when the diatom has an elongated form, the water enters through depressed points in its extremities which are free from silex.

Fig. 12. Biddulphia pulchella.

Fig. 13. Pleurosigma angulatum.—A, entire frustule; B, its hexagonal areolation; C, the same more highly magnified.

Numerous as these plants are, the valves of each genus have their own peculiar ornaments, consisting of the most beautiful and symmetrical designs, which are impressed upon the young valves when they are in a plastic state. The genus Navicula and others are marked with the finest striÆ, some diagonally, others transversely. Rows of round or oval spots disposed in parallel lines are peculiar to some; the valves of others are covered with hexagonal forms of the most perfect structure, as those of the Pleurosigma angulatum, fig. 13, where A is the magnified diatom, and B and C its hexagonal areolations, seen under higher and higher microscopic powers; but the figures on the discoid genera are the most beautiful of all. There is generally a small ornamented circular space in the centre of the valves, from whence rays extend to the circumference, dividing the surface of the valves into eight, ten, or more equal parts, the alternate segments being differently and highly ornamented, as in the Actinocyclus undulatus (fig. 14), where A is the side view, and B is the front view. The Arachnoidiscus Ehrenbergii takes its name from the likeness of the figures on its circular valves to a spider’s web. According to the observations of Mr. Shadbolt, each valve is formed of two superposed layers; on the uppermost of these, which is a thin horny transparent substance, the spider’s web is engraven; and the undermost silicious layer, which forms the supporting frame-work, is like a circular Gothic window. The genus Triceratium, nearly allied to the preceding in general characters, though differing in having a triangular shape, has many species in a fossil state, while others are still existing in the ocean, and in tidal rivers. The Triceratium favus, one of the largest and most beautifully marked, occurs in the mud of the Thames, and that of the estuaries of other rivers on our coasts; it is also frequently found on the surface of uncleaned shells.^[32] From the few examples given, a faint idea only can be formed of the variety and beauty of the engravings on the diatoms. It had long been doubted whether those on the valves of Coscinodiscus, Triceratium and others, were elevations or depressions, but Professor Rood of New York, United States, has proved them to be depressions by an optical arrangement which will be useful for the investigation of microscopic forms.

Fig. 14. Actinocyclus undulatus.

Diatoms increase by spontaneous bisection, by conjugation, and by the resolution of their endochrome into minute spores, called gonidia. When bisection is about to take place the cell elongates, the hoop increases in breadth, the endochrome divides into two equal parts, and the coating of the cell bends in between them, which gives the diatom the appearance of an hour-glass. At last they separate, and upon each of the new surfaces a new silicious half is formed, usually the exact counterpart of the old one, so that there are two diatoms instead of one; and the process may be continued indefinitely. In most cases, the new diatoms thus produced are free and independent. Sometimes, however, they adhere to one another by a fragment or connecting membrane, and if they happen to be slender and rectangular, and attached side by side, they form a slender filament, or if attached by alternate angles they form a zigzag chain, as in fig. 11.

The Meridion circulare (fig. 15) is a diatom of exquisite beauty, millions and millions of which cover every submerged stone, twig, or blade of grass, and even form the mud at the bottom of the streams at West Point, in the United States of North America. Its frustule or single diatom is long, slender, and rectilinear, but being broader at one end than at the other, by continued bisection and adhering to one another they form a circular, spiral, or flattened helical screw of several turns. The individual frustules of some marine diatoms have a precisely similar form, being rectilinear and broader at one end than the other, but each frustule is attached by its narrow end to the extremity of branching cellulose stems fixed to sea-weeds or stones, and by a continuous subdivision of which the stem does not partake, they are spread out at their free ends like a fan.

Fig. 15. Meridion circulare.

By continual bisection a diatom is propagated through many generations, but at some stage or other, owing to an unknown cause, propagation by conjugation takes place. When two frustules are near to each other, two little swellings arise in one, which meet two little swellings in the other opposite to it. These soon unite and elongate, the septum or division between them is absorbed so that they form two tubes in which the endochrome of the two frustules becomes mixed, and a spore is formed in each of the two connecting tubes, which increase in size and change in form till they resemble in every respect the parent, except in being much larger. As these young diatoms swell, they split the two parent frustules, become free, and lay the foundation of a twin series of generations. In the Fragillaria only a single spore is formed.^[33]

In Surirella and Epithemia the manner of conjugation is somewhat different. In the former the valves of two free adjacent frustules separate from each other at the suture or line of junction and the two endochromes are discharged; they coalesce and form a single mass, which becomes enclosed in a gelatinous envelope, and in time this mass shapes itself into a frustule resembling that of its parent, but larger. In Epithemia, however, the endochrome of each of the conjugating frustules divides at the time of its discharge into two halves; each half of the one coalesces with each half of the other, and two frustules are formed which become invested with a gelatinous envelope and gradually assume the form and markings of the parent frustules, but grow to a much larger size, for the spore masses have the power of self-increase up to the time that their envelopes are consolidated. This double conjugation seems to be the ordinary type of the process among the diatoms.^[34] But these plants multiply also by gonidia. It is thought probable that as long as the vegetative processes are in full activity diatoms multiply by self-bisection, but when a deficiency of warmth, of moisture, or of some other condition, gives a check to these, that they increase by gonidia, some of which becoming encysted, possess a greater power of resisting unfavourable circumstances, and thus the species is maintained in a dormant state till a change enables them to germinate. It is even thought they may be the origin of distinct species.

A peculiar spontaneous locomotion is exhibited by some diatoms of a long narrow form, as the NaviculÆ, which by a succession of jerks in the direction of their length, go to a certain distance, and then return nearly by the same path. The motion of the Bacillaria cursoria is still more unprecedented. The frustules, which are narrow, lanceolate, and acute, are joined end to end in a long line by some highly elastic invisible medium. One of the terminal frustules remains at rest while all the others slide over it till the line is so much stretched that they are nearly detached from one another; then they all slide back again in the same manner, and this alternate motion is continued indefinitely at regular intervals of time. The velocity of the diatoms at the free end of the row is very considerable; in the Bacillaria paradoxa it is ¹/₂₀₀th of an inch in a second; the impetus of one has been observed to upset and even to push aside a plant as much as three times its size which obstructed its path. If the frustule at the free end gets entangled, the fixed frustule takes the lead and continues the motion till the other is free. Minute particles in the vicinity are sometimes attracted and dragged after the frustules, sometimes they are repelled, possibly by some invisible organs; but the whole motion of the diatoms themselves may perhaps be attributed to the action of light and heat upon the highly contractile substance, whatever it may be, which connects their frustules, since their motion is exactly in proportion to the quantity of light and heat received, for it ceases during darkness, and is renewed on the return of light; ultimately it may disperse the individual frustules, which are not more than between the ²⁸/_10,000th and the ⁸⁴/_10,000th of an inch in length and the ⁴/_10,000th of an inch in breadth.

Fig. 16. Bacillaria paradoxa

This Bacillaria paradoxa (fig. 16) differs from the preceding species in its motion; each half of the row of frustules moves in an opposite direction on each side of a central stationary frustule, and the alternate motion is so regular as to time, that if in advancing, the frustules meet with an impediment, they wait till the proper time comes for their retreat. The jerking motions of the NaviculÆ are ascribed by Prof. W. Smith to forces acting within the plants, originating in the vital operations of growth, by which the surrounding water is drawn in at one end of the frustule, and expelled at the other.

Some species of diatoms are so universal that they are found in every region of the globe; others are local, but the same species does not inhabit both fresh and salt water, though some are found in brackish pools. The ocean teems with them. Though invisible as individuals to the naked eye, the living masses of the pelagic diatoms form coloured fringes on larger plants, and cover stones or rocks in cushion-like tufts; they spread over the surface as delicate velvet, in filamental strata on the sand, or mixed with the scum of living or decayed vegetable matter floating on the surface of the sea; and they exist in immense profusion in the open ocean as free forms. The numbers in which they exist in all latitudes, at all seasons, and at all depths—extending from an inch to the lowest limit to which the most attenuated ray of light can penetrate, or at which the pressure permits—are immeasurably in excess of what we have been in the habit of assuming. Temperature has little to do with the distribution of diatoms in the tropics; it decreases with the depth at a tolerably fixed rate till it becomes stationary. It increases in the polar regions with the depth, and approaches the standard, which is probably universal, near the bed of the ocean.

Nothing can exceed the vividness of colour or massiveness of the endochrome or soft internal matter of the floating diatoms, that matter which diminishes their specific gravity and makes the plant buoyant which otherwise would be weighed down by its silicious coat. At those periods in which the structural and reproductive phenomena proceed most vigorously, their position in depth must be fluctuating; hence they approach and vanish from the surface. Their growth is perfected by the heat and light which penetrates the sea in calm weather.

Diatoms are social plants crowded together in vast multitudes. Dr. Wallich met with an enormous assemblage of a filamental species of Rhizoselenia, which is from six to twenty times as long as it is broad, aggregated in tufted yellow masses, which covered the sea to the depth of some feet, and extended with little interruption throughout six degrees of longitude in the Indian Ocean. They were mixed with glistening yellow cylindrical species of such comparatively gigantic size as to be visible to the naked eye.

Other genera constitute the only vegetation in the high latitudes of the Antarctic Ocean. Dr. Hooker observes, that without the universal diffusion of diatoms in the South Polar Ocean, there would neither be food for the aquatic animals, nor would the water be purified from the carbonic acid which animal respiration, and the decomposition of matter, produce. These small plants afford an abundant supply of food to the herbivorous mollusca and other inhabitants of the sea, for they have been found in the stomachs of oysters, whelks, crabs, lobsters, scallops, &c. Even the Noctiluci, those luminous specks that make the wake of a boat shine like silver in a warm summer night, live on the floating pelagic diatoms, and countless myriads are devoured by the enormous shoals of salpi and other social marine animals.

The silicious shells of the diatoms form extensive fossil deposits in various parts of the globe, containing species which have long ceased to exist, and others that are identical with those still alive even in their most minute and delicate engravings. The polishing slate of Bilin in Bohemia, which occurs in beds 14 feet thick, and the Tripoli and Phonolite stones on the Rhine consist entirely of the silicious coats of diatoms, while the city of Richmond in Virginia stands upon a marine deposit of the debris of diatoms 13 feet thick, and of unknown extent. Near the Mediterranean, very extensive strata, consisting almost entirely of marine DiatomaceÆ, alternate with calcareous strata chiefly formed of Foraminifera, the latter being a race of microscopic mollusca. The fossil DiatomaceÆ at Oran in Algeria are particularly perfect and beautiful. In many of these deposits existing species are found.

The trade winds bring over large quantities of dust mixed with diatoms, which sinks through the upper into the lower current, blowing over America, and at last falls in Europe. Professor Ehrenberg found that this dust contained chiefly true American species, many of which were identical with forms existing at the bottom of the Antarctic Ocean, where an area of 4,800 square miles was discovered by Sir James Ross skirting the volcanic coast of Victoria Land, consisting of the remains of these microscopic plants, which have deposited their silicious valves at death for countless generations, producing geological changes of enormous magnitude; while a still greater area of sea-bed in the North Atlantic is the perpetual grave-yard of myriads of microscopic mollusca. Thus the Supreme Being, whose power is stupendously manifested in the motions of the celestial bodies, creates generations of infinitesimal creatures, adorns them with exquisite beauty, and makes them His agents to form future continents.

The ConfervaceÆ are a numerous tribe of pretty little plants, usually of a green colour, growing in fresh and salt water, on moist ground, wet rocks, and thermal springs. There is scarcely a gently running stream in which they may not be seen, like bunches of green threads, attached to stones and waving in the current; some are so soft as to become almost a mass of jelly when taken out of the water. They are sometimes branched, but more frequently simple, formed of cylindrical cells, joined in a single long row by their flat ends, and they increase in length by the bisection of their terminal cells.

Cell multiplication in Conferva glomerata:—A, portion of filament with incomplete separation at a, complete partition at b; B, the separation completed; C. formation of additional layers of cellulose wall.

In unicellular plants bisection is an act of reproduction; in the multicellular ConfervÆ it is an act of growth and extension which is accomplished as follows:—The terminal cell of the plant grows to twice its length, the matter within the primordial cell spontaneously divides into two equal parts, and both the film and cellulose coat which cover it, bend round, and form a double layer or cellulose division between them. This cellulose layer extends over the whole exterior of the primordial cell, so that the new cellulose division or septum becomes continuous with a new layer which is formed throughout the interior of the cellulose wall of the original cell. In this manner two perfect cells are formed out of one, and as the extreme cell may undergo the same process, the growth of the plant may be continued indefinitely. Branches are sometimes formed by buds springing from any part of the stem; though apparently so different, it results from the subdivision of the cell which produces the bud.

Fig. 18. Zoospores.

The ConfervaceÆ are generally reproduced by zoospores. In most cases the endochrome within a cell divides itself into numerous segments, each of which becomes a minute zoospore, and escapes into the water through a rupture in the cell wall. This is the case in a very graceful genus of ConfervÆ, of which the ChÆtophora elegans is a species. It consists of filamental strings of cells, ending in a capillary bristle, with lateral branches like narrow fronds. It is reproduced by zoospores. One half of each zoospore is round, opaque, and full of matter; the other half hyaline, and tapering to a beak furnished with four cilia. It frequently happens in this genus of ConfervaceÆ, where the filaments are divided at equal distances into little joints or compartments, that the zoospores issue from the terminal cell first, then from the next, and so on in succession till the upper part of the branch is left empty, while the lower part is still forming zoospores. After moving in water for a time, the zoospores retreat to a shady place, fix themselves to some substance, and begin to grow. These plants rapidly cover a large surface of water; for each individual cell may produce 100 zoospores, and as the development and dissemination of them continues during the whole summer, one plant may yield an enormous number.

The SphÆroplea annulina is a rare and very remarkable Conferva, whose cinnabar-coloured spores make the surface of the water in which it floats, like a pool of blood. It has no root, being merely a filament with capillary extremities, formed of elongated cells joined end to end. The spores only grow on the filaments that are exposed to the sun and air; the filaments that are below the water are green and barren. The spores are filled with red matter, grains of starch, and red oil, the outer or cellulose coat being so plaited, that the spore looks like a red star with white rays.

When a spore germinates, it produces a minute cell ending in capillary fibres, which increases in length by the continual bisection of its central cells, while the other ConfervÆ grow by the bisection of those that are terminal. During this growth, the red contents of the spore are so changed by a remarkable succession of chemical processes, that the primordial cells in the filament of the young plant are filled with a colourless viscous matter, an aqueous liquid, granules of starch, and chlorophyll. In some of the cells the starch disappears, while the green matter and the other materials arrange themselves into a series of rings, alternating with empty spaces or vacuoles. After a time, the green changes to a yellowish red, and then each ring in succession resolves itself into a multitude of minute active particles, which move with incredible velocity in the void spaces of the cell, till at last the whole cell swarms with them. They are analogous to the pollen of flowering plants, and thence are called spermatozoids. Their form is cylindrical, thick, broad, and yellow at one end, sharp at the other, with a colourless beak, and long cilia. The parent cell is at last pierced by their united efforts, and out they rush in great confusion into the water; some whirl round their centres, others swim in a circle, many describe cycloidal curves by a series of leaps, and a few swim in straight lines.

During the preceding changes another process is in progress, within what may be called female cells. In these the starch, mixed with green matter and a plastic substance, arrange themselves also into green and vacant rings, and after various and complicated changes, each green ring forms itself into a kind of plastic primordial free cell, which, after being fertilized by the moving bodies, gets a stronger coat. The green matter becomes first of a red-brown colour, then red; and after leaving the parent cell it is invested with a plaited cellulose coat, and becomes a star-like resting spore which may produce a new plant. No cryptogamic plant exhibits a greater variety in the modes of action of the vital forces, none more activity in the motile powers. In some of these ConfervÆ, the moving male filaments, or spermatozoids, instead of escaping singly from their prison cell in confusion into the water, are discharged in a mass enclosed in a capsule furnished with cilia, which moves with its lively burden like a zoospore, till a lid falls off which sets them free.

Some pretty plants allied to the ConfervÆ are called BatrachospermeÆ, from the resemblance which their beaded filaments bear to the spawn of a frog. They are all inhabitants of fresh water, chiefly of gently flowing streams, and are so flexible that they yield to every movement of the water, and when taken out of it are like a mass of jelly. Their colour is usually a brownish-green, but sometimes it is of a reddish or bluish purple. The central stem of the plant, though originally formed of a single row of large cylindrical cells placed end to end, gets an investment of cells, or rather branches, which ultimately becomes a thick cylindrical stem, bearing, at nearly regular intervals, whorls of short radiating branches, each composed of rounded cells, arranged in a bead-like row, and sometimes branching again. Some of the radiating branches grow out into transparent points, which may possibly be antheridia, and contain motile bodies; for within certain cells in other branches resting spores are found, which are agglomerated and form the large dark globular masses that are seen in the midst of the whorls.

The Hydrodictyon utriculatum is another allied plant of singular structure, which grows in fresh-water pools in the midland and southern counties of England. It resembles a regularly reticulated green purse, from four to six inches long, and is composed of a vast number of tubular cylindrical cells, which adhere to one another by their rounded extremities, the points of junction corresponding to the knots or intersections of the network. Each of these cells may form within itself from 7,000 to 20,000 gonidia, which at a certain stage of their development are observed to be in active motion in its interior; subsequently, by mutual adhesion, they form into groups which lay the foundation of new net-plants, when set free by the dissolution of their envelope. Besides these groups, there are certain cells which produce from 30,000 to 100,000 more minute bodies of a longer shape, each of which is furnished with four long cilia, and a red spot. These escape from their cell in a swarm, move freely in the water for a time, then come to rest, and sink to the bottom, where they remain, heaped together in green masses. Their future fate is unknown, but they are believed to be male filaments similar to those described, and are generally called spermatozoids.

The NostochineÆ are either an assemblage of cells loosely united into numerous green chaplets, or distinctly beaded filaments, generally twisted, and occasionally branched; they are imbedded in a firm gelatinous frond of different form, sometimes globular, sometimes spreading in branched masses, often of considerable size. They are frequently seen on damp shady walks in gardens: they shrink to a film in dry weather, and reappear so suddenly in rain that they have been called fallen stars. They are reproduced by spontaneous division of their filaments; the segments escape from the gelatinous mass, move slowly in the direction of their length, after a time come to rest, secrete a gelatinous envelope, and not only grow in length by transverse bisection, but split longitudinally into new filaments which are separated by their gelatinous secretions. These movements, discovered by M. Thuret, are evidently intended to disperse the plant.

Vesicular cells, destitute of endochrome, sometimes furnished with cilia, and of a larger size than the others, are occasionally seen at the end or middle of a filament of the Nostocs, sometimes situated at intervals along their length; and near to these are sporangial cells, a little larger than the ordinary cells. From analogy, it is believed that the vesicular cells are antheridia, and that the sporangial cells contain germs which, after being fertilized by the spermatozoids, are set free and become resting spores. In some species, the sporangial cells are oblong, and contain vividly green matter; in others, the cells are elliptical and brown.

The species are widely distributed. Hormosiphon arcticus, a species consisting of a modification of cellulose, abounds to such a degree in the herbless polar regions, that it affords a welcome variety of food. Each plant lies on a small depression of the snow, which covers the soft and almost boggy slopes bordering the arctic seas, but it is carried by the winds in every direction, rolling over the snow and ice to a distance of several miles. Two northern species of Nostoc were found by Dr. Hooker in Kerguelen’s Land, growing on wet rocks near the sea; one of them was the common Nostoc commune. Other species occur in the warm springs in India, as well as in the arctic and antarctic regions, and an aquatic species is much used in China as a wholesome food. The genus Monormia forms floating masses of jelly on the surface of brackish water. The necklaces are of vast length, and, together with the jelly in which they are imbedded, wave with the slightest motion of the water. Floating masses grow on large ponds or lakes, which give the water a green tint.

The structure of the OscillatoriÆ is microscopic. They are minute filiform plants closely allied to the Nostocs; and consist of transparent colourless tubular filaments containing colour cells of various forms, more or less separated from each other, and visible through their transparent tubes; the colour is usually some shade of green, yellowish, or purple. In the genus Rivularia these tubular filaments have a globular transparent cell at the base, and are closely packed into little balls, either forming small groups, as in the Rivularia nitida, or singly attached to stones and rocks. In Rivularia nitida, the filaments radiate from a centre. Some OscillatoriÆ form velvety cushion-like patches upon rocks, others are attached in tufts as parasites to other sea weeds, while many are arranged in free or attached stratified bundles. Lingbya furnishes a beautiful specimen of the latter. The filaments in the stratified group are usually much twisted and interwoven, and some of them exhibit singular oscillating motions, as the Oscillatoria littoralis and spiralis, Spirulina tenuissima and others; one end of the filaments remains at rest, while the other extremity is in constant vibration. With a microscope the movement in some species is seen to be from side to side like a pendulum, in others it is spiral or twisting, and when a fragment of the plant is set free when vibrating the movement is progressive. If a fragment be put into a glass of water, its edge in a little time becomes fringed with short filaments radiating from central points with their tips outwards. They soon detach themselves from the fragment by their oscillations, and as their vibrations continue after they are free, they swim with a spiral motion to the edge of the water, and even ascend the glass till arrested by the dry part above.^[35] During these motions there is a corresponding alteration in the form of the filamental tubes believed to arise from rhythmical periods of vital contractibility, which are affected by light and heat, because the motions are more rapid in sunshine than in shade; besides, they are checked by strong chemical agents. Some of the species have a tuft of delicate cilia at the extremities of their filaments.

The free stratified bundles contain the simplest form of the OscillatoriÆ. Each filament is a straight or slightly curved chain of cells, full of coloured matter, and enclosed in a common transparent colourless tube. Multiplication takes place in these by division; when about to multiply, two adjacent coloured cells, or the two halves of a divided cell, recede from one another, and the outer tube contracts at the point of division, and separates them into two distinctly new filaments. Sometimes the transparent outer tube does not yield, so that the divided parts retain their places in the tube, which dilates when these new parts are again divided. The manner of division varies with the species, and the generic characters of the OscillatoriÆ depend upon the different conditions of the external tube, and the form and arrangement of the coloured cells within it. The tube often contracts to the finest point during division, and frequently consists of distinct coats, the number of which increases upwards, sometimes with such regularity as to produce a beautiful streaked effect. Like their allies, the OscillatoriÆ are reproduced by zoospores. While these parts are growing, but especially during their dissolution, the endochrome undergoes various changes of colour, staining the water they die in, and rendering it putrid; some of the common kinds emit a strong odour of sulphuretted hydrogen.

In the compound gelatinous OscillatoriÆ, the jelly is of very different degrees of tenacity. The mass of the Dasygloea is so slippery that it can scarcely be taken hold of; Rivularia nitida (fig. 19) is equally so, its tubes being so thick and tender. Many species of the genus Rivularia have a peculiar mode of oblique alternate branching; species of that genus grow on the stems of aquatic plants, on rocks in rapid streams, on cliffs when washed by cataracts, or sometimes in calcareous water, in consequence of which crystals of carbonate of lime are deposited on their substance. The Rivularia nitida occurs among AlgÆ exposed at low tides, and a species of another genus floats on fresh-water lakes like green stars.

Fig. 19. Threads of Rivularia nitida.

The OscillatoriÆ are found in every part of the world, most abundantly in the temperate zones. They chiefly inhabit fresh water, but these minute plants attain their greatest size in the sea. Numerous species grow in warm springs, and one species, Trichodesmium erythrÆum (fig. 20), spreads for many square miles over the surface of the Indian seas in faggots of red-brown threads, like fragments of chopped hay; the same species is said to abound in the Red Sea also.^[36]

The ConjugatÆ are fresh-water plants of numerous species, which have almost the same structure as the ConfervÆ, but the green endochrome within the cells of their articulated threads is more highly organized, and the manner of reproduction is altogether different and very peculiar.

These plants consist of strings of cylindrical cells joined end to end by their flat ends, and generally float freely on or near the surface of still water, especially when buoyed up by the bubbles of gas which are liberated from them by the heat and light of the sun. In the early stage of their life, while as yet the cells are undergoing multiplication by self-division, the endochrome is diffused pretty uniformly in each cell; but as the plant approaches towards maturity, it undergoes various modifications, according to the species. In some it consists of large granules disposed in rows; in others it is formed into broad spiral bands with large granules in binary or stellar groups placed at intervals on it; and, in the oedogonium capillare and others, the granules are united in spiral lines which cross one another and form a network.^[37]

Fig. 20. Trichodesmium erythrÆum.

The act of conjugation by which spores are formed, usually takes place between the cells of two distinct parallel filaments which happen to be adjacent to each other, and all the cells of the two filaments generally take part in it at once. The cells that are opposite to one another put out little protuberances, which come into contact with each other; the intervening partitions disappear, so that a tube is formed which establishes a free communication or passage between the cavities of the conjugating cells. In the genus Mesocarpus and others, the conjugating cells pour their endochromes into a dilatation of the passage that has been established between them, and it is there that the matter mingles to form a spore or embryo cell. But in the Zygnema (fig. 21), which is the commonest form of these plants, the endochrome of one cell passes entirely over into the cavity of the other, and within the latter the two endochromes coalesce into a single mass, round which a firm coat is developed, and it becomes a spore. All the cells of one filament are thus left empty, while spores are formed in all the cells of the other.^[38] Sometimes cells in the same filament conjugate, and occasionally the endochrome in a cell divides into two parts, each of which becomes a spore.

Fig. 21. Conjugation of Zygnema quininum:—A. two filaments in the first stage of conjugation; B, completion of the act of conjugation.

Some of the spores are quiescent, others have cilia and are motile, but both after a time become attached at one end by two or three root-like fibres, and grow into filaments by repeated bisection. According to the observations of M. Itzigsohn, the endochrome in certain filaments of Spirogyra breaks up before conjugation into little spherical aggregations, which are gradually converted into nearly colourless spiral filaments, having an active spontaneous motion, and therefore corresponding precisely to antherozoids. With the exception of South America, the ConjugatÆ are widely dispersed in warm and temperate climates.

The genus Vaucheria may be assumed as a type of the SiphoneÆ, whose essential character is, that the plant consists of one single tubular cell, however branched and complicated its form may be. The Vaucherias form tufted masses of branching tubes, filled with bright green granular matter, on mud and damp soil; they abound in fresh-water pools, and some grow in the sea. When about to produce fruit, the extremities of some of the tubes swell out in the shape of a club, in which a portion of the green matter collects, takes a darker hue, and is separated from the rest by a transparent space and a new envelope. After various changes, the darker green matter forms itself into a zoospore, which is so active that it breaks open the top of its club-shaped cell, and comes into the water; sometimes several come, one after another. They are egg-shaped, with a colourless beak, and as their whole body is bristled with cilia, they leave a long current in their wake when they swim, which they do with such impetus that they are flattened against any obstacle they meet with, even to the discharge of their green endochrome. They escape from their cell about eight in the morning, move for two hours, then come to rest, and begin to grow into a new plant.

M. Pringsheim discovered another mode of reproduction in the Vaucherias, which are monoecious plants, that is to say, the same plant produces snake-like fertilizing spermatozoids and female germ cells. For example, the Vaucheria sessilis consists of one long branched cell; on the same side of it two swellings appear near to each other, one of which elongates, curls round like a horn, and is soon filled with snake-shaped filaments having long cilia at their thin end, with which they move rapidly both within the horn, and after they come out of it into the water. They are perfectly colourless, and correspond to the pollen of flowering plants. The other protrusion which swells into a globose germ cell, and which corresponds to the pistil of a flower, contains a mass of green endochrome, which, after being fertilized by the snake-like filaments, becomes a primordial cell which has no motion, but after having secreted a strong coating of cellulose, it sinks to the bottom of the water, becomes a winter or resting spore, and lays the foundation for a new generation of plants. The resting spores produce new forms, while the zoospores, like buds, only multiply the type of the individual plant with all its peculiarities.

The marine genus Bryopsis grows in New Zealand, the Falkland Islands, and the seas about Cape Horn. The species are mostly parasites on other AlgÆ, and produce innumerable zoospores. The genus Codium is found in high latitudes, and appears under four different forms on the British coasts; one of these inhabits turfy banks exposed to the spray of the sea, the others grow in deep water, or on rocks never uncovered but at spring tides. Species of this genus are found as far south as Kerguelen’s Land, and in most of the intervening latitudes. The Caulerpas inhabit the warmer districts in the northern hemisphere, and furnish five species in New Zealand. The numerous species afford almost the whole food of turtles on many coasts, and other genera furnish nutriment to a host of smaller animals.^[39]

The Achlya prolifera is also a unicellular plant, much smaller than the Vaucheria, but whether an Alga or a Fungus is not very clearly settled. To the naked eye it appears as a cluster of colourless threads on dead flies floating in water, on the gills of fishes, and sometimes on frogs. With a microscope the tufts are seen to consist of tubes extending in all directions, filled with a nearly colourless granular matter, the particles of which are seen to move slowly in streams along the walls of the tubes, the currents sometimes anastomosing with each other. When the plant is about thirty-six hours old, the endochrome begins to accumulate in the dilated ends of the tubes, and is cut off from the remainder by a transverse division, the motion of the particles being still visible in the part cut off. The endochrome breaks up into a number of long masses, each of which acquires a cell wall and two cilia, and begins to move about within the parent cell; when mature they are set free by the rupture in its wall, and germinate, and produce a facsimile of the parent. It appears that, in some species, the transverse dividing film becomes convex as soon as the motile bodies are discharged, a new fertile articulation is formed and new motile spores are set free, and this process is continued till the vital powers of the plant are exhausted. The Achlya has resting spores, which may remain long in the water without change, but if a dead insect be put into it, they fix on it and germinate immediately. It is supposed that these resting spores are fertilized by filamental bodies. The Achlya prolifera goes through all its changes in an hour and a half or two hours. It is found in the thermal springs at Vichy, Nevis, and Vaux, where it contains an alkaline iodide.

The whole of the plants which have been described in the preceding pages belong to the group of green AlgÆ, although many are inhabitants of fresh water. The structure of the marine AlgÆ is entirely cellular. Deprived of vascular tubes, they can have no circulation of sap, consequently they derive their nourishment by absorption throughout their whole surface from the medium in which they live, for their root, or rather fulcrum, only serves to fix them to the rocks and stones to prevent them from being buffeted by the waves. Since solar light and heat decrease rapidly with the depth, each family of AlgÆ has a zone peculiar to itself. The first zone extends from high to low water mark, and is inhabited by plants periodically exposed to the atmosphere, to the direct light and heat of the sun, and occasionally to rain. Some of the AlgÆ that are long left dry are believed to derive some nourishment from the substances to which they are fixed. The second zone, which extends from low water mark to a depth of fifteen fathoms, is the region of the great marine forests which encircle the globe in both hemispheres. Other two zones follow at greater and greater depths, but all are divided into various minor regions, below the last of which the AlgÆ decrease as the depth increases, till, as far as we know, vegetation ceases altogether; that depth, however, must be very great, as diatoms are sometimes found, and in great quantities, three hundred fathoms deep.

The marine Confervas, like those growing in fresh water, are slender-jointed filaments formed of one series of cells joined end to end. The cells become more or less flattened on the surface of contact, while the side walls retain their natural curvature, which may be cylindrical or oval. The filament may, therefore, be cylindrical or beaded. The cells are almost always longer than they are broad, and for the most part equal and similar in the same plant, although there are exceptions to uniformity of size. The cells contain a transparent liquid through which minute solid particles of various shades of green are pretty evenly scattered. The conversion of these particles into zoospores has already been described. Since these AlgÆ have no roots, and the cell wall no opening, each cell of a Conferva elaborates independently the nutriment it absorbs from the water. Some species form a fleecy layer over rocks, and on the bottoms of salt-water pools and estuaries, others extend in bundles in salt-water ditches, and some are found on rocks, between tide marks, rising in long, straight, stiff, and wiry tufts, from three to eight or twelve inches high.

The genus Hormotrichum, which forms tufts several inches long, of bright grass green, differs from the Confervas in being soft and gelatinous, and even more by its mode of increase, which, however, is still by zoospores. The H. collabens may be taken as the type of this genus. It forms a long and large tuft of soft gelatinous and slippery filaments of glossy green. The joints of the filaments are once, or once and a half, longer than they are broad, and the green granular matter within them is collected into a round sac or sporidium in the centre of each, and after being converted into zoospores, the sac comes through a rupture in the joint into the water, opens, and sets the zoospores free.

The genus Cladophora, which has twenty-five species in the British seas alone, forms tufts of jointed filaments from four to eight, ten, or even twenty inches high. In some species the filaments are rigid, bristly, and wiry; in others they are soft and silky; but they are always richly, variously, and sometimes densely, branched and rebranched. In some the branches and branchlets are forked, in others tripartite; the Cladophora pellucida, which is a rigid, wiry plant, combines both these forms.

The genus Bangia consists of purple filamentous jointed and unbranched AlgÆ, which are distinguished from all others by the microscopic arrangement of their endochrome, which is enclosed in little cells placed according to a definite plan within the transparent and tubular joints of the filaments. In the Bangia fuscopurpurea, whose blackish purple tufts, several inches long, cling closely to the rocks near high-water mark, the tubular joints contain rows of minute colour cells radiating from a centre. In the narrow filaments there is but one colour cell in a joint, but in the broader filaments there are from three to five, forming a tesselated line across it. In this plant one spore is produced in each joint. The Bangia ciliaris forms a scarcely perceptible rosy pink fringe of hair-like jointed filaments on the Zostera marina, and also on other AlgÆ. The filaments are not more than the tenth or fifth of an inch long, consequently their joints are most minute, yet the microscope shows that they contain from two to three colour cells set as if radiating from a centre, and that the granular endochrome in each cell is converted into two zoospores. The Bangia ceramicola, which forms purplish pink tufts on small AlgÆ in rock pools, differs from both of the preceding. The joints of its filaments are once or twice as long as they are broad, and contain colour cells like long upright lines. By aided vision zoospores are seen to be formed within the linear colour cells, then the cells run together into a globular mass, which bursts through the cell wall, leaving the joint empty. The whole genus is soft and sometimes gelatinous.

The Enteromorpha genus is characterized by a cylindrical and tubular stem and branches. These plants form two groups, one whose filaments and branches swell from a narrow base upwards and terminate in a blunt extremity, while in the other group the tips of the branches are pointed. The Enteromorpha intestinalis, which is an inhabitant of many seas, has a thin membranous, tubular, cylindrical, and unbranched stem, inflated upwards into a broad round head, being more or less wrinkled and curled throughout. Downwards it tapers to a fine thread, and although attached at first, at last it becomes floating. Several of these plants rise from the same root, sometimes to the height of two feet, at others not more than an inch, and they are of every width, from the tenth of an inch to three inches, their colour being grass green. The typical form of the other group is much branched, and all the branchlets are finely pointed.

The three genera Codium, Bryopsis, and the marine Vaucherias are all soft plants characterized by their filaments being tubular, however much they may be branched. They agree also in being reproduced by zoospores developed from the green matter within little sacs attached to the exterior of their filaments. The species of the genus Codium differ much, although formed of similar elements. In the C. tomentosum, which is from three to twelve inches long, the dark green stem is thicker than a crow’s quill and much branched; while Codium Bursa, on the contrary, is a dark green round spongy lump of tubular filaments, densely interwoven and matted together. These masses, which are from one to eight inches in diameter, become hollow when old, and different sizes and ages grow together in a group.

The Bryopsis is a yellowish green tubular plant, from two to four inches high, plumed like a feather, and sometimes replumed. It is a rare plant in England, and grows on the larger AlgÆ in deep water.

The Vaucheria marina forms soft limp tufts of hair-like filament filled with bright green matter, which often runs partially out. It is from one to three inches high, and has a few long upright branches, to which are attached small stalked pear-shaped sacs containing zoospores. Both this plant and the Vaucheria velutina, grow on muddy shores.

Fig. 22. Ulva latissima: a, portion of ordinary frond; b, cells in which the endochrome is beginning to break up; c, cells from the boundary between the coloured and colourless portion, some containing zoospores; d, ciliated zoospores; e, development of zoospores.

The Ulvas, which are the grass green layers seen on all our coasts, originate in the simple vegetable cell, whatever form their foliaceous fronds may ultimately assume. When the cell is divided in one direction only, a confervoid filament is the result; and if the filament should increase in breadth as well as length, according to a determinate law, a ribbon-shaped frond may be produced; but when the original cell is divided into four cells, and each of these four and all their successors undergo similar division, the increase being as the series 1, 4, 16, 64, &c., a membranous expansion is formed, in which all the cells are firmly attached to one another, and every portion is the exact counterpart of another. The cells of the Ulvas frequently exhibit an imperfect separation of the granular endochrome into four parts preparatory to multiplication by double division, and the entire frond or leaf shows the groups of cells arranged in clusters containing some multiple form of four, as in fig. 3, page 171.

The frond membrane of the true Ulvas, as that of the Ulva lactuca, is formed of but one layer of cells; the frond itself is thin as cambric paper, almost transparent, and of a pretty light green. When young it is a puckered inflated bag, which afterwards bursts and opens into a flat, ribless, wavy, more or less rounded expansion, three to six inches long, and as many broad. This plant, which is attached to the rocks between the tide marks on our shores, is rare in the Mediterranean; nor is it so common in Britain as the Ulva latissima (fig. 22), which is cosmopolite, and abundant everywhere. It is found as a ribless irregular expansion of a full bright green in deep water, and of a yellow apple green when in shallow water, and exposed to the light. The base and stem are very short, and the frond, which the microscope shows to be formed of two layers of cell membrane, spreads so rapidly into crisp wide-lobed foliations, that the parts often overlap each other in stiff bulging folds. It is from six inches to a foot in height, and from three to twelve inches wide. The frond of the Ulva Linza is also formed of two layers of cells, but so small and so closely pressed together that the two layers can only be detected by the microscope. This plant, which is from six inches to two feet long, is a ribless, narrow, ribbon-shaped expansion with curled wavy edges tapering to a base, and either blunt or pointed at the top. Its colour is the same as that of the Ulva lactuca.

In the Ulvas, which are multilocular plants, some cells are selected to bear fruit, and others not. The granular endochrome of these chosen cells divides into several parts, which are at first in close contact and at rest; then they become restless, acquire four or a greater number of cilia, and pass through a fracture in the cell wall into the water, in which they swim freely as zoospores. After a time they come to rest, attach themselves to some object, and begin to grow. The walls of the cells which have thus discharged their endochrome in the form of zoospores, remain as colourless spots on the frond. The whole colouring matter of a portion of the frond may escape as zoospores, leaving behind it nothing but a white membrane. With a microscope, this process may sometimes be observed in all the different stages of its progress.

Every full-grown Ulva has its own precise and definite form, but whatever that may be, the young plants on their first appearance from the shore are in all respects similar to Confervas; the top cells soon divide, and a plane or sac-like frond is formed.

Certain Ulvas, which have a yellow tint, produce small zoospores with only two cilia, but in the Ulva bullosa and the Ulva latissima four zoospores are produced in the same cell, each having four cilia. The same fructification prevails also in the purple Ulvas—Porphyra laciniata and vulgaris. The latter is seen in winter and the early spring, covering the rocks near high water mark, with its tiny bright purple lanceolate leaves. Later in the season it grows into a flat narrow ribless frond with a pointed end, and about two feet long, the margin of the frond becoming waved and plaited as the plant increases in growth. At a later period, it is seen mixed with the Porphyra laciniata, which is a ribless flat frond of a dull purple; sometimes it is very thin, divided or torn, and occasionally growing in a circle round its root. Both forms are sold as laver.^[40]

The RhodospermeÆ, FlorideÆ, or Rosetangles, are the most beautiful of the marine vegetation. No sea plant surpasses them in delicacy and grace of form or richness of colouring, but the most beautiful are seldom seen, because they grow below the line of ebb tides, or under the shelter of other sea weeds in the rock pools left at low water, their crimson tints being deepest when sheltered from strong light. The Rhodosperms, which have representatives in every sea, are much more numerous than the green AlgÆ both in genera and species. Thirteen orders, comprising sixty-seven genera, inhabit the British coasts. Many are exceedingly minute, forming patches and velvety cushions on rocks and other AlgÆ; a vast number have jointed filamentous fronds, while others consist of tubular filaments, and many exhibit a shrub-like collection of firm branches; some are flat and foliaceous expansions without a midrib, either thin and delicate, or thick and strong, while a very brilliant group of both narrow and spreading fronds possess a midrib as a distinguishing character. The structure of the frond varies from a simple membranous to a cartilaginous or even horny substance, caused by a greater development of the cellular tissue, which in the higher kinds of FlorideÆ divides the epidermal layer or skin from the parenchyme or spongy matter within.

The mode of reproduction by tetraspores, as well as by simple spores, distinguishes the Rhodosperms from the other two great divisions of the marine AlgÆ. These bodies are produced by the division of the red or crimson endochrome into four parts, which remain in the cells till they acquire an envelope; their form, which is much varied, depends upon that of the endochrome. Some are produced by the breaking up of a globe of endochrome from the centre into four pyramidal segments; or should the endochrome be elliptical, by dividing it into four by three parallel segments, or a mass may be divided into four by horizontal and vertical sections. Some of these are represented, greatly magnified, in fig. 23. The tetraspores are lodged in wart-like excrescences, immersed either partially or wholly in some part of the frond.

Fig. 23. A, Polyides rotundus:—a, thin slice showing the wedge-shaped spores; b, tetraspores.
B, Furcellaria fastigiata:—c, thin slice showing a nucleus with the dividing spores; d, one of the large cells; e, a tetraspore.

The simple spores are produced within colourless tubercules called nuclei, variously situated upon the plant, as at fig. 23 a, c. These nuclei contain many microscopic spores. Sometimes the nuclei are enclosed in conceptacles or ovate sacs, which are either perforate or not at the apex. These contain many microscopic strings of cells like jointed threads, and the endochrome in each joint of these threads is converted into a spore successively from the summit downward. Sometimes the endochrome in one or two joints only, becomes a spore whether terminal or central, and when the spores break through the joint wall and fall off from the threads, they are collected without any definite order into a mass within the nuclei. Sometimes new joints or cells are produced on the threads when the old ones have yielded their fruit. Occasionally a globose nucleus contains several secondary nucleoli full of spores. In every instance, the perfect spore is a dense grumous mass surrounded by a hyaline sub-gelatinous coat consisting of at least two membranes. The situation, mode of growth, and structure of the nuclei vary almost infinitely, and together with the structure of the frond afford the distinctive marks by which the genera are separated from each other.

The spores and tetraspores are equally capable, like buds, of reproducing their species; but the spores are believed to be in some cases fertilized by spindle-shaped particles, and consequently are considered to be the true fruit. Antheridia, or sacs containing these particles, have been discovered in various genera of Rhodosperms. Although, as a rule, the red AlgÆ have two modes of vegetative reproduction, yet there are various species in which tetraspores only have hitherto been met with.

A large proportion of the higher Rhodosperms is distinguished from those possessing the preceding mode of fructification by the internal structure of their reproductive nuclei. In some of these AlgÆ the nuclei are divided into two equal chambers by a fibro-cellular substance to which the spores are attached; in others, pear-shaped spores radiate from a fibro-cellular substance at the base of the nucleus. There are, moreover, AlgÆ which have nuclei containing conical spores whose broad bases radiate from the centre, and other arrangements occur.

The Rhodosperms are comparatively small plants. Some which form velvety cushions on stones, or minute tufts on small AlgÆ, are only the fraction of an inch high, but the larger kinds range from one to four, six, ten, or twenty inches; probably none exceed two feet. In thickness, some fronds are fine, like jointed and branched hairs, while others are thick, like hog’s bristles or crow quills. Numerous as the forms are, the simple jointed filamentous frond is connected by a series of forms with the highest order of the class.

A great portion of the Rhodosperms on the British coasts is composed of the exquisitely beautiful order of the CeramiaceÆ. They abound in every rocky pool, on every piece of wood that has been long exposed to the waves, on rocks and stones, and, above all, they fringe the Zostera marina, or sea wrack, as well as the firmer AlgÆ, with every shade of red from bright crimson to purple. They are articulated filiform plants, approaching in simplicity of form to the Confervas. The genus Callithamnion, which has thirty species in the British seas, consists of cylindrical jointed threads more or less profusely branched, and distinguished by having the divisions between the joints opaque and of various shades of red and purple, while the joints themselves are transparent and colourless, so that the stem and branches appear to be striped across by alternately white and coloured bands which are often visible to the naked eye, notwithstanding the smallness of the plants and the delicacy of their filaments, as the C. sparsum,—which is a soft purple tuft of jointed threads scarcely one-tenth of an inch high.

The Callithamnion corymbosum has a soft jointed filamentous stem, hair-like below, fine as a cobweb above, and excessively branched, with dichotomous branches. In fig. 25 a represents a thread of this plant with tetraspores, much magnified; b, a portion of the same, more highly magnified; c, a thread with naked nuclei, gongylospermous, that is, filled with a mass of spores, magnified; and d, a spore, magnified more highly. The nuclei are naked in all the CeramiaceÆ.

The genus Ceramium, some species of which have spinulose branchlets, is characterized by the tips of the forks of its terminal branchlets being hooked inwards, and by the stems and branches being striped by alternate hyaline and coloured bands as in the preceding genus, though the arrangement of the colours is somewhat different. The Ceramium ciliatum, which is a dense tuft of capillary jointed filaments, from two to six inches long, repeatedly and regularly forked, has the tips of the last forks so much hooked inwards, that the extremities of the branchlets look as if they were heart-shaped. It has minute spores in globular nuclei, sessile on the branches with two or three branch-like hairs beneath them, and tetraspores set in the coloured parts of the joints with a thorn between each, for in this plant the centre of the joint is hyaline, the rest coloured.

The genus Griffithsia contains various species of bright rose-coloured plants, which become bleached when put into freshwater, and form a circle when spread out. Soft, tender, and gelatinous, they form dense tufts of jointed and branched filaments on rocks at low water mark. The filaments are slender below, capillary and forked above, and the joints contain one linear upright rose-coloured tube, which is seen throughout their transparent walls: a distinguished mark of the genus. Tetraspores are borne on the hair-like jointed ramuli, and spores are amassed in coated roundish sessile nuclei, surrounded by minute hair-like fibres. Several species once called Griffithsia differ so much from the others, that they are by some referred to Halurus, which has the stems and branches thickened by overlapping whorls of tiny forked jointed and curved ramuli. They are propagated by spores, enclosed in clusters of nuclei borne on the tips of short branches, with a mass of curved ramuli folding over them, and by tetraspores attached to the inside of another set of curved ramuli. Antheridia have been discovered in several genera of the group CeramiaceÆ, especially in Ceramium, Callithamnion, Griffithsia, and Halurus. They consist of little clusters of cells variously arranged, in which the active particles known as spermatozoids are generated.

The Polysiphonia are AlgÆ, seen in tufts from ten to twelve inches long, of usually much branched jointed filaments, on rocks, corallines, and the smaller AlgÆ at low water mark. The joints of the filaments contain upright tubes, full of purple or reddish brown matter, which is seen through their transparent walls. The number of these colour tubes vary from four to ten, eighteen, or even twenty, and form the characteristic of the genus. Thus there is a similarity of structure between the Polysiphonia, a genus of the highest order amongst Rhodosperms, and the Griffithsia, which is one of the lowest. The Polysiphonia elongata, which is from six to twelve inches high, has four primary and several secondary colour tubes in the transparent joints of its filaments. Like many of its congeners, this plant does not come to perfection or bear fruit till the second spring. In its youth, it resembles the full grown plant but is smaller, and the colour tubes are not formed in the capillary threads of the tufts, which with many of its branchlets are deciduous, leaving the plant in its naked winter state. With returning warmth, it assumes its perfect form, and in March and April bears fruit, which consists of nuclei in conceptacles, sessile on the branches, either clustered or scattered. The spores are at the top of jointed threads rising from a substance at the base of the nuclei. In some species of this genus, tetraspores only have been found.

The CryptonemiaceÆ are the most numerous and diversified of all the orders of the Rhodosperms. Thirty-five genera are widely dispersed throughout the world, chiefly in the northern hemisphere; twenty-four genera at least occur on the east coast of North America; and fifteen genera have representatives in the British seas. This multitude of generic forms is divided into two groups of gelatinous structure, the one having inarticulate fronds composed of articulate threads closely incorporated, the other membranaceous, formed of cells closely incorporated into a foliaceous expansion. Most of these plants have a stratum of cellular tissue, interposed between a spongy matter in the interior of the frond, and the epiderm or external skin, which for the most part consists of a simple layer of minute cells firmly united by their sides, generally forming a mere film; but it may be thin and flexible, thick, tough, or leathery, according to circumstances.

The Furcellaria fastigiata (fig. 23 B) has an intermediate layer of cellular tissue between its skin, and a pulpy interior. The frond is cylindrical, smooth, strong, and opaque, repeatedly forked with long narrow forkings. The root is fibrous, and the stem short and tapering. Masses of spores nestle under the skin and swell out the upper forkings, and oblong tetraspores are deeply imbedded in the same.

In the Dumontia filiformis the simple undivided stem and branches are filled with a watery jelly.

The stem of the Chylocladia kaliformis is a cylindrical tube, from four to eighteen inches high, constricted at intervals of half an inch or more into long hollow joints; branches of the very same construction but smaller spring from each constriction either opposite to one another or in whorls; these again have lesser branches, all tapering more or less to each end. The plant, which is of a pink colour fading to greenish yellow, is propagated by tetraspores imbedded in the branches, and by transparent conceptacles sessile on the branchlets, enclosing nuclei containing pyramidal spores. We neither possess the Constantinea rosa marina, nor the C. sitchensis, some of the largest and finest plants of the group, both being inhabitants of high latitudes, but there are some very pretty species on the British coasts. They are supposed to be annuals.

The red dulses belong to the foliaceous and gelatinous part of this order. The Chondrus crispus, or Irish moss, sold as Carrigeen, is very common on rocky coasts in the northern seas. It is from three to eight inches high, and exceedingly varied in form. The frond is thickish, firm, and elastic, with a stratum of cellular tissue under the skin, which is probably much developed, as the plant becomes horny when dried. It is reproduced by tetraspores, in large oval groups scattered all over the surface, often prominent on one side only, and, in some rare instances, spores in prominent oval conceptacles are immersed in the lesser frond divisions. Besides these are warts composed of radiating threads, possibly antheridia, but not made out.

The RhodymeniaceÆ are sometimes filiform, but for the most part they are compressed flat cellular fronds, spreading widely from a short delicate stem. They are usually of a blood red, but Rhodymenia palmata, or common Scotch dulse, is of a dark purple. The tetraspores are variously disposed, and simple or compound globular conceptacles containing nuclei are either attached externally to the filiform fronds, or partly immersed in those that are foliaceous. The spores are produced in the joints of moniliform threads within the nuclei, which are sometimes divided into two chambers by threads running from wall to wall. Rhabdonia (fig. 26 A) belongs to this group.

The WrangeliaceÆ are filiform, many species consisting of a central thread coated more or less with smaller ones, sometimes so disposed as to form a most elegant lacework. Each joint of the stem, branches, and branchlets is beset with whorls of short slender forked and jointed ramuli. They have clusters of spores in stalked capsules. The spore threads of Wrangelia penicillata (fig. 26 C) are surrounded by a whorl of ramuli composed of radiating pyriform spores arising from the endochrome of terminal cells.

The SquamariÆ resemble lichens in spreading themselves in a red crust over stones and rocks. They have roots below, and warts, on their upper surface, in which there are tufts of moniliform spore-bearing threads. The tetraspores of Cruoria pellita are shown in fig. 26 D; its repeatedly forked filaments taper upwards, and the tetraspores are formed in the swollen centre cell of the filaments. The Peyssonnelia grows on shells and other marine objects, and extends from the Mediterranean to Ireland, and the east coast of North America.

The Polyides rotundus (fig. 23 A), representing the only genus of the SpongiocarpeÆ, has a dark purple solid gristly cylindrical stem, repeatedly and regularly forked, all being of the same thickness. The tips of the last forkings, which are small and equal, give the top of the plant a rounded form. The microscope shows that the stem and branches are composed of a central column of interlaced threads and radiating cells; it shows, moreover, that hyaline nuclei containing a cluster of conical spores whose broad bases radiate in all directions from a centre, as in fig. 23 a, are scattered among the articulated threads of oblong irregular spongy warts which clasp or embrace the stem and branches. The tetraspores are buried in the ends of the last forks. This plant is so like the Furcellaria fastigiata (fig. 23 B), that it affords a remarkable instance of similarity of form and total diversity of fructification, not only in the spores and their arrangements, but in the form of the tetraspores; for in the Polyides they are formed by two sections, one vertical and the other horizontal, while in the Furcellaria the endochrome is divided by three annular sections, as in fig. 23 E.

The Gelidium corneum, common in Britain and almost everywhere, representing the group GelidiaceÆ, is opaque, firm, and of a dark purple. The axis with its alternate and repeated branches lying all in one plane, is composed of confervoid threads. This plant is distinguished by having its spore-cases or nuclei divided into two chambers by a fibro-cellular substance; the spores are either attached to this or to a network of threads; these bodies and the tetraspores are lodged in the tips of the branchlets. It is one of the most variable of all AlgÆ.

The SphÆrococcoideÆ comprise some of the most common and beautiful AlgÆ, remarkable for their brilliant rose and purple tints. This section consists of those red AlgÆ which have their nuclei lodged in an external subglobose conceptacle, the spores being formed at the tips of jointed threads rising from a substance at the base of the nucleus. A portion of a nucleus of SphÆrococcus coronopifolius, and a single spore magnified, is shown at fig. 26 b. The tetraspores are variously disposed. The fronds in this family are either gristly or membranaceous, and totally different from those which follow. They often assume a leafy aspect from the regularity of the nerves, which sometimes perform the functions of a stem when the membraneous border has decayed, and then they give rise in turn to new fronds. That happens in some species of Nitophyllum: a very short stem rises from a minute disc, and spreads widely into a flat ribless expansion, more or less deeply slit into broad rounded divisions. Wavy nerves from the top of the stem spread through the fronds, which are left bare in winter, and give rise to new fronds in spring. The leaves of the Delesseria sanguinea, from two to eight inches long and from one to six inches broad, are of the richest colour and most delicate structure, with evenly curled edges, and a firm solid stem, with prominent midrib and nerves. In winter globose stalked spore conceptacles are borne on the skeleton midribs of the summer’s leaves from which the margin has decayed, which thus become the stems of the next year’s plant. In this plant tetraspores in small special stalked leaflets fringe the skeleton midribs; in the Nitophyllums they are either scattered in dots over the frond, confined to the centre, or in lines round the margin. As regards the internal structure of this order, nothing can be more various, but they never acquire a truly articulate form. The genera and species of this group are widely distributed; they have many representatives in the Mediterranean.^[41] The genus SphÆrococcus is confined to Europe, while numerous genera are exclusively tenants of the southern hemisphere. The Gracilaria lichenoides, the Ceylon moss, is celebrated for its gelatinous qualities; and the Gracilaria compressa on our own shores is excellent as a pickle or preserve, and very ornamental. One of the most beautiful AlgÆ known is the Grinnelia americana, which abounds on the eastern coast of North America; fig. 24 b, is a vertical section of its conceptacle, showing the rudimentary placenta and spore threads. It differs singularly from the Delesseria sanguinea, of which it is an exact analogue, in the capsules being scattered over the surface of the frond instead of being situated on the midrib.^[42] The Delesseria sanguinea is now known as the Wormskioldia sanguinea.

The Corallines are florid AlgÆ, which absorb such a quantity of carbonate of lime from the surrounding water, that they become rigid, hard, and often stony. They are purple or pink when fresh, white and sometimes brittle when dry, and are propagated by strings of spore threads rising from the base of the nuclei which are enclosed in conceptacles or spore cases, open at the top. Some are articulate, composed of closely compacted threads, as the Corallina officinalis, a pretty little branched and bushy plant, most luxuriant in deep water, and particularly abundant in the rocky pools. Its urn-shaped spore sacs are attached to the tips or sides of the branches; fig. 24 c is a vertical section of one of them magnified, and d is a membrane of the same, more highly magnified, with impressions of the external cells. The joints of the articulate corallines, which are flexible and vary much in length, are either free from carbonate of lime, or ornamented with calcareous plates; it is through these open spaces that the plant is believed to obtain nourishment. The forms of the corallines are varied beyond description; many are mere amorphous crusts on stones and sea weeds, increasing from the centre outwards as in the lichens, others are lobed and branched like real corals. Corallines ascend to very high latitudes, but abound most in warm and tropical seas: either free, or coating pebbles at vast depths, they form the last zone of vegetable life.

The LaurenciaceÆ have fronds which are soft and thread-like, or solid, fleshy, and inarticulate; both are repeatedly branched. The colour of these plants is purple or a dullish red, but they are extremely sensitive to the influence of light and air, changing through every shade of orange, yellow, or green, according to the exposure, and like many other florid AlgÆ they lose their colour in fresh water. They are amongst our commonest sea weeds. The Laurencia pinnatifida is the pepper dulse of Scotland, and is also native on the eastern and western coasts of North America. Species have been found at the Cape of Good Hope, Australia, and New Zealand. The fructification in this section is quite peculiar. They have tetraspores lodged in the branchlets; and egg-shaped conceptacles with a terminal pore, enclosing nuclei with pear-shaped spores radiating from a fibro-cellular mass at their base. The antheridia, which differ in the different species, attain a greater degree of complication than in other tribes. In Laurencia tenuissima they form curious lateral twisted plates of a greyish tint, bordered with large cells. The plate is occupied by the productive cells of a much smaller size, evidently springing from a cellular branched axis. In Laurencia pinnatifida instead of a plate there is a somewhat hollow cup-shaped disc, formed of dart-like vertical groups of pale cells surmounted by two or three larger oily-looking sacs filled with yellow pigment. These bodies are sometimes forked, and appear to shoot out from the mass. L. dasyphylla presents a third modification, the antheridium being a sac, and the dart-like groups of cells being ejected from the minute terminal orifice. The moving particles produced in the cells of these three forms, differ a little in shape, and as they do not germinate they are believed to be spermatozoids, though no cilia have been found on them.

The RhodomelaceÆ, the last and highest family of florid AlgÆ, are, as the name implies, of a rich red brown colour. None of the other Rhodosperms can vie with them in peculiarity or variety of structure. The fronds may be areolate or reticulate, filiform or variously leafy, articulate or inarticulate.

Some genera, as for example Dasya, have slender, often elegantly branched threads, while such genera as Amansia and Odonthalia have instead a flat and pinnatifid frond. The latter, which has a very conspicuous cellular reticulation, is a genus of high latitudes, but is common on some parts of the Scotch and North American coasts. The British seas are rich in many genera of this order, and analogous forms occur in the southern hemisphere, where there are at least twenty-three genera. Many are remarkable for their singularity of structure: the Claudea for example, which is one of the most elegant of the AlgÆ, has a cancellated frond and is the ornament of warm seas; the Amansia and Leveillea which are distinguished by the beautiful reticulation of their fronds caused by large hexagonal cells; and the Dictyurus, in which the net forms a spiral web round the principal stem. Fig. 27 shows a portion of the network of Dictyurus purpurascens magnified. All the genera of this order possess free areolate hollow conceptacles perforated above, and containing nuclei, from the base of which short tufts of threads arise, each bearing a large obovate spore at its apex. The tetraspores are arranged in series either within the frond, or in distinct pod-like receptacles called stichidia. Fig. 28 shows the Polyzonia cuneifolia with its tetraspores arranged in rows in their pod-like stichidia, together with the areolated conceptacle and spores, all highly magnified. The antheridia differ in form in the different genera. In the Dasya they assume that of pods full of cells, in which the motile particles are generated; in the RytiphlÆa tinctoria the antheridia resemble those of the Dasya except in being elliptical, and in the RytiphlÆa pinastroides they are cellular bodies, without any investing membranes, clothed with delicate hairs.

The form of the Rhodosperms, as well as the limits of the species, like those of other AlgÆ, are affected by many circumstances known and unknown, such as the depth, temperature, saltness, and currents in the water. The Gelidium corneum varies to such an extent that its forms may not only be considered as distinct species, but even as belonging to different genera. The Delesseria alata is sometimes destitute of its margin, and then its midribs alone being left, it has the form of the Delesseria angustissima. Several species of the florid AlgÆ, which in their natural state have the tips of their fronds even and straight, occasionally produce hooked and clasping tips.

Brackish water is often a cause of change. The Irish moss, Chondrus crispus, when exposed to the fresh water of an estuary acquires great breadth and thickness, while at low water mark it is thin and has narrow forked branches, and there are many intermediate forms. The fruit rarely varies with these changes; its disposition and intimate structure, as well as that of the frond, are the points of prime importance for the determination of genera and species in the AlgÆ.

The MelanospermeÆ, or Melanosperms, are olive-green AlgÆ, sometimes inclining to brown. They have fewer species than the Rhodosperms, but the individuals exceed in abundance and in magnitude all the other AlgÆ.

These large Melanospermous AlgÆ, which form marine forests in both hemispheres, are excessively strong and tough on the exterior but of a looser texture within, so that the cells of their tissue are of different sizes and forms, according to the degree of pressure. The stems and branches are more dense than the leaves. This highest order, however, has small and delicate AlgÆ united to the largest by many intermediate forms. The Melanosperms are either monoecious or dioecious, and bear their olive-green spores in cases, that is cysts, variously disposed on the plants. Many have two kinds of zoospores differing in nothing but size; they are produced in different organs; in some species both are fertile, in others only one, and, in these cases, the other is therefore supposed to be a fertilizing body, but however that may be, there are certainly antherozoids in this group of AlgÆ, especially in the order FucaceÆ.

The EctocarpeÆ have many representatives on our coasts, all of which are tufts of articulated threads from one to eighteen inches long, branched or simple. They are generally soft, some so flaccid that they cling together, but sometimes they are firm and stiff. The cysts which are attached to these threads have various forms; they are spherical, siliquose (that is, like long pods), or of other shapes, according to the species; but whatever form they may assume, they are filled with a dense endochrome. Besides these they have active granules contained in other distinct organs. M. Thuret has decided beyond a doubt that the latter are small zoospores, and it is presumed that the endochrome in the cysts is resolved into zoospores, but of a different order, as in the Ulvas. These two organs are for the most part situated on different individuals; in Ectocarpus pusillus (fig. 29 b) they are on the same. The different forms of fruit carpels are represented magnified in fig. 29.

The EctocarpeÆ contain little or no gelatine, whereas the genera of the group ChordariÆ have soft gelatinous fronds of many forms, either incrustations, convex lumps, or tubers, like the Leathesia so common on our coasts; small plants as the Mesogloias, which have soft slippery filiform stems beset with myriads of moniliferous worm-like branches; or lastly the Chorda filum, a simple unbranched slimy cylindrical cord, varying from a quarter of an inch to the thickness of a pencil, and from one to twenty or even forty feet in length in deep water. The cord is tubular, divided into chambers by transverse partitions, formed of interlaced vertical and horizontal articulated threads. It tapers at each extremity, and the exterior, which is brown, is clothed with pellucid hairs. Vertical spores are immersed throughout the whole surface of the cord, and Dr. Harvey says that, mixed with these, there are numerous narrow, elliptical, transversely striated cells, which according to M. Thuret produce zoospores. Each plant rises solitary from its own little disc, but as the Chorda filum is a social plant, vast assemblies of it cover extensive areas of sand and mud, and form dense thickets in our northern seas. There are bands of it in the North Sea 15 to 20 miles long, and more than 600 feet wide; there is a submarine forest of it in Skapta Bay, Orkney; and in passing through the sounds of the western islands, as between Kerrera and the mainland, there are others. The long cords always lean in the direction of the tide, and must oscillate between two zones of rest, one at the turn of the flood, and another at the turn of the ebb. When dried the people use them for fishing lines. In the Chordaria divaricata both kinds of spore cysts are external, and give rise to zoospores.

In the preceding divisions of the Melanosperms the fronds consist of articulated threads; in the succeeding divisions the fronds are inarticulate. The latter comprise four very remarkable groups, of which the DictyoteÆ are distinguished by a leathery or membranous frond, sometimes cylindrical, but mostly flat, the surface of which is reticulated and sprinkled with groups or little patches of naked spores or cysts. The endochrome in the cysts is sometimes quadripartite, or even divided into eight parts. In one of the genera only, anything like antheridia have been found. The zoospores produced from the quadripartite endochrome are large, of a dark colour, and have two lateral cilia, while the bodies in the filiform much divided antheridia seated variously in the tufted threads are far more minute and pale, but with similar cilia. This order obtains its maximum of development in the tropical and subtropical regions; several species are found in the Mediterranean, while a few occur on our coasts, and on those of North America.^[43]

The genus Dictyota begins the zonarioid group, whose structure is very curious. Every band (lacinia) of the frond terminates in a single cell, by the constant division of which at the lower side, the other cells of the frond are formed, the terminal cell of the frond being thus continually pushed onwards. Hence it results that the longitudinal lines of superficial cells converge, thus affording a ready method of ascertaining the genus in default of fructification. When a new centre of growth is to be made, that is, when the frond is to become forked, the terminal cell divides longitudinally and then each half-cell grows according to its own law. Fig. 30 shows the tip of the frond of the Dictyota dichotoma magnified; the cells on its surface are square, and the interior of each has a spiral structure.

The Padina Pavonia, or Peacock’s-tail laver of our southern coast, and those of North America and the Mediterranean, is sometimes included in the genus Zonaria. The species is remarkable for its wedge-shaped fronds, which are olive green shaded with rust colour, and, when in fruit, they are striped across with dark concentric zones, which are merely lines of spores immersed in the frond and seen through its transparent superficial membrane. Each zone is ornamented with a fringe of orange-coloured hairs. Parallel to, or rather concentric with, the spores, is a row of articulated threads, which bear so strong a resemblance to the antheridia of the Cutleria that a similarity of function is suspected by Mr. Berkeley. Species of Zonaria, Padina, and Haliseris, which is the most highly developed of the DictyoteÆ, are most abundant in tropical and low latitudes.

The Cutleria multifida is a small plant not exceeding eight inches in length, of an olive green varied with rusty tints. The frond is a flat ribless expansion many times variously slit in the upper part. It is beautifully marked by prominent dot-like tufts of fructification scattered over both sides of the frond, and grows on rocks and shells in from four to fifteen fathoms water.^[44]

The great LaminariÆ form the principal part of those vast submarine forests which encircle the globe in the arctic and antarctic oceans. None of these gigantic AlgÆ are to be met with in low latitudes, but there are several smaller species. The Laminaria debilis of the Mediterranean is not more than five inches high, and we have some ribbon-shaped species also of small size. Besides, many small individuals of the large species grow on our coasts at low water mark or below it; but the largest individuals are only found at depths suited to their size, so that the great Laminaria, or tangle forests, extend from low water mark to a depth of fifteen fathoms.

The fronds of these AlgÆ are for the most part leathery and of a fibro-cellular consistence. The Laminaria bulbosa is the largest of our sea weeds. Mr. Berkeley says that individuals are sometimes found which are a sufficient load for a man to carry. A flat stem, often more than a foot long, rises with a twist from a round hollow bulb a foot in diameter, throwing out numerous stout fibrous roots below; the stem is bordered by a thin wavy membrane, whence these plants are commonly called sea furbelows. At the top of the stalk there is a broad leafy expansion cut into straps or segments, twelve or more feet long, and from one to two feet wide.

The Laminaria digitata, commonly called the great tangle, oar weed, or sea girdle, has a fibrous root, a stem six or more feet long, with a wide expansion at its top cut into very long narrow segments. The fronds of some LaminariÆ are deciduous; the stem increases in size year by year, a new frond springing from the apex and replacing the old one, which at last separates from the point of junction with the new frond, to which it is attached till the latter has attained its natural form and dimensions.

The Laminaria saccharina, called the devil’s apron on our northern coasts, is of a greenish olive when young, brownish when old. It has a fibrous root, a stem several feet long, ending in a flat ribless ribbon-like expansion, always very much longer than the stem, and terminating in a point. The margin of the frond is even, but wavy or puckered.

‘The fruit of these three great LaminariÆ is imbedded here and there in the surface of the frond, thickening it and forming cloudy patches.’^[45] It consists of thick club-shaped perpendicular cells in which the endochrome is ultimately divided into four parts. This is certainly the case in the Laminaria bulbosa, and also in the Alaria Pylaii, a species of which latter genus, the Alaria esculenta of our own coasts, is a much esteemed British dulse.

Abundance of colossal AlgÆ are found in the North Pacific, about the Kurile and Aleutian Islands, and along the deeply indented and channel-furrowed northwestern coast of America. The Nereocystis Lutkeana forms dense forests in Norfolk Bay, and all about Sitka. Its stem resembles whipcord, and is sometimes 300 feet long. It is exceedingly slender at the top, where it terminates in an enormous air-bladder six or seven feet long, and about four feet and a half in diameter at its widest part, the lower extremity passing into the stem. This huge air-vessel, which is the usual seat of the sea otter, is crowned with a tuft of twin leaves mostly rising on five stalks. These leaves, which are membranous and lanceolate when young, and from one to two feet long and two inches broad at the centre, are only marked with a few faint nerves, but they ultimately split lengthwise, cover a large space, and attain a length of twenty-seven or thirty feet, or even more. The growth of the Nereocystis must be enormously rapid, since it is an annual, and must therefore develop its whole gigantic proportions in one summer.^[46] Boats cannot pass through the floating masses of this plant, whose stem is used for fishing lines, and whose cylindrical air-vessel serves as a siphon for pumping water out of boats.

The Thalassiophyllum Clathrus is also an inhabitant of the Russian coast of North America. It is about six feet high, very bushy and branched, each branch bearing a broad leaf at its extremity which unfolds spirally, and by this gradual development produces the stem with its branches and lateral divisions. A spiral border wound round the stem indicates the growth of the frond, which presents a large convex bent lamina without nerves, or a leaf of which one-half is wanting. Numerous long narrow perforations, arranged in a radiating form, give it the appearance of a cut fan.

The Macrocystis pyrifera and the Laminaria radiata are the most remarkable of marine plants, for their gigantic size and the extent of their range. They are met with on the antarctic coasts two degrees nearer the pole than any other vegetable, except the DiatomaceÆ. The stem of the Macrocystis is slender, smooth, round, and slimy, rising from a fibrous root, like other LaminariÆ, and bearing at its tip a lanceolate or oblong lanceolate frond. This frond divides at the base; the fissures extend upwards so as to form two petioles, each of which swells into an oblong or pyriform air-vessel. Another fissure is formed in a similar way a little above, and so on, till a single frond may at the same time have eight or ten fissures, each of which will ultimately gain the common apex. The margins of the fissures are at first perfectly smooth, but they soon become ciliated like the outer edge. The continuity with the fibrous base is at last broken, and the divisions of the leaves going on indefinitely, the whole reaches the length of some hundred feet, forming enormous floating masses which are wafted by the waves hundreds of miles from their origin. Fructification only takes place in young plants; consequently in such as are still attached to their native rocks. Even in that youthful state, Mr. Darwin mentions that such is the buoyancy of this powerful weed, that there is scarcely a loose block of stone on the coasts of Cape Horn that is not buoyed up by it.^[47] The Macrocystis is native on the shores of the Atlantic, from Cape Horn to 43° S. latitude; but on the Pacific coast, according to Dr. Hooker, it extends to the river San Francisco in California, and perhaps to Kamschatka. The plant is reproduced by pyriform cells, full of endochrome, in nearly parallel rows imbedded in the fronds.

The rocky coasts of the Falkland Islands are covered with a vast growth of the gigantic Macrocystis mixed with forests of the arborescent Lessonia, which forms large dichotomous trees with a stem from eight to ten feet high and a foot in diameter. The leaves are two or three feet long, drooping from the forked branches like weeping willows. In the Lessonia nigrescens the quadripartite endochrome, ultimately resolved into spores, is contained in thickened club-shaped cells springing vertically between the surfaces of the frond.

A transverse section of the stem of many of the larger sea weeds presents zones, formed period by period, corresponding with the development of the laminÆ, roots, and branches. The stem of the Lessonia bears a strong analogy to that of dicotyledons in having rings of growth, though there is a great difference. As increase in Lessonia takes place by the constant division of a flat leaf, the basilar portion of which becomes the petiole and ultimately swells into a branch, the stems have always a more or less elliptical form, and their section exhibits an elliptical core. This form of the core is not however peculiar, but exists in other AlgÆ. It is probable that the LessoniÆ, although attaining so large a size, are really of rapid growth.^[48]

The Ecklonia is essentially a southern genus, though one species ascends to Spain and the Canaries. The frond is pinnatifid, the segments arising from the evolution of marginal teeth. The stem of the Ecklonia buccinalis, which is three or four inches thick and strongly inflated above, exhibits rings of growth with an orbicular central pith.

The group of the FucaceÆ exhibits the highest structure of all the olive-green AlgÆ, and forms a large portion of the sea weeds on our coasts, but they abound more in individuals than in the number of genera and species. A few have cylindrical stems and branches swelling out at intervals into large oblong inflated air-vessels, which gives them buoyancy in the water. The rest have a flat, ribbon-like stem, and for the most part dichotomous branches with a decided midrib, but no air-vessels, because they chiefly grow at half-tide level, and are exposed twice every twenty-four hours. The most common of our fuci, the Fucus vesiculosus, or bladder-wrack, has a midrib with air-vessels, generally in pairs on each side of it, formed by the inflation of the frond; these vessels, however, are frequently wanting, for it is the most variable in form and most widely spread of the Fuci. The fructification of this group is contained in large clavate receptacles or expansions of an orange or greenish yellow colour situated at the extremities or borders of the branches.

MM. Thuret and Decaisne discovered, by microscopic investigation, that the fuci have a truly sexual fructification, consisting of male and female cells inclosed in these receptacles. In the common Fucus vesiculosus it was found that the male and female cells are either in different individuals, or in different conceptacles on the same individual; whilst in the Fucus platycarpus, both the male and female cells were found to be contained in a globular cavity enclosed in the flattened receptacles which grow at the extremities of the branches. The cavity is lined with jointed hair-like filaments formed of cells, some of which are so long as to project through a pore on the surface of the receptacle in a spreading brush (see fig. 31, where the whole is highly magnified). Towards maturity, the cells of some of these filaments assume an ovoid form; the white viscous, granular matter in their interior acquires an orange hue, and is divided into a multitude of hyaline particles, each having an orange spot and two cilia of unequal lengths, which enable these spermatozoids to swim with great vivacity in the water as soon as they are set free by the rupture of the cell in which they are inclosed. Besides these, dark olive-green female cells, of a large pyriform shape, are fixed to the walls of the same cavity by very short stems; their contents spontaneously divide into eight spore cells, never more; each contains a colourless viscous liquid, which is mixed with protein and yellow-green matter, and is inclosed in a double coat. ‘The coats are united at the base, and when the spores are ready for dispersion, the inner coat bursts through the apex of the outer one, dragging with it a portion of the latter in the form of a little peduncle. The immediate covering of the spores at length bursts, and they are set free.’^[49] In Fucus serratus, vesiculatus, and nodosus, swarms of spermatozoids are produced, but M. Thuret has proved by experiment that they never come to anything of themselves, and the unfertilized spores perish.

Fig. 31. Vertical section of receptacle of Fucus platycarpus.

When a fertilized spore begins to grow, it assumes a pear shape, and sends out from its narrow end filaments or footstalks containing solid yellow grains at their extremities, where a hook or claw is formed by which it fixes itself to rocks or stones. The spore then divides itself into four equal cells of a brown colour, and by the continued subdivision of these into four, the plant increases in size, and assumes a form corresponding to the genus and species of the spore. Dr. Carpenter mentions that in the FucaceÆ there is also a multiplication by zoospores. These bodies are produced within certain of the cells that form the superficial layer of the frond, and swim about freely for a time in the water after their emission, until they fix themselves and begin to grow; but these are merely gemmÆ.

All the FucaceÆ are tough leathery plants. This is even characteristic of the genus Cystoseira, various species of which may be seen on our coasts at low water mark, or in the tide pools. They are little shrub-like and somewhat thorny plants, not more than three feet high, with a cylindrical stem and many branches, near the extremities of which there are inflated air-vessels, sometimes two or three together; in some species they are lower down. Long spiny conceptacles are situated at the tips of the branches, but the endochrome does not divide in the germ cells as it does in the Fuci, so that each cell produces but one spore.

‘Throughout all latitudes the two divisions of FucaceÆ—FucoideÆ and CystoseireÆ, form the prevailing marine vegetation to which the name of sea-weed is commonly applied, and the different genera so arrange themselves as to present, with a few exceptions, a most harmonious assemblage.’ ‘None of these approach the tropics; the FucoideÆ abound towards the poles, and there attain their greatest bulk, diminishing rapidly towards the equator, and ceasing some degrees from the line itself; while the immense genus Sargassum finds its maximum in lower latitudes and under the equator itself. In the opposite cold and frigid zones the waters are inhabited by certain genera of FucoideÆ, which are in a great measure representatives of one another.’^[50] The huge D’UrvillÆa and the Sarcophycus in the Antarctic Ocean represent the Himanthalia and Fucus proper in the north, and the CystoseireÆ and Halidrys of the northern seas are represented by the Blossevillea and Scytothalia in the southern.

The frond of the Himanthalia lorea is a knob about an inch high, somewhat like a small mushroom; by degrees the top of the knob sinks in, and the frond becomes cup-shaped. In the second year of growth it throws out from its centre strap-shaped receptacles from two to three feet long and the sixth of an inch wide; they are slimy, forked, and entirely covered with fruit. The true frond sometimes becomes hollow and swells into a bladder. This singular plant, which grows on our coasts, extends from Norway to Spain. In the D’UrvillÆa, its representative in the southern hemisphere, the frond and receptacle are united, for the plant, which is of large dimensions, has dichotomous fronds ten feet long, and an inch or more in breadth. Their surface is ornamented with large cavities like a honeycomb, and the fruit imbedded within them consists of antheridia and club-shaped germ cells with four spores in each. These plants form a large portion of the wrack and also of the living AlgÆ which surround the Falkland Islands and Cape Horn; and they extend to Western Chili, where the poorer class make a sweet mucilaginous soup of them. The Sarcophycus potatorum, the only species of its order, is nearly allied to the D’UrvillÆa by the structure of its fruit, and is so named from pieces of its frond being used to carry water. Many other olive-green AlgÆ are peculiar to the southern hemisphere—among them the Hormoseira, in which the frond, at first even and filiform, becomes inflated so as to produce moniliform chains of vesicles, parts of which are at length rough with the apertures of the conceptacles; this plant has bladder-like air-vessels formed by swollen parts of the frond, like many of our FucaceÆ.

Those genera which have distinct organs containing air, as the Sargassum, of which there are numerous species, are either of low latitudes or tropical, but are sometimes drifted by currents to the extra-tropical shores. The Sargassum vulgare, however, grows on the rocks in the Mediterranean. The whole plant is of a translucent reddish brown; the stem has alternate branches, bearing lanceolate serrated leaves with a midrib, and generally dotted with dark pores. The air-vessels are small translucent round balls about the size of a currant, borne on flat stalks in the axils of the branches, and the spores are in conceptacles borne on the branchlets just above the air-vessel. In one variety of this most variable plant, the Uva di mare, the main stem ends in a loose bunch of these little air-balls.

The Sargassum bacciferum is often found in the Mediterranean, but only as a wanderer drifted in from the Atlantic, where masses of it, like floating meadows, occupy an area west of the Azores equal in extent to that of France, which has never changed its position since the time of Columbus, on account of the surrounding currents. Fields of it cover the seas near the Bahama Islands, and another permanent area of Sargassum of great extent occurs in the South Pacific. The Sargassum bacciferum is of a pale translucent olive colour, having branched stems, with lanceolate, midribbed, and serrated leaves, destitute of pores, and little stalked air-balls in the axils of the branches. The same individual continually produces new branches and leaves, and thus multiplies its species, but it never produces fruit; consequently its habits exactly resemble those of the Macrocystis, and as that plant becomes detached and floats after fructification, it is supposed that the Sargassum bacciferum may grow on rocks at the bottom of the Atlantic, between the parallels of forty degrees north and south of the equator, and when detached after fructification that it is uniformly drifted to particular spots which never vary. ‘Multiplication is so rapid in the floating beds of the Sargassum and Macrocystis, as to render fruit needless; and even the common Fucus vesiculosus occurs in the Mediterranean under a peculiar form consisting entirely of specimens derived from sea borne weed carried in by the current which sets in towards the Mediterranean from the Atlantic.’^[51]

Kelp, the ashes of sea weeds, is the commercial source of iodine. AlgÆ growing in deep water contain most of that substance; consequently the kelp made at Guernsey, consisting chiefly of the ashes of Laminaria digitata, is richer in iodine than that made elsewhere.

Marine vegetation varies both horizontally and vertically with the depth, and it seems to be a general law throughout the ocean, that the light of the sun and vegetation cease together. It consequently depends upon the power of the sun, and the transparency of the water; so that different kinds of sea weeds affect different depths, where the weight of the water, and the quantity of light and heat, suit them best. One great marine zone lies between high and low water marks, and varies in species with the nature of the coasts, but exhibits similar phÆnomena throughout the northern hemisphere. In the British seas this zone does not extend deeper than thirty fathoms, but it is divided into two distinct provinces, one to the south and another to the north. The former includes the southern and eastern coasts of England, the southern and western coasts of Ireland, and both the Channels; while the northern flora is confined to the Scottish seas, and the adjacent coasts of England and Ireland. The second British zone begins at low water mark, and extends below it to a depth of from seven to fifteen fathoms. It contains the great tangle sea weeds or marine forests mixed with fuci, and is the abode of a host of animals. A coral-like sea weed is the last plant of this zone and the lowest in these seas, where it does not extend below the depth of sixty fathoms; but in the Mediterranean it is found at seventy or eighty fathoms, and is the lowest plant in that sea. The same law prevails in the Bay of Biscay, where one set of sea weeds is never found lower than twenty feet below the surface, another only in the zone between five and thirty feet, another between fifteen and thirty-five feet. In these two last zones they are most numerous; at a greater depth the kinds continue to vary, but their numbers decrease. The distribution in the Ægean sea was found by Professor E. Forbes to be perfectly similar, only that the vegetation is different and extends to a greater depth in the Mediterranean than in more northern seas. He also observed that sea weeds growing near the surface are more limited in their distribution than those that grow lower down, and that with regard to vegetation, depth corresponds with latitude, as height does on land. Thus the flora at great depths in warm seas is represented by kindred forms in higher latitudes. There is every reason to believe that the same laws of distribution prevail not only throughout the ocean, but in every sea.