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 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 be more correctly described as that portion which vegetates downwards. The point of junction be tween the stem and the root bears the name of the neck, or crown:-from this point they proceed in opposite directions, so that the part the nearest to this is, in both organs, the oldest, and in general, the thickest. The office of the root is double; it both serves to fix the plant in the soil, and to imbibe its requisite nourishment. Roots are never green excepting at their extremity, where it has been shown (8) that they perform their function of absorbing water through their spongioles. As soon as a plant begins to exist, a principal, or tap root, may always be perceived, growing in an opposite direction to the stem: it is very remarkable in the seed, and is there called the radicle; this principal root, after having sent out branches in all directions, often perishes, and the ramifications frequently take a horizontal course. Besides affording nourishment by direct. absorption from the soil, the roots are often storehouses of nutritive matter. Such are those of the Dahlia, which abound in starch, the orchis, &c. &c. ; such roots are generally much swelled or thickened. In their anatomical structure roots principally differ from stems by the absence of stomata, and, in the Exogenes, by the want of a central pith or medulla (27). 26. The Stem (caulis). This organ is never really wanting in vascular plants, though in some it is hidden beneath the earth. 66 The stem is pro by the Pandamus or Screw Pine-this stem is smaller at the bottom than it is above, and as this form is of course unfavorable to the steadiness of the plant in the ground, it sends out roots at various distances up the stem which find their way into the earth, and thus act as buttresses for its support. Such is also the well-known method by which the Banyan, from a single tree becomes a grove. duced by the successive development of leaf buds (35), which lengthen in opposite directions." The stems of Exogenous plants possess the most complicated organization, but as they are much better understood than those of the Endogenous and Cellular tribes, and as the Exogenes comprise all the trees of our own part of the globe, they are more interesting to us. Four distinct parts are observed in Exogenous trees-the Pith, or Medulla, in the centre; the Wood surrounding the pith; the Bark which envelops the whole, and the Medullary Rays, which run horizontally across the wood and bark, from the centre to the circumference. To these may be added the Medullary Sheath, which is but the first annual layer of wood. 27. The Pith, or Medulla, is composed of cellular tissue, whose cells are large, regular, and spongy; it contains starch which is afterwards converted into mucilage,* and its office seems to be that of nourishing the young buds; when this function is performed, it perishes. Around it is the Medullary Sheath which differs from the succeeding annual layers only in having its vessels usually capable of being unrolled, and consequently truly spiral; it envelops the pith like a case, and its fibres often branch into the substance of the pith itself, where they appear as scattered spiral vessels. The medullary sheath has been supposed to be the channel by which oxygen, liberated by the decomposition of carbonic acid, is conveyed to the leaves. 28. The Wood immediately surrounds the central pith, and is formed of concentric layers of vessels, * See "Introduction to Practical Organic Chemistry," p. 49, $36. |