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though capable of being employed to indicate the state of comparative dryness or dampness of the air, from their power of attracting moisture to a certain extent, are nevertheless, under ordinary circumstances, insoluble in water. It is the same with several vegetable productions, which can, consequently, be similarly employed. Vegetable tissue is in general more hygrometric in proportion as it is less loaded with extraneous substances: the woody fibre is, in this respect, very different from the bark; this latter being scarcely hygrometric, while the woody fibre imbibes moisture with great facility. This absorption of water occasions an enlargement of the woody portion, which thus presses itself, as it were, against the bark, and it is in consequence of this pressure, that the gums contained in and under the bark of certain trees are forced outwards, as in the cherry, plum, &c. Senebier has greatly exaggerated the effects of this power in attempting to account by its agency for the ascent of the sap, and for some of the most important phenomena of vegetation. The fact, that the sap ascends in plants which live in water, and that it does not rise in dead plants, might alone prove his theory to be erroneous. 20. “Connected with the hygroscopicity of vegetable membrane, we may here mention a property" of all membrane, which has probably a considerable influence in the economy both of animal and vegetable life. When a membrane is viewed under the highest powers of the microscope, it appears to possess a perfectly homogeneous texture, without pores of any kind; and yet water, milk, and other fluids, placed under certain circumstances, are capable of passing through it with considerable facility. The

* This property is called Endosmosis.

conditions required for producing this effect are these: —Any two fluids which exert a mutual affinity towards each other, being placed on opposite sides of a membrane, their immediate intermixture will commence, each of them passing through the substance of the membrane. If, for instance, a little treacle be enclosed in a piece of bladder, and this immersed in water, a portion of the treacle will soon be found to have exuded, while a still larger quantity of water will have penetrated into the bladder; and this action will continue until the fluids have acquired the same density. The remarkable circumstance attending this phenomenon is the fact of the lighter fluid having penetrated the membrane with greater velocity than the denser fluid.” (Henslow's Principles of Botany, p. 159-60.) 21. Vegetable existence has been supposed to possess three vital properties, so termed from their analogy with the powers similarly named in the animal economy; viz., 1. Eaccitability. 2, Irritability, and 3. Sensibility: by the first is understood that peculiar state of the vegetable tissue, which enables it to resist decomposition by water much more energetically while living than after death, and which also renders it capable of supporting the action of air and heat during life, in a manner totally different from that in which their agency affects it afterwards. Many phenomena common to all plants concur to prove that this difference is inexplicable without the admission of vital excitability; such are the rapid mounting of the sap in the living plant, compared with the slow absorption of water in the lifeless tissue; the influence of light on the ascent of the sap, &c. 22. The quality to which the term Irritability has been applied by some physiologists, is that by

which certain portions of some plants respond to the agency of external objects, in a manner somewhat similar to the sudden contraction of the muscles in the animal body: for example, when the base of the stamen of the Berberis is pricked with a needle, it is seen to depress itself towards the pistil. If the hairs of the Drosera are irritated, they press themselves close to the leaf; and one instance, especially, must be familiar to most persons, viz., the closing of the leaves of the Mimosa pudica, or sensitive-plant, on the slightest touch. It has, however, been conjectured that all this class of facts may be referred to vital excitability alone; and with respect to the third quality, which some persons have attributed to plants, sensibility, or more properly sensation, until much more positive proof of it shall be adduced than has yet been offered, it can only be classed with those phenomena which are referable to excitability. The same argument, from analogy, which leads us to suppose that the lower orders of animals are far less sensitive than the higher, is against the idea that plants, wholly unprovided as they are with any apparatus of nerves, can be susceptible of those impressions, whether of pain or pleasure, which in the animal economy we have every reason to refer to a particular portion of the nervous system:—nor can we see in the general order of things any sufficient cause to lead us to an opposite conclusion. Although it may be a poetical and an agreeable idea to imagine the whole vegetable world welcoming and rejoicing in the return of spring, and basking in the warm beams that are so congenial to our own nature and necessities, yet the satisfaction this notion might afford would be far more than counterbalanced by the reflection that we could not pluck a rose or gather a peach without inflicting pain; and that the pruning

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26 STRUCTURE, ETC., of vegetABLE TISSUE.

knife was an instrument of torture. One strong reason to conclude against the sensibility of plants, arises from the great contrast between the provision made for them and for animals during the winter. It is known that animals liable to exposure to cold are well defended against it by their fur or down ; while trees, stripped bare at the season when all sentient beings look for shelter, would indeed undergo a heavy penalty if they could feel the chill blasts that howl around them. 23. It was formerly supposed that vital excitability was seated exclusively in the vessels, but M. de Candolle’s reasoning is conclusive against this theory, as he shows that the power is possessed by plants wholly formed of cellular tissue ; that is to say, they offer the same facts from which the existence of vital excitability in vascular plants has been deduced. The immediate cause of these phenomena appears to be that the cells and vessels of the tissue are endued with a contractile power, analogous to that of the heart in animals, or rather, perhaps, to the contraction and dilatation observed in certain microscopic infusoria: there are cases in which this action (though ordinarily confined to parts so minute as to escape observation), becomes visible: for instance, if a branch of the Euphorbia, or any other milky plant, be cut across, the milky juice exudes from both surfaces. If it flowed by an impulse given either from below or from above, it would only appear on one half of the severed plant; if it issued forth by its own weight by the law of gravity, it could only flow when turned downwards, and if the lower half were held upright, the fluid would stand as in a cup; but it exudes let the branch be held in whatever direction it may, and it must therefore be owing to some contractile power within.

The agents which occasion or modify vegetable excitability, are light, heat, and perhaps electricity; and in addition to these, accidental causes of excitement, such as blows, the action of certain chemical substances, &c., will in some cases produce the phenomena by which it is manifested.

CHAPTER II.

NUTRITION.

24. THE general structure and properties of Vegetable tissue having been explained, it becomes desirable briefly to describe the organs by which plants are nourished, and enabled to perform the functions of growth and secretion, as the physiology of this part of the subject, which is, in fact, nothing more than the active agency of those organs, cannot be well understood without some distinct idea of their form and nature.

The organs which are indispensable to the nutrition of all vascular plants, are three, i.e., the Root, the Stem, or Trunk, and the Leaves. In cellular plants these are often so united that the parts are scarcely distinguishable. It will be desirable to consider them in detail as they are found in vascular plants, in which they are generally well defined.

25. The Root (radix). This term is commonly applied to that part of a plant which is beneath the earth; but this is not an exact definition, as there are roots which exist out of the soil altogether;" it may

* Such are the curious braces, as they may be called, sent out

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