DIFFERENT FORMS OF STEMS.

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exhibit any distinct division by nodes, the Botanist is accustomed to regard them as existing near the points from which leaves or branches arise; and to consider as internodes the spaces between these.

149. Many parts are commonly regarded as roots, which are in reality stems. Their position, whether above ground, or beneath the surface, is no criterion as to their real nature. It has been seen (§ 110.) that roots sometimes grow in the air, and it is equally true that stems frequently grow in the earth. What are ordinarily called bulbous roots, for example, such as those of the Onion, Hyacinth, Lily, &c. are in reality underground stems. It was formerly stated (§ 111-113.) that the real distinctions between the root and the stem consist in the contrary directions of their growth, and in the tendency of absorbing fibres to arise from the former, whilst the latter gives origin to leaf-buds. Now the base of the bulb is the real point of division between the stem and root; for, whilst all below it,—namely the fibres which really constitute the roots,—has a tendency to grow downwards, the mass of the bulb, together with all above it, has a tendency to grow upwards. Further, the scales of the bulb are in reality but leaves, changed from their usual character and aspect, or metamorphosed; and at the base of every one of these scales, is found a little bud, occupying the same position in regard to it as the buds to the leaves on the higher parts of the stem. (§ 301.) Thus we perceive that here the stem is in a very contracted state, the internodes not being developed, and the leaves and buds of several nodes arising close together. The difference between one of these scaly bulbs, therefore, and the solid fleshy expansion of the root which constitutes a turnip or turnip-radish, is at once evident.

150. But stems are sometimes so completely changed in their direction as well as form, that they can scarcely be recognized as such, except by their tendency to produce leaf-buds. Thus they sometimes creep along the ground, or even just beneath it, send.. ing up buds, which develope themselves into branches, at intervals. Of this kind is the rhizoma or root-stock of most British Ferns, which creeps above ground in some species, and below in others;

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DIFFERENT FORMS OF STEMS.

and the Ginger-plant of tropical countries has a stem of the same character, which really furnishes Ginger, although this is commonly spoken of as the root of the plant, being partly buried beneath the ground. The runners of the Strawberry, again, are but trailing stems, which send down roots and develope buds at intervals, and thus extend the plant. This tendency, which is serviceable to man in this instance, is very troublesome to him in another; for in the same manner the Couch-grass overruns pasture lands, exterminating, if its increase be not checked, their original vegetation. As every internode of these trailing stems possesses the power of developing both roots and buds, it is useless to attempt to destroy the plant by chopping the stem into pieces; for this is in reality only multiplying it. It is well to mention, however, that, though usually regarded as a very rank weed, the underground shoots of this plant constitute a peculiarly nutritious food for cattle.

151. One of the most distorted forms of the stem is that which presents itself in the Potatoe. This plant grows with an underground stem, sending up its flowering branches into the air, and sending its roots downwards into the earth; but on this stem it forms at intervals the tubers or knobs, which constitute such an important article of food to man. That these tubers are still parts of the stem, is shown by their power of originating buds, from the points commonly known as the eyes of the Potatoe. When, therefore, we divide the tuber into pieces, keeping an eye in each, from every one of which we expect a young plant to spring, we follow in fact the same plan as that adopted in planting SugarCanes, which are not propagated from seed, but by dividing the stem into its internodes, and laying each of these separately in the ground. And thus it is seen that the division of the creeping stem of the Couch-grass effects in reality the same end. The quantity of fleshy matter deposited in the Potatoe serves for the nourishment of the growing buds before their roots are formed; and thus it is that, if exposed to a warm and moist atmosphere, they are liable to sprout, without the contact of earth. It is remarkable that in their native climate (the tropical part of South

GROWTH OF ROOTS FROM BRANCHES.-BANYAN TREES. 103

America) the tubers of the Potatoe are extremely small, and that they become so when plants are raised from British stocks in any countries equally hot.

152. In all these instances it is seen that not only buds but roots may arise from different parts of the stem and branches. But this tendency is by no means confined to such as grow on or beneath the ground. There are many trees of which the branches naturally hang downwards; and if these reach the ground, they give origin to a new set of roots, which serve for their own nourishment, and for that of the shoots they send off, so that they become so many secondary stems. The most curious examples of this kind are the Banyan trees of the East Indies, of which one individual sometimes constitutes a miniature forest. The most celebrated specimen is that of Cubbeer-bur, which, many years since, possessed 350 principal trunks, and smaller stems amounting to more than 3000, every one of which was casting out new branches and hanging roots, to form future trunks. The space of ground which it covered was such, that it was estimated that 7000 persons might have found ample room to repose beneath its shade. These trees are held by the Hindoos in superstitious reverence, and are dedicated to religious observances. Our own sacred poet, Milton, has given a beautiful delineation of it.

"The fig-tree; not that kind for fruit renown'd;
But such as at this day to Indians known
In Malabar or Deccan, spreads her arms,
Branching so broad and long, that in the ground
The bending twigs take root, and daughters grow
About the mother tree, a pillar'd shade,

High overarch'd, with echoing walks between."

153. The only Cryptogamia at present existing, which form true woody trunks, are the Tree-Ferns of tropical climates. In these, the stems which creep along or under the ground in the species inhabiting temperate climates, erect themselves into the air, and bear a beautiful crown of leaves. These stems are sometimes hollow, and sometimes contain a sort of spongy pith. Their

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leaves.

STEMS OF CRYPTOGAMIA.

mode of growth is different from that of either Exogens or Endogens; and appears of a simpler character. The stem, when cut across, is seen to consist of a number of hard woody plates adhering rather loosely together; and these, if traced upwards, are found to be either continuations of the flattened footstalks of the leaves which crown the summit, or to be the remains of those which have dropped off. Every year the leaves decay away, and are replaced by a new set formed above; so that the stem continues increasing in length, but undergoes little change in diameter. The marks seen on the exterior of the trunk are the scars of the former leaves; and by the relative position of these it is seen that, though the portion of the stem first formed increases but little in diameter, it receives some addition to its length,-its scars being separated from each other by a much wider interval than in the newly-formed part. However, it is the general rule in these and other Cryptogamia, that the portions first produced undergo little subsequent change; hence, whilst the names Exogens and Endogens are used to indicate the modes of growth respectively peculiar to the two chief divisions of flowering plants, the flowerless plants may be included under the general term ACROGENS, which intimates growth by the point, or by addition to the extremities only.

CHAPTER VI.

OF THE FOOD OF PLANTS, AND THE MANNER IN WHICH IT IS OBTAINED.

154. A plant or tree can no more exist without food than can an animal; and it is only because the mode in which they receive it is less evident to us, that we do not commonly think of vegetables as equally dependent with animals upon the materials supplied to them by the elements around. We are constantly witnessing the act of feeding in all the animals that are under our notice; but the growth and reproduction of plants seem to take place with so slight an introduction of solid matter into their system, that it cannot be comprehended without further examination how they derive the the means of uprearing the gigantic masses of wood and foliage which many of them present to our admiring view. It cannot be shown that any solid matter is ordinarily taken up by the roots, except certain mineral ingredients which most plants require, and the use of which will be presently stated. How then, do they obtain the materials of the firm wood of their stems, roots, and branches, of the soft but still firm tissue of their leaves and fruits, -of the fleshy seeds they generate in their flowering system,and of the various hard substances which they produce in their different tissues? This question will now be answered.

155. In the first place it may be laid down as a fact beyond doubt, that neither plants nor animals have the power of creating or producing matter which did not before exist. Living beings are entirely dependent upon the supplies they obtain from without, for the maintenance and enlargement of their own structures;they greatly alter the form and properties of the elements they take in ;-but they can create nothing. It is easy to say whence