« AnkstesnisTęsti »
vided with such a store-house, in which to lay it up, they would be frequently in danger of perishing.
Vegetables have no stomach; they do not require such a magazine, since they find a regular supply of nourishment at the extremity of their roots. The food of plants is not of a complicated nature, like that of animals: but consists of the simplest materials-water, and the solid and gaseous matter contained within it.
The second distinction between the animal and vegetable creation is, that the latter are not endowed with sensibility.
Some ingenious experiments have, however, been recently made, which tend to favour the opinion that plants may be endowed with a species of sensibility; and seem to render it not improbable, that there may exist in plants, something corresponding with the nervous system in animals. There are certain vegetable poisons, which are known to destroy life in animals, not by affecting the stomach, but merely by acting on the nervous system. These poisons were administered to different plants, either by watering them with, or steeping the roots in, infusions of these poisonous plants. The universal effect was, to produce a sort of spasmodic action in the leaves, which either shrunk, or curled themselves up; and, after exhibiting various symptoms of irritability, during a short time became flaccid, and the plant, in the course of a few hours, died. When we see plants thus acted upon by vegetable-poisons, which are known to be incapable of destroying the animal fibre, or of injuring the frame, but through the medium of the nerves, we may be led to suppose, that certain organs may exist in plants, with which we are totally unacquainted, and which bear some analogy to the nervous system in animals.
It is certain that some plants possess a power of irritability or contractibility. There are some flowers, such as those of the barberry, whose stamens will bend and fold over the pistil, if the latter be pricked with a needle; and there is one instance of a plant the leaves of which move without any assignable cause: this is the hedysarum gyrans, which grows only on the banks
of the Ganges. It has three leaflets on each footstalk, all of which are in constant irregular motion. The leaves of the sun-dew, near the root, are covered with bristles, bedewed with a sticky juice. If a fly settles on the upper surface of the leaf, it is at first detained by this clammy liquid, and then the leaf closes, and holds it fast, till it dies. Plants in general, turn their leaves towards the light; and, when growing in a room, they spread out their branches towards the windows, as if they were sensible of the benefits they derive from light and air.
Plants appear also to be susceptible of contracting habits. The mimosa, or sensitive plant, if conveyed in a carriage, closes its leaves, as soon as the carriage is put in motion; but after some time, it becomes accustomed to it, the contraction ceases, and the leaves expand.
Plants, which are brought from the southern hemisphere, faithful to the seasons of their native country, make vain attempts to bud and blossom, during our frosty winter; and seem to expect their sultry summer at Christmas.
These, and many other phenomena, exhibited by plants, do not permit us positively to say, that plants are wholly devoid of sensibility; but the evidence against that opinion is so strong, as to amount almost to proof. Had Providence endowed plants with the sensations of pleasure and of pain, it wo at the same time, have afforded the means of seeking the one, and of avoiding the other. Instinct is given to animals for that express purpose, and reason to man; but a plant rooted in the earth, is a poor, patient, passive being: its habits, its irritability, and its contractibility, all depending on mere physical causes.
The properties of plants may be separated into two classes: first, those which relate to their structure; such as their elasticity, their hygrometic power: these properties may continue after death. Secondly, those which relate to their vitality; such as contractibility: which, consequently, can exist only in the living state. The elementary organs of vegetables are of three
kinds. First, the cellular system; consisting of minute cells, of an hexagonal form, apparently closed and separated by their partitions, somewhat similar to the construction of a honeycomb. These cells in plants are marked by small spots, which have been conjectured to be apertures, through which fluids are transmitted from one cell to another; but these marks are so minute, as to render it hazardous to venture on deciding for what purpose they are designed.
The vascular system forms the second set of elementary organs. It consists of tubes, open at both ends: they are always situated internally. The organs of plants are so extremely small, that, though aided by the most powerful microscope, it is frequently difficult to examine the structure of their parts, with a sufficient degree of accuracy, to be able to ascertain their functions. It has long been a disputed point, whether the sap ascends through the vascular or the cellular system of organs; the latest opinion is, that it passes through neither; but that it rises through the interstices which separate the different cells.
The third system of elementary organs, is the tracheae; so called from their conveying air both to and from the plant; they are composed of very minute elastic spiral tubes. Air is so essential an agent, in promoting the nourishment and growth of plants, that it is scarcely less necessary to their existence, than to that of animals.
The whole of the vegetable kingdom consists of masses of these several elementary organs with the exception of fungi, mosses, and lichens, whose vessels are all of a cellular form: they have no vascular system whatever; and this affords a strong argument against the passage of the sap through the vascular system.
The layers of wood, which are seen in the stem or branch of a tree cut transversely, consist of different zones of fibres, each the product of one year's growth.
The bark consists of three distinct coats, the cuticle, the cortex, and the liber or inner bark; of these, the cuticle is that which is external. It covers the leaves
and flowers, with the exception of the pistiles and anthers, as well as the stem and branches. The cuticle of a young shoot, after it has been for some time exposed to the atmosphere, becomes opaque, dries, and being distended by the lateral growth of the branches, splits, and after a year or two, falls off. A second membrane is then formed, by the desiccation of the external part of the cellular integument; but it differs from the former, in being thicker, and of a coarser texture. This envelope is distinguished from the former by the name of epidermis.
The root not only supports the plant by fixing it in the soil, but affords a channel for the conveyance of nourishment. At the extremity of each fibre of a root, there is an expansion of the cellular integument, called spongiole, from its resemblance to a small sponge; being full of pores, it absorbs the water from the soil. There are pores in every part of a plant, above ground, but they are almost wholly for the purpose of exhalation. The roots have no pores, except in the spongioles at the extremities. It would be useless for them to be furnished with evaporating pores, since they are not exposed to the atmosphere, where alone evaporation could take place.
The tendrils of vines, and of other climbing plants, which serve to fix them against a wall, or the trunk of a tree, cannot be considered as roots; since, though they answer the purpose of sustaining the plant, they are unable to supply it with nourishment. But there are some parasitical plants, such as the misletoe, which, having no immediate communication with the earth, strike their fibres into the stems or branches of a tree, and derive their nourishment from this richly prepared soil: yet, as the absorption in this case is not carried on
by the regular mode of spongioles, their fibres are not denominated roots.
The spongioles act only by capillary attraction, and suck up moisture, just as a lump of sugar absorbs the water into which it is dipped. As a proof of this it has been shown, that if roots, saturated with moisture, be transplanted into very dry earth, the latter will absorb the moisture from the roots.
Absorption does not immediately cease upon the death of a plant, as the blood ceases to circulate upon the expiration of animal life; but when the vessels, through which the fluid should pass, have lost their vital energy, that susceptibility of irritation and contraction, which enabled them to propel the fluid upward, ceases, and it can no longer ascend into the roots, but remains stagnant in the spongioles, which soon become saturated. Disease and putrefaction follow; and that nourishment, which was designed to sustain life, now serves only to accelerate disorganization. The fluid is, however, still performing the part assigned to it by the Creator; for if it be necessary to supply living plants with food, it is also necessary to destroy those which have ceased to live, in order that the earth may not be encumbered with bodies become useless, and that their disorganized particles may contribute to the growth of living plants. Thus, the putrefaction of leaves, straw, &c. which reduces the bodies to their simple elements, prepares them to become once more component parts of living plants.
Botanists distinguish several kinds of roots. The radix fibrosa, or fibrous root, is the most common in its form it consists of a collection or bundle of fibres. The roots of many grasses, and most annual herbs, are of this description. The couch-grass is an example of the radix repens, or creeping-root. If an attempt be made to eradicate such roots, a succession of bunches of fibres are met with, springing from an apparent root which grows horizontally, and appears to be endless. This long horizontal fibre is, however, not a root, but a subterraneous branch, for it has no