THE FLORAL AXIS—INFLORESCENCE—FRUIT—SEED—NUTRITION OF PLANTS—ABSORPTION OF CONSTITUENTS.

There are certain arrangements and mutual relations of the various portions of the flowers which we have mentioned that it is useful to consider. The floral axis refers to the position of the verticils, and inflorescence signifies the arrangement of the flowers on the stem. Flowers which possess both stamens and pistil are hermaphrodite; those with only stamens are male; those with the pistil female flowers. If both organs be absent the flower is neutral.

Plants bearing flowers in clusters form several distinct groups, to which appropriate terms are applied indicative of their respective form of flora arrangement.

In the examination of this kind of inflorescence (indefinite or axillary inflorescence), the first object of remark is the general or primary peduncle, termed rachis, and which bears numerous leaflets called bracteoles or bractlets, from whose axils arise the pedicellate or sessile flowers. The lower bracts often produce no flower-buds in their axils, and form instead a whorl surrounding the heads of flowers on the primary axis, and which is called involucre (as in the sun-flower, for instance).

The different varieties of axillary inflorescence are determined principally by the elongation or depression of the axis, the presence or absence of stalks to the flowers, and the form and nature of the bracts. We distinguish—

1. (1) The spike (fig. 777). In this form of inflorescence, sessile or short-stalked flowers are arranged along the rachis in the axils of the bracts; the spike is said to be compound when small spikes or spikelets arise again from the bracts of the secondary axis. (2) The catkin or amentum (fig. 777 [2]); a spike, usually pendulous, which falls off, rachis and all, by an articulation, as in the willow or hazel. 3. The spadix, a thick fleshy spike (fig. 777 [3]); examples, arum and calamus. 4. The cone, a fruit-bearing spike, covered with scales (fig. 777 [4]); examples, the coniferÆ. 5. The raceme or cluster, a spike with the flowers on longer pedicels (fig. 778); examples, the currant. 6. The panicle, a branching raceme (fig. 779, Yucca gloriosa). 7. The thyrsus, a dense panicle, with longer peduncles in the middle than at the extremities; example, lilac. 8. The corymb, a raceme, in which the lower flower stalks are elongated and raised to nearly a level with the upper (fig. 780)—example, cerasus mahaleb. 9. The compound or branching corymb, a corymb in which the secondary axis again sub-divides; example, Pyrus terminalis. 10. The umbel: in this form the primary axis is greatly depressed, and the peduncles arise from a common point, and spread out like radii of nearly equal length, a whorl of bracts (involucre) surrounding the common base. In the compound umbel (fig. 781), Daucus carota, the secondary axis ends in small umbels surrounded by bracts, which is termed an involucel. This is observable in the umbelliferous plants—carrot, parsley, hemlock, etc.

A very peculiar kind of inflorescence, which characterises the great family of the compositÆ, is illustrated by fig. 783. We see here the enlarged floral axis or receptacle, a, surrounded by several whorls or bracts, b b, which constitute a general involucre; the membranous bracts, (paleÆ), b´ b´, seen in the receptacle, bear in their axils the sessile florets, c and d, which either have a calyx, e e, or not. The florets on the receptacle are either all of them tubular (d) or ligulate (tongue or strap-shaped); florets (c) are associated with the tubular ones. The receptacle is not always flat, but frequently presents a convex, globular, conical, concave, etc., shape.

In the absence of paleÆ the receptacle is said to be naked. The florets at the margin, or circumference, are termed marginal flowers, or flowers of the ray; the florets in the disc (centre), central flowers, or flowers of the disc.

Some plants bear male and female flowers on the same stem. These are termed monÆcious plants. The oak is an instance. The diÆcious plants are those which bear stamens and pistil, or separately, on different plants, like willows. We will now glance at the functions of the stamens and pistil. The ovule has been mentioned as a tiny body in the ovary, and it consists of a nucleus, and cellular tissue surround it, leaving a small hole called the micropyle, into which the pollen tube enters after passing through the ovary. As in the animal creation, the unions of different families succeed best; no close relationship will fertilize so well as with other flowers.

Fertilization is accomplished in two ways; (1) by the action of the wind, by which the pollen is carried away to other plants; and (2) by means of insects—the bee particularly. These flowers have distinctive qualities relatively. In the case of the pine the pollen is powdery; so those plants which are thus fertilized are the diÆcious species, which include the poplar, the oak, and the birch, as well as the pines. These are all wind-carried pollen plants. The nettle is illustrated here with male and female flowers.

Plants fertilized by insects are visited by them, and they carry away upon their heads, or bodies, the pollen, which is then thrust into the stigma by the insect; or perhaps birds may carry the pollen in the same way after sipping the nectar, and thus playing an unconscious, but most important, part in the economy of nature.

We always find the ovule at the termination of an axis; it is unable to form a seed alone. The pollen grains must fertilize it, and in consequence many ovules come to nought. The ovule is produced in the pistil, which, as before stated consists essentially of two parts—the ovary and the stigma; the latter secretes a fluid to hold the pollen. We annex the representation of a highly-magnified pistil (vertical section, fig. 786a). The pollen grains are indicated by d, attached to stigma, c, projecting through the style, b, into the ovary, a, and passing through the ovules.

With the transmission of the pollen to the ovary of the pistil, the functions of the anther and stigma terminate; accordingly these parts of the flower rapidly wither and decay after fertilization. The filaments, the style, and the petals speedily participate in the decay, but the sepals remain sometimes persistent in an altered form. The ovary and its contents alone proceed in their further development, and undergo material changes, in which, however, the bracts and the calyx often participate.

The carpellary leaves occupy the summit of the floral axis. The axis terminates either in one single carpel, in which case the ovary is one-celled, or unilocular; or the axis is surrounded by several carpels, in which case the manner of their union determines the number of cells in the ovary.

The Fruit.

The carpels are the chief agents in the formation of the fruit, and they form the endocarp (core), and sometimes the whole pericarp, or seed-vessel. Upon the nature of the various parts and the changes they undergo during the ripening of the seeds the nature of the fruit depends. The fruits are classified, some being the produce of a single carpel, others of several united carpels.

Fruit, in botany, is by no means limited to the juicy products of trees or plants which are so refreshing in the summer weather, and so acceptable in any form. In plant life the herb yielding seed produces a fruit equally with the orange or the apple. The fruit is the outcome of the varied processes of the plant. We may trace the plant from its tiny, sometimes very minute seed, through stem to flower and seed again. “In the final struggle, even when life is hopeless, and starvation, in consequence of drought, is imminent—when all is hopeless and barren, the plant will make an effort to produce its fruit and flower.” This is a very touching and interesting fact in nature—this last attempt to beautify the earth and to propagate its species for the use of man.

Fruit, then, is not limited to the market and the stall.

This statement scarcely needs proof; but if we consider for a moment the number of “wild” fruits—the parents, probably, of our table-fruits—we find many we cannot eat. In short, out of the hundred thousand plants which bear flowers scarce one two-hundredth part serve us as producers of edible fruits.

The fruit is the result of the flower, and if any objection be made by readers on the part of the common fig, it will be found that this appreciated fruit really consists of male and female flowers that are fertilized by the action of minute insects, which enter and depart (sometimes they die, and are found dead and black in the figs). No blossom is perceived on the tree, because within the green sac the so-called “seeds” (really the fruits) are developing. A fig is a sac full of fruits.

The legume or pod is formed of a single carpel bearing seeds. We annex illustrations of the pod. The covering is called the pericarp, and the parts when opened separate into valves. Dehiscent fruits shed their seeds, indehiscent fruits do not; they lie within the seed-vessel, like the acorns and nuts. These are dry fruits, but there are others of a soft nature, such as apples or gooseberries.

Fruits are variously named, and underneath will be found a list. We have the aggregate, like the mulberry, etc.; the dehiscent fruit of one carpel like the pea, etc.; the simple fruits as cherry, nettle, wheat, etc. The dandelion fruit is often a precious object in children’s estimation, as it is blown away to ascertain the time. There are indehiscent fruits with many carpels,—the common buttercup, for instance, and the strawberry. A list is added.

a. Fruits which are the Produce of a Solitary Carpel.

1. The gymnospermous fruit, where the seed lies naked in the axils of the ligneous bracts, as in the cone of the fir and spruce tribe.

2. The legume or pod, which is formed of a solitary carpel bearing seeds on the ventral suture. It characterises the pea and bean tribe (leguminosÆ).

3. The follicle is a mature carpel containing several seeds, and opening by the ventral suture. There are usually several follicles aggregated together; examples, larkspur, monkshood, evergreen.

b. Fruits which are the Produce of Several Carpels United.

4. The capsule consists of two or more carpels, either simply laterally united (one-celled or unilocular capsule), or folded inwards towards the axis, but without reaching it (spuriously multilocular capsule), or uniting with the axis (bilocular, trilocular, multilocular capsule). Examples of capsular fruit—mignonette, balsam, violet, poppy, etc.

5. The siliqua or long pod is formed of two carpels, and longitudinally divided into two parts by a spurious dissepiment called the replum; examples—cabbages, stock, wallflower, etc. The silicula is a broad and short pod; examples—Iberis, shepherd’s-purse, etc.

6. The cariopse (caryopsis, having the appearance of a nut), is a monospermous or one-seeded fruit, with an indehiscent membranous pericarp, closely investing the seed or incorporated with it; examples—rye, wheat, and other grains.

7. The achÆnium is a dry, monospermous, indehiscent fruit with one seed; examples—cashew, ranunculus, strawberry, etc.

8. The nut or glans is a one-celled, indehiscent fruit, with a hardened coriaceous or ligneous pericarp; examples—hazel-nut, acorn, etc. The nucula, or little nut, is a cariopse, with a solid coriaceous pericarp; examples—buckwheat hemp, etc.

9. The berry (bacca) is a pulpy, succulent fruit, with soft rind; examples—the gooseberry and the currant. The pepo or peponida (pumpkin), illustrated by the fruit of the gourd and melon, and the hesperidium, illustrated by the fruit of the orange and lemon, are modifications of the berry.

10. The drupe (drupÆ, unripe olives); the mesocarp is generally pulpy and succulent, the endocarp hard; examples—the cherry, the peach, the plum, etc.

11. The pome (pomum, or apple); the outer parts of the pericarp form a thick cellular, eatable mass; the endocarp (core) is scaly or horny, and encloses the seeds within separate cells; examples—the apple, pear, etc.

Fruits consisting of the floral envelopes and the ovaries of several flowers united into one, are termed multiple or anthocarpous; the sorosis (cluster-fruit: example—the pine-apple, the breadfruit, the mulberry), the sycosis (fig-fruit), and the strobilus (fir-cone), form varieties of the anthocarpous or multiple fruit.

Non-Flowering Plants.

The cryptogamia or acrogens is the botanical term for these plants, of which we must be very brief in our description,—not that the subject is not worthy of a much larger space than we can devote to it, but our pages are not elastic.

There are numbers of plants without pistils or stamens properly so called. They are hidden from human observation—buried out of sight; and in the fern, moss, and other primitive plants they are thus hidden. There are several families of the cryptogamia, but two main sections include them all—viz., the cormogens and thallogens. These are sometimes known as cormophytes and thallophytes, but the former will be our terms, and they include the ferns, algÆ, lichens, and mosses, with many other families, which we do not propose to examine in this summary sketch. The microscope will here be a great aid if not always absolutely necessary for any close investigation.

We are all familiar with the appearance of ferns, and we may commence with a few observations concerning them. They are an extensive family and very beautiful, some of the tropical species being particularly noticeable for elegance. We are here mostly concerned with the development of the plant. The polypod ferns fructify under the leaves or “fronds,” which open from a ball. The seed-cases or sorri are situated at the back of the fronds in brown spots, and when examined they will be found to be collections of capsules like tiny cases. There is a kind of band at the upper part which at the proper time is extended, and tearing open the capsule releases the seeds. These seeds or “spores” are very minute, and not properly seeds but buds, every one of which can generate seeds. So if we try to grasp in imagination the generating powers of a few fern fronds, we shall miserably fail in the attempt.

Some ferns have the “spores” upon the summit of the frond. The osmundas belong to this family, and are known to all as the “flowering fern,” a contradiction palpable enough under the circumstances. The beautiful dust upon some ferns has been mistaken for “spores” by many people, but it is merely a natural ornament of the plant. The venation and vernation of ferns are very curious, but in the determination of ferns the only sure way is to consider the sorri and the venation. The differences that puzzle may be little or great, but when the sorri have been examined all doubts will be set aside.

There are about three thousand varieties of ferns known, and we give a few illustrations of them, although any detailed description is out of the question, for we have to mention the beautiful mosses of which there are in Britain more than five hundred different species, all extremely beautiful, perfectly innocuous, and even beneficial.

The Mosses and AlgÆ.

These plants are extremely lowly in the score of creation, and also in stature. Very few mosses attain any elevation, only the “sporangia” shoot up, and the plants are very delicately formed, the leaves being all of the same pattern. They are common in damp situations, and thrive in woods, streams, and banks. The Fontinalis is a river moss, while the Hypnum is found in hedges. The LycopodiaceÆ or the club-moss family is intermediate between ferns and mosses. They are found in warm, moist climates, and contain a sort of brimstone. They grow well with ferns under glass.

The Musci or moss-family proper are useful in various ways. We have also the liverworts, which bear some resemblance to lichens. They grow between stones near water, or in damp situations. There are two distinct families, both beautiful when examined, and are named MarchantiaceÆ, and JungeramanniaceÆ, or scale moss.

The Thallogens or ThallogenÆ include algÆ, lichens, and fungi, which are the lowest of the plants, and all very much alike. The algÆ are termed “protophytes,” and consist of living cells propagating by subdivision, or union. The thallogens have therefore no distinct axes, leaves, or stomata.

The algÆ are thus simply cellular plants found in salt or fresh water, hot and cold. They sometimes appear as “slime.” Some contain silicia, and are termed DiatomaceÆ, and these propagate by subdivision, and when they die their shelly covering remains, and we find the shells or cases in all earthy formations. These diatomaceÆ have been raised from the beds of oceans, and Atlantic soundings have revealed their presence,—as mud, when examined, proves to be these remains of vegetable shells. Thus the infinitely little in the animal and vegetable worlds meet at the bottom of the sea, as well as on dry land.

There are marine and fresh-water algÆ—the former familiar to us as seaweeds which possess air-bladders that children love to explode, and which assist the algÆ to float. They attach themselves to rocks, generally at the base; the lovely colours of seaweeds are well known. They will be recognized under the name of “tangle” (fucus), which, when burned, gives kelp and barilla, which is full of iodide and sodium. The Sargasso Sea is composed of miles of algÆ which live in the open ocean. The Carrageen or Irish moss is very nutritious and useful in consumptive cases. Indeed, all algÆ, if not absolutely useful, are certainly not deleterious. The “bladder-wrack” was formerly useful for the production of soda.

“The life-history of one of these uni-cellular plants in its most simple form, can scarcely be better exemplified than in the Palmogeoea macrococca, one of those humble forms of vegetation which spreads itself as a green slime over damp stones, walls, etc. When this slime is examined with a microscope, it is found to consist of a multitude of green cells, each surrounded by a gelatinous envelope; the cell which does not seem to have any distinct membranous wall is filled with granular particles of a green colour, and a ‘nucleus’ may sometimes be distinguished through the midst of these. When treated with tincture of iodine, however, the green contents of the cell are turned to a brownish hue, and a dark-brown nucleus is distinctly shown. Other cells are seen, which are considerably elongated, some of them beginning to present a sort of hour-glass contraction across the middle; in these is commencing that curious multiplication by duplicative subdivision which is the mode in which increase nearly always takes place throughout the vegetable kingdom.”37

Lichens are numerous, and may be found upon the bark of trees in dry forms of grey and yellow growth, and on walls and old stones in our graveyards. On the hills we find them growing upon the granite, and it would appear that they prefer stone to any other holding ground. The Arctic lichens form the principal food of the useful reindeer, and “Iceland moss” is represented as wholesome for man. Lichen is derived from the Greek term for “wart.”

The Fungi are very important, and with them we will close our summary. They include the favourite mushrooms and poisonous toad-stools, with many other “fungous growths,” from the “mould” on the jam pot to the mushroom.

Some of these fungi are peculiar to the substances upon which they exist, and are in numerous instances destructive. The microscopic fungus Puccinea graminis is the parasite which fixes itself to corn, and produces the disease known as mildew, and the Uredo segetum (another microscopic fungus) causes the “smut”; the “bunt” is caused by the Uredo foetida, and the “spur” or “ergot,” which attacks rye, is caused by the Acinula clavis. These fungi completely destroy the grain of corn in which they form, and propagate in the most rapid manner; the ergot is moreover a dangerous poison to those who eat the bread made of rye infected by it. The truffle is a kind of underground fungus, and is esteemed a dainty. Mushrooms are also fungi, and several species are sufficiently wholesome; these are the field mushroom and the fairy-ring mushroom.

Any organic substance will shortly become covered with this “mould” or mildew. The air is so full of the germs of animal and vegetable life that, as it penetrates everywhere, the smallest supply must contain some germs; and these, under a powerful microscope, present most beautiful forms and colours. We annex (fig. 822) some of these forms highly magnified. They are deposited by the air, and the substance into which they happen to fall determines the kind of life which is to inhabit it. A few of these spores only come to maturity.

We again take the liberty to quote Dr. Carpenter on this subject. He says:—

“There are scarcely any microscopic objects more beautiful than some of those forms of mould or mildew which are so commonly found growing upon the surface of jams and preserves, especially when they are viewed with a low magnifying power and by reflected light; for they present themselves as a forest of stems and branches of extremely varied and elegant forms, loaded with fruit of singular delicacy of conformation, all glistening brightly on a dark ground.

“The universality of the appearance of these simple forms of fungi upon all spots favourable to their development, has given rise to the belief that they are spontaneously produced by decaying substances, but there is no occasion for this mode of accounting for it, since the extraordinary means adopted by nature for the production and diffusion of the germs of these plants adequately suffices to explain the facts of the case.

“The number of sporules which any one fungus may develop is almost incalculable; a single individual of the “puff-ball” tribe has been computed to send forth no fewer than ten millions. And their minuteness is such that they are scattered through the air in the finest possible dust, so that it is difficult to conceive of a place from which they should be excluded.”

Pure water exposed to the air does not afford nourishment to the germs which fall into it, till a sufficient number of them shall have been deposited to form a food for those which come after them; but if we mix with the water any soluble vegetable or animal matter, in a short time the microscope will detect the growth of the germs that are being deposited, for where nourishment is, there only can they be developed. These germs are capable of existing for an indefinite period, either floating in the water, or blown about by the air, and have been detected hundreds of miles from land; the rigging and sails of ships far away from shore are often covered with what sailors suppose to be sand blown from the land, but which are organic substances, either vegetable or animal. According to Humboldt, the Red Sea has derived its name from the fact that at certain seasons the surface of the water has a reddish appearance, and this (as he says) he was fortunate enough to observe, which colour he found to be due to myriads of red fungi, which had formed on the surface. The seeds of some plants are furnished with minute wings or plumes, which cause them to be borne on the air or floated on the water (fig. 824), to fertilise some barren spot, perhaps a coral reef, which has at length reached the surface of the water, and which ascends no higher, for the little creatures which built it are aquatic, and cannot live exposed to the air; this coral reef now becomes a receptacle for seaweed and fungi, which float on the surface of the ocean are washed on to the reef, die, decay, and leave behind a thin layer of mould, which process being repeated again and again, forms an elevated edge to the reef, enclosing a lake, or “lagoon” as it is called, the waters of which evaporate, and the space is filled up in the same way as the edge was formed, together with the excrements of birds, etc., forming layer after layer of mould, and the surface becomes fit for the growth of larger seeds, as the cocoa-nut, banana, etc., which are drifted on to it by the waves; in this way a coral reef becomes an island fit to be inhabited by man and other animals.

It is impossible for any person not accustomed to observe the manner of the propagation of the fungi, to understand a written description, for the fructification of these plants are very varied in the manner of the development of the spores. They are not generally hurtful, but much caution should be observed in the matter of the mushroom, which may be distinguished by the pale pink and black of the under part. There are many poisonous fungi, but the greater number are harmless, though they are not intended for food. They simply clear away the decaying growths, and act as safety-valves to Nature by carrying away what is not required, to give it to the air again to be renewed into life.

The vegetable kingdom forms the link between the minerals and the animals. The vegetable derives food and nourishment from water, carbonic acid, and ammonia, which are, as we already know, made up of certain elements, and thus supply us all with food. They give out oxygen for the use of animals, and are thus, in another sense, the source of life. The growth of a plant is very interesting, and we may conclude by following it.

The seed is sown, and the cells of the “cellular tissue” become developed, passing some upwards, some downwards, to form a radicle or plumule, as explained. The latter carries up the cotyledon, which begins to decompose carbonic acid from the atmosphere, and fixing the carbon as woody fibre. The leaves are then formed and more fibres, and so on for every leaf; thus the number of woody fibres which form the trunk of a tree is in proportion to the number of leaves which that tree has borne, from which we come to the conclusion that the size of the trunk of a tree is the sum of all its branches. While all this is going on, the cellular tissue of the downward part or radicle also becomes developed and divides out into roots, on the surface and at the extremities of which are minute cellular bodies called “spongioles” (from their power of absorbing moisture), which take up the fluid of the earth which surrounds them; this moisture ascends through the vessels of the plant till it arrives at the surface of the leaves, where it is exposed to the action of light and sunshine. The ascent of the moisture of the earth was first correctly explained by Du Trochet, and is owing to a peculiar power which he discovered, and which is called “Endosmose”; this consists in the tendency which a fluid has to penetrate a membrane on the other side of which is a fluid of greater density than itself. This may be seen by the following experiment: obtain a piece of glass tubing about a foot long, having the end blown out into the form of a bell, as in fig. 825, tie a piece of bladder over the expanded end and fill it partly with syrup or gum-water, so that this shall rise in the stalk about an inch; place this in a glass of water with the bladder downwards, and the fluid will be seen slowly to rise in the stalk, so that in perhaps an hour it will rise to the top. This apparatus resembles one of the spongioles at the extremity of the fibre of a root.

The rain falling through the air carries with it a certain amount of carbonic acid and ammonia, which the air always contains, and it is the whole source of the nitrogen which forms a very important part of the bodies of plants and animals. When the rain arrives at the surface of the earth, it sinks down into it and carries with it all soluble vegetable or animal matter which it meets with, together with any soluble earthy matter which may exist in the soil; this forms the sap of the tree. When it arrives at the surface of the leaf, the watery part of it combines with the carbonic acid of the air (through the influence of light), and appropriating its carbon, gives out the oxygen; this is the true respiration of plants, and is exactly the reverse of what takes place during the respiration of animals, in which case oxygen is absorbed and carbonic acid given off. The carbon thus retained by the plant combines with the elements of the water to form the solid green substance called chlorophyl, which is the basis of all the tissues of the plant; the ammonia is also decomposed, and its nitrogen combining with the oxygen and hydrogen of the water, and the carbon of the carbonic acid forms those compounds which constitute the most nourishing parts of vegetables, such as albumen, gluten, etc., and of which all the animal tissues are built up, for the production of these organic substances takes place in the vegetable only, animals simply appropriating them for their food. The sap which reaches the leaf is not all converted into chlorophyl, but also into those peculiar juices which are found in plants, some of which contain sugar, some gum, others (as the pine tribe) turpentine, and in the laurel tribe camphor, all of which are substances containing much carbon; moreover the solid wood and bark are deposited from these juices as they descend from the leaf after having been acted on by light (or the actinic power associated with it). Now, as the water, ammonia, and carbonic acid which descend with the rain are from the air, and as the vegetable is formed wholly by their absorption, it may be fairly said that the vegetable kingdom (and therefore the animal) feeds upon the air, and that the trees do not grow out of the earth, but into it.