Part 17

Dr. Carpenter in his work on the microscope 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 develope 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. 4), 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 sea-weed 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, &c., forming layer after layer of mould, and the surface becomes fit for the growth of larger seeds, as the cocoa-nut, banana, &c., 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.

The vegetable kingdom forms a mysterious link between the mineral and animal kingdoms, and binds all creation into one grand and unspeakably beautiful whole. There are certain substances from which the vegetable derives its nourishment, namely water, carbonic acid, and ammonia, which, though strictly inorganic, are yet not simple or elementary substances, but consist of pairs of elements combined. Thus water consists of oxygen and hydrogen, carbonic acid of oxygen and carbon, and ammonia of hydrogen and nitrogen. All these aliments of the vegetable kingdom exist in the air, and in sufficient quantities to supply all the requirements of vegetation; so that the air, together with a few metallic oxides (salts) derived from the earth, furnishes food for the whole vegetable tribe, from the highest to the lowest, and these vegetables in their turn supply all the food which the animal tribes consume, for although one animal feeds upon others, yet these must previously have derived their nourishment from vegetable matter. Vegetables not only supply food properly so called, but likewise that which is essential to respiration, for besides separating all noxious excess of carbonic acid from the air, they are an inexhaustible source of oxygen, the element essential to respiration, and consequently animal life. This supply of oxygen by vegetables compensates for that which is consumed in respiration by animals, and thereby maintains the atmosphere in a state proper to be breathed. That plants do thus absorb carbonic acid and give out oxygen, can be proved in the most certain manner, if the green parts of a plant be placed in water holding carbonic acid in solution, and exposed to sunshine; the carbonic acid will shortly be found to have disappeared from the water, while oxygen gas is evolved and can be collected, its quantity being exactly equal to the carbonic acid which the water contained.

Besides furnishing food and oxygen for the nourishment of animals, vegetables afford a shelter from the burning rays of the sun in hot climates, not only to man, but to those hundreds of wild animals whose proper home is in the forest. Humboldt says he found in South America forests composed of such close growth, that it was quite impossible even for the wild animals to penetrate into them, except at a few places, and that the jaguar often lives for weeks in the trees without descending to the ground, preying upon the monkey tribes and other animals, which are found in almost incredible numbers there; and Dr. Livingstone thus describes the forests of Africa:--"The forests became more dense as we went north. We travelled much more in the deep gloom of the forest than in open sunlight. No passage existed on either side of the narrow path made by the axe. Large climbing plants entwined themselves round the trunks and branches of gigantic trees, like boa-constrictors, and they often do constrict the trees by which they rise, and, killing them, stand erect themselves."

These woods, as well as the grass and herbage of the plains, afford an enormously extended evaporating surface, and act, in the heart of hot countries, as inland seas, giving out clouds of vapour, which ascend and are condensed in the form of rain; in this way the surface of the earth is cooled, the fluid (far away from its proper source, the ocean) is economised and fertilises a greater space of surface. Vegetation also acts in preserving the surface of the earth from those ravages which both wind and rain would otherwise effect; the great deserts have been levelled by these causes, the earth not being of a quality to support vegetation. Cattlin says, that in North America he frequently met with great conical-shaped mounds of earth devoid of vegetation, and which were fast being levelled by the rains, at every fall of which gullies were formed on their sides, down which the mud poured in streams; now, had these been of a quality to support a covering of grass, their size and form would have remained unchanged for centuries. Railway embankments are often sown with grass seeds, to prevent injury to them, especially where this covering is not likely to come soon, as in the vicinity of large towns.

Thus it is seen that the vegetable kingdom far exceeds the animal in quantity, that it is everywhere distributed, that it affords nourishment and shelter to animals, preserves the air fit for respiration, keeps down the excess of heat, and preserves the form of the surface of the earth.

MODE OF GROWTH--AGENTS AFFECTING--CONSTITUTION AND POSITION IN CREATION.

The green colour of plants is due to the formation and deposition in their cells of a peculiar compound called "chlorophyll," and it is ascertained that this compound is produced in the plant in consequence of its exposure to light, for no plant grown in the dark is green, but will become so when light is admitted. This is shown in many cultivated vegetables, as celery, lettuces, &c., in which by banking up the celery, or tying together the lettuce, the light is excluded, and the stalks of the one and the heart-leaves of the other become mild and white, whereas the same grown in a state of nature are so green and rank as to be almost poisonous.

The effects of light and heat in favouring vegetation being nearly always found in union, causes that which is really due to light often to be attributed to heat, as in the growth of tropical plants; where both are combined, there will be the greatest powers of vegetative growth. The absence of light and air often causes the upward growth of a plant to seek them; a curious instance of this, altering the habits of a tree, is mentioned by Dr. Livingstone, who says:--

"As we traverse a succession of lawns and open forests, it is interesting to observe something like instinct developed even in trees. One, which when cut emits a milky juice, and if met with on the open lawns grows as an ordinary umbrageous tree, and shows no disposition to be a climber, when growing in a forest still takes the same form, then sends out a climbing branch which twines round another tree until it rises 30 or 40 feet, or to the level of the other trees, and there it spreads out a second crown, where it can enjoy a fair share of the sun's rays. In parts of the forest still more dense than this, it assumes the form of a climber only, and at once avails itself of the assistance of a tall neighbour, by winding vigorously round it, without attempting to form a lower head. It does not succeed so well as parasites proper, but when forced to struggle for space, it may be mistaken for one which is invariably a climber."

The absence of heat with plenty of light is shown in the vegetation of mountainous districts, where there is great size, but the absence of leafy expansion, as in the pine tribe (fig. 5). Where both light and heat are deficient, nothing but fungi, mosses, and lichens of the most stunted nature will grow. Experiments in hothouses prove that although tropical plants be supplied with their proper amount of heat, yet without the very greatest supply of light that can be obtained, they do not come to any degree of perfection. The influence of light is chiefly upon the leaves, that of heat upon the earth and root, which becomes more fully developed. When a seed is placed in the earth, moisture is absorbed by it from the earth, and a development of the cells of the cellular tissue of which the seed is composed takes place, and from the peculiar nature of these cells they divide into two sets, one of which passes upwards towards the light, the other passes downwards into the earth; these last (which pass downwards) form a fibre called the "radicle" (fig. 6), those which pass upwards form also a stalk or fibre called the "plumule," which carries up with it a part of the seed called the "cotyledon," which (like a leaf) has the power of decomposing the carbonic acid of the air and fixing its carbon in the form of wood. This begins as soon as the cotyledon has reached the light, and thus the formation of fibres of woody matter takes place, which fibres descend from the cotyledon to the radicle. Meanwhile the formation of other cells of the plumule takes place, until the first leaf is formed, when other fibres of wood are sent down, and so on for every leaf, so that 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. 7, 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 this 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 chlorophyll, 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, &c., 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 from their food. The sap which reaches the leaf is not all converted into chlorophyll, 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.

With respect to the position which the vegetable kingdom occupies in creation, there can be no doubt that it is subordinate to the animal kingdom, and takes a place between it and the mineral world, inasmuch as it prepares food from the one kingdom and transfers it to the higher. It has been supposed that the vegetable and animal kingdoms aid and support each other equally and mutually; this is true with respect to respiration, but not as regards nutrition, for although in the decomposition of animal matters, food is given off for the vegetable, yet they are quite independent of this source of nourishment. A forest of trees would be quite as well nourished if there were no animals, but on the other hand the animal kingdom would shortly cease to exist if there were no vegetables.

It was formerly supposed that the lowest grades of the animal kingdom were higher than the highest of the vegetable kingdom; this is not strictly true, the best way of viewing the connection between the two kingdoms is to approximate the lowest of each, when it will be found that our most acute physiologists are only just beginning to determine their distinctive characters.

Dr. Carpenter says:--"In the present state of science it would be very difficult, and is perhaps impossible, to lay down any definite line of demarcation between the two kingdoms, since there is no single character by which the animal or vegetable nature of any organism can be tested.

"Probably the one which is most generally applicable, among those lowest organisms which most closely approximate to one another, is not, as formerly supposed, the presence or absence of spontaneous motion, but the dependence of the being for nourishment upon organic compounds already formed, which it takes (in some way or other) into the interior of its body, or its possession of the power of obtaining its own alimentary matter by absorption from the inorganic elements on its exterior. The former is the characteristic of the animal kingdom as a whole, the latter is the attribute of the vegetable."

Both vegetables and animals begin with a simple nucleated cell, having certain vital properties, a double chain ascends from this simple type, one branch of which is developed more and more till it arrives at a perfect vegetable, and the other branch is developed till it arrives at a perfect animal. Thus, by the addition of distinctive and characteristic appendages, one being acquires properties regarded as vegetable, while by the addition of other appendages equally characteristic, the other being obtains those properties which cause it to rank as an animal; but it must not be inferred from this that all organic forms have been a simple cell at some former period, but that there are two classes of organic beings, the vegetable and the animal, and that each embraces forms, ranging from a simple cell to the highest, and that each of these forms (from the lowest to the highest) is and always was, from their first creation, the same as they now are, in individual shape and size. Some of the lower members of the animal kingdom resemble the higher members of the vegetable kingdom, both in outward appearance and in intimate structure; Dr. Darwin, who wrote scarce half-a-century ago, says that a tree should no more be considered as one plant than a branch of coral as one animal, for as it is found that in the coral hundreds of separate beings exist associated in one habitation, so (he says) should every bud on a tree be considered as a separate being. Even Dr. Carpenter, one of our most eminent physiologists, seems in a great measure to favour this idea. He says: "The radiata possess many points of affinity with the vegetable kingdom, and of these the circular arrangement of their parts is one of the most evident. Many species of sea-anemones, for instance, present an appearance so much resembling that of various composite flowers, as to have been commonly termed animal-flowers, a designation to which they seem further entitled from the small amount of sensibility they manifest, and the evident influence of light upon their opening and closing.

"But it is in the tendency to the production of compound fabrics, each containing a number of individuals, which have the power of existing independently, but which are to a certain degree connected with one another, that we recognise the greatest affinity in structure between this group and the vegetable kingdom. Every tree is made up of a large number of buds composed of leaves arranged round a common axis; each bud has the power of preserving its own life and reproducing the original structure when removed from the parent stem, if placed in circumstances favourable to its growth, and yet all are connected in the growing tree by a system of vessels which form a communication between them. This is precisely the nature of those structures which are formed by the animals of the class that may be regarded as the most characteristic of the group. Every mass of coral is the skeleton of a compound animal, consisting of a number of polypes, connected together by a soft flesh in which vessels are channelled out; the polypes are capable of existing separately, since each one, when removed from the rest, can in time produce a massive compound fabric like that of its parent, but they all contribute to the maintenance of the composite structure so long as they are in connection with it. In some instances the skeleton is stony, and is formed by the deposition of calcareous matter either in the centre of each fleshy column, so as to form a solid stem, or on its exterior so as to form a tube. In other cases it is horny, and then it may be a flexible axis in a delicate tube. Both the stony and horny corals often possess the form of plants or trees, and as their skeletons are often found with no obvious traces of the animals to which they belong, they have been accounted vegetable growths."

DESCRIPTION OF A VEGETABLE AND ITS PARTS.

As has been said before, it is extremely difficult to make any distinction between the lower tribes of the vegetable and animal kingdoms, and physiologists are not yet agreed, with respect to some members, as to which kingdom they belong. Their whole substance is made up of cellular tissue, and there are but few distinctions of parts, forming generally a broad foliaceous expansion called the "thallus," as in lichens and sea-weeds, or a sort of root composed of fibres and called the "mycelium," as in the fungi. But, a very few steps higher, the distinctive characters become so evident that they are impossible to be mistaken, the following description will therefore apply to vegetables of the more elevated character, such as trees or flowering plants.

Plants are fixed to the earth, and receive nourishment from it by imbibing its liquids, which circulating upwards through the porous structure of the plant itself, and becoming exposed to the air in the leaves, attract to themselves nourishment from that source also. The part of a plant which grows into the earth is called the root; this has a variety of forms, in some it is branched like the upper part, and these branches divide into rootlets or fibres penetrating deep into the ground, and absorbing nourishment in all directions, but this absorption does not take place from the whole surface of the root but from spots covering it, and from the slightly expanded ends of the fibres, these portions are formed of new and porous cellular tissue, and are called "spongioles." The part of plants which springs upwards from the earth is called the stem, if large the trunk, this divides into branches and twigs; stems in some cases continue for a distance more or less underground. The part of a potato plant, usually called the root, and from which the tubers or potatoes grow, is in reality an underground stem, and the fibres which spring from it are the roots; the underground suckers of mint are also portions of stem, and in some cases these are greatly expanded, they then obtain the names of "tubers" (as the potato), or "corm," as in the crocus and meadow saffron; when the stem is thin and runs along the ground, sending in roots at intervals (as in the strawberry), it is called a "runner," when thicker and running horizontally under the ground, it is called a "rhizome."

The stem consists of a central portion, either made up of long bundles of woody fibre running side by side, as in the endogenæ, or deposited in rings and having a central cylinder of pith, as in the exogenæ. In these the wood at the central part (or the oldest) is called "duramen," or "heart-wood," while that at the outer part or nearest to the bark (the newest), is called "alburnum" or "sap-wood." The former is the harder and the latter the softer portion. From the pith in the centre, through the woody part, rays or laminæ of cellular tissue similar to that which constitutes the pith itself, are sent outwards through the woody rings to the inner part of the bark, which they form; these are called medullary rays, and may be seen in wood which has been cut across the grain, they are often called the "silver grain," and are very evident in oak, beech, and elm, the inner part of the bark is called the "liber." The bark itself is made of cellular tissue dried and hardened by age; the outer portion (called the "epidermis") is in many cases shed, and may be constantly seen hanging loosely from the bark of the birch and other trees in loose white silvery portions. The outer part of the bark of some trees is so largely developed as to be of considerable thickness, this is especially the case in the Cork oak (Quercus suber), which is the cork of commerce. The bark cannot stretch as the circumference of the tree increases, it is therefore split up and cracked, which accounts for the rough state of it on those trees which do not shed the outer part.

The chief appendage of the branches, is called a leaf, it grows from a small projection called a leaf-bud, which contains the leaf rolled up. The method in which this occurs is different in different plants, in some it is folded backwards and forwards, in others doubled up with the opposite leaf alternately, and in various forms in other plants. The leaf consists of two parts, the stalk (petiole) and the blade (lamina); the blade is of different forms, and has ribs and veins covering it, in some of a reticulated or network pattern, these belong to the exogenæ, and in others running parallel, which is the kind of venation found in the leaves of the endogenæ. When leaves are placed on a stalk, they are said to be "petiolate," when without one, "sessile."

When leaves are not separated into different portions (although they may be much notched) they are said to be "simple," as in the oak or willow, but when divided into separate portions, as in the ash, they are said to be "compound."

The following are the chief forms of leaves, named according to variations of their several parts:

Lanceolate (Privet).