Page images
PDF

parallelograms of cellular tissue, connect the centre and circumference of the trunk: they strengthen the tissue, and convey secreted nutritive matter in a horizontal direction. They are distinctly perceptible in a section of a woody stem. Sometimes they can be traced from the central pith to the extreme circumference, but ordinarily the line is interrupted. 31. Stems vary extremely in appearance in different plants—sometimes they run under the ground, and are improperly called creeping roots; occasionally they lie prostrate, and send roots into the earth underneath them;-the term rhizoma is then applied to them;-and sometimes they are much swollen, and called a tuber;-or if they (or rather their leaf buds) (35) thicken below the ground, a corm. All these forms of stem have been called roots; but there are two marked distinctions between these and true roots. They have what are termed modes, which are the points at which the leaf buds are formed, as well as leaf buds, which are never found on roots properly so called. Scales being the rudiments of leaves, no proper root can be scaly. 32. The stems of Endogenous plants, considered generally, have these common characters. 1. They are composed of one single homogeneous naaSS. 2. They have no true medullary channel nor distinct medullary rays. 3. Their older fibres are on the circumference, and the newer deposits in the centre, from which latter circumstance they take their name. They are less marked in character, and present less regularity of structure than the Exogenes. Thus one species, the Palm, will afford a sufficient idea of the whole class. This stem is generally upright, strong, simple, regularly cylindrical, and crowned at its summit with a bunch of leaves; transversely divided, it appears to be composed of scattered fibres, mixed with cellular tissue, which unites them together. At a glance it is obvious that the fibres of the circumference are more close, of a firmer consistence, and older than the inner ones, which are distant, soft, and surrounded by a loose cellular tissue. Each fibre consists of a bundle of tracheae, and rayed and dotted vessels. The difference in consistence between the circumference and the centre of the trunk is always perceptible, sometimes very remarkable; for instance, there are some palms whose exterior is so hard that a hatchet can make no impression on it, while the inside is a loose spongy tissue, quickly decaying in a humid air. The circumference of the palms corresponds to the wood of our trees, while the centre is a species of alburnum. It is from this central alburnum that the leaves and flowers spring, or in a word, it is from the centre that the development of all the parts takes place. Immediately on the appearance of the plant a first row of leaves is put forth, attached to the crown by a layer of fibres—-the next year a second row is produced within the former, and distends them—it is the same with the succeeding seasons, till the period when the outer layer, having acquired by age the hardness of perfect wood, and no longer admitting of further distension, is incapable of any increase of diameter.

33. A Leaf has two distinct parts—the Petiole, or stalk, and the Lamina, called also the blade or limb; the former consists of fibres proceeding from the stem, and enclosed in a cellular integument; the latter is formed by the ramifications of the fibres of the petiole, and the expansion of its cellular tissue. In exogenous plants the veins branch in various directions, so as to form a kind of network; in the endogenes they run parallel to each other, and are simply connected by transverse veins. When the petiole becomes lengthened so as to curl up, it is called a tendril, and many curious forms, such as that of the Pitcher Plant, are but expansions of this portion of the leaf. The limb of a leaf presents three distinct parts ; the superior and inferior surfaces, and the mesophyllum, or substance contained between the nervures. Both the surfaces are ordinarily furnished with stomata, the under side much more abundantly than the upper; but in leaves which rest by their under surface on the water, this relation is reversed, their upper surface (that which is exposed to the air) being alone furnished with stomata. In like manner, leaves which are constantly immersed have no stomata. The nervures of the superior surface are supposed to be the channels by which the juices are conveyed from the stem to the limb; those of the lower surface conduct them back to the bark. If we attempt to twist a leaf so that the naturally superior surface shall be undermost, it endeavours to regain its original situation ; and if the force used, prevent it from doing so, the leaf quickly perishes. 34. Stipules. This name has been given to small leafy organs, whose only essential character is their lateral position at the base of the leaf. They are occasionally changed into true leaves, and one of them is sometimes wanting; they vary exceedingly in appearance. 35. Leaf Buds are those vital points, surrounded by scales, which are usually found in the axils of the leaves, and from whose growth a branch is formed.*

* “ Leaf buds are always formed from the cellular portion of the stem or branches, on which the function of extending the growth of the individual seems especially imposed. They may be distinctly traced, in young branches, to the pith; and where

The scales, as the vegetation proceeds, are replaced by leaves. When leaf buds are found under ground, and become swollen and large, like the crocus, &c., they are called bulbs or corms (31). In both cases young bulbs are produced in the axils of the scales, and feed on the old bulb. Some of the latter tribe raise themselves out of the earth by a very curious process. “In some Gladioli,” says Professor Lindley, “an old corm produces the new one always at its point; the latter is then seated on the remains of its parent, and being in like manner devoured by its own offspring, becomes the base of the third generation.” Leaf buds are divided into regular and adventitious, the former being always found in the axil of the leaves, none of which, in fact, are ever really without them, though in some cases they are undeveloped; so that the arrangement of the branches of a plant would always be the same as that of its leaves, were it not that the buds are very unequally matured: and this regularity is found to exist in reality through every part of a plant, although from the obliteration of some portions, and the non-development of others, it cannot always be traced throughout. “It has been distinctly proved, that while roots are prolongations of the vertical or woody system, leaf buds universally originate in the horizontal or cellular system.” 36. The nutritive organs of cellular plants are far less defined than those of the vascular tribes, and it even appears as if the whole mass of the former were composed of one homogeneous substance, capable of taking diverse forms, and fulfilling different functions, without being separated into distinct or

this has dried up, they may be seen to arise from the medullary rays.” (Carpenter's Weg. Phys., p. 197.)

gans. They are analogous in many cases to those of vascular plants, but never consist of vessels. They vary so much in the different species of the cellular tribes, such as the Mosses, Hepaticae, Lichens, &c., that it would be impossible to describe them here without entering into details far exceeding the limits of this work. 37. On considering the phenomena of vegetable nutrition, one fundamental principle meets us at the outset; viz., that no aliment ever penetrates the plant, unless water serve it for a vehicle. Without water there is no vegetation. The first thing then to inquire is how it enters into the system. The habitual and vital absorption of water is performed by the spongioles of the roots (8), although under certain conditions, such as rain, heavy dew, artificial watering, &c., the surfaces of the leaves have also the power of imbibing it. Plants being utterly without locomotion, and unable to seek their own food, it follows that their nourishment must be so abundant in nature as to be almost universally within reach, and so easy of absorption as to offer no resistance to their comparatively feeble powers of action. These necessary conditions are beautifully fulfilled by the spongioles and by the nature of water. The spongioles make no selection of healthy material for the nourishment of the plant; and the absorption of fluid through their medium appears to be regulated merely by the readiness with which certain solid substances held in solution can be received along with the water. Thus the action of the spongioles separates a portion of the water from a solution of gum arabic, leaving the gum behind in the remaining solution, in an increased state of saturation; but sulphate of copper in solution—one of the substances most injurious to vegetation—is rapidly absorbed. Dr. Car

« PreviousContinue »