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required by the young shoots of the succeeding year. The whole tissue of a plant will therefore become distended with fluid food by the return of spring, and the degree of distension will be in proportion to the mildness and length of the previous winter. As the new shoots of spring are vigorous or feeble in proportion to the quantity of food that may be prepared for them, it follows, that the longer the period of rest from growth, the more vigorous the vegetation of a plant will become when once renewed, if that period is not excessively protracted.

35. Powerful as the absorbing action of roots is found to be, those organs have little or no power of selecting their food; but they appear, in most cases, to take up whatever is presented to them in a sufficiently attenuated form. Their feeding property depends upon the mere hygrometrical force of their tissue, set in action in a peculiar manner by the vital principle; this force must be supposed to depend upon the action of capillary tubes, of which every part of a vegetable membrane must, of necessity, consist, although they are, in all cases, invisible to the eye, even aided by the most powerful microscopes. Whatever matter is presented to such a set of tubes will, we must suppose, be attracted through them, provided its molecules are sufficiently minute; and, as we have no reason to believe that there is, in general, any difference in the size of the molecules of either gaseous matter or

fluids consisting principally of water, it will follow that one form of such matters will be absorbed by the roots of plants as readily as another. For this reason, plants are peculiarly liable to injury from the presence of deleterious matter in the earth; and it is probable that, if in many cases they reject it, it is because it does not acquire a sufficient state of tenuity; as in the case of certain coloured infusions.

36. But, although this appears to be a general rule, there are some exceptions of importance. If a Pea and a grain of Wheat are placed side by side in earth of the same kind, and made to grow under the same circumstances, the Wheat plant will absorb silex in solution from the earth, and the Pea will absorb none; whence it would seem that the Pea is unable to receive a solution of flint into its system, and that, consequently, it possesses what amounts, practically, to a power of selection. In like manner, Dr. Daubeny has proved that Pelargoniums, Barley, and the Winged Pea (Tetragonolobus) will not receive strontian; and it is mentioned by Saussure, that he could not make Polygonum Persicaria absorb, by its roots, a solution of acetate of lime, although it took up muriate of soda (common salt) freely.

37. It is a curious fact that the poisonous substances which are fatal to man are equally so to plants, and in nearly the same way. So that, by

presenting opium or arsenic, or any metallic or alkaline poison, to its roots, a tree may be destroyed as readily as a human being.

38. The natural food of plants consists of carbon in the state of carbonic acid, of nitrogen, certain earths and salts, and water. The latter, if distilled, has little power, by itself, of sustaining vegetable life but, as in nature it is universally mixed with various other substances, it conveys to the roots the organisable matters that are required; and it furnishes, by its decomposition, a considerable supply of the oxygen consumed in the formation of carbonic acid, and all the hydrogen that is incorporated in the tissue of plants. It has been proved, experimentally, that plants cannot long exist upon pure water; but, if they are so circumstanced as to be able to obtain and decompose carbonic acid, they will grow in the absence of other matters. It is only, however, when the peculiar principles, whether earthy or saline, on which they naturally feed, are presented to them, that they become perfectly healthy; and especially when they have the means of obtaining nitrogen, which appears, from its great abundance in the youngest parts, to be indispensable to plants upon the first formation of their tissue. *

Mr. Rigg states that those seeds of the same kind, which contain the largest quantity of nitrogen, germinate the earliest. He found nitrogen in young roots having the proportion of one

39. In addition to their feeding properties*, roots are the organs by which plants rid themselves of the secreted matter which is either superfluous or deleterious to them. If you place a plant of Succory in water, it will be found that the roots will, by degrees, render the water bitter, as if opium had been mixed with it; a Spurge will render it acrid; and a leguminous plant mucilaginous. And, if you poison one half of the roots of any plant, the other half will throw the poison off again from the system. Hence it follows, that, if roots are so circumstanced that they cannot constantly advance into fresh soil, they will, by degrees, be surrounded by their own excrementitious secretions.

40. It would also seem to follow that, under the circumstances just named, they would be poisoned, because they have little power of refusing to take up whatever matter is presented to them in a fitting state (35.). But it is by no means certain that the excrementitious matter of all plants is poisonous either to themselves or to others; and therefore the consequences of roots growing in soil from which

to five of carbon.

Theodore de Sausure also ascertained that germinating seeds absorb this gas.

* According to Mr. Knight, the roots of trees retain the original vigour of the variety, after the trunks have become debilitated; or, to use his own words, the powers of life do not become expended so soon in roots as in bearing branches. (See Hort. Trans, vol. ii. p. 252.)

they cannot advance are uncertain, and only to be judged of by actual enquiry into the nature of the

secretions.

41. In general, roots have no buds, and are, therefore, incapable of multiplying the plant to which they belong. But it constantly happens, in some species, that they have the power of forming what are called adventitious buds; and, in such cases, they may be employed for purposes of propagation. There is no rule by which the power of a plant to generate such buds by its roots can be judged of; experiment is therefore necessary, in all cases, to determine the point.

CHAP. III.

GROWTH BY THE STEM.

Origin of the Stem.

Wood, Bark, Pith,

The growing Point. - Production of Medullary Rays.- Properties of Sap

wood, Heart-wood, Liber, Rind, &c.- Nature and Office of Leaf-buds. - Embryo-buds. — Bulbs. — Sap, and its Nature.

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Conveyance of

42. As soon as the root is fully in action, which is shortly after it has begun to lengthen, the vitality of the living point that exists at the bottom of the seed leaves is excited, and a stem begins to be formed. At first the stem is a mere point of

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