alburnum of the stem (fig. 1. b); it is, therefore, placed in the most favourable position possible for communicating to the general system of circulation the fluids taken up by its highly absorbent tissue.

23. It is the opinion of most vegetable physiologists, that the absorbing or feeding powers of roots are conducted principally at these points; and that the general surface of the root possesses little or no power of the kind. And, indeed, it seems highly probable that this is so, when we consider that the bark of the root, through whose thickness all fluids would have to pass before they reach the alburnum, has at least two offices to perform, either of which might be interfered with by a current of fluid setting through it. One of those offices is to convey in a downward direction, or to store up, the matter which has descended to the roots from the branches and leaves, the other is to give off such superfluous matter as it is necessary for its health that the plant should part with.

24. But although there can be no doubt that the spongelets act as absorbents with more force than any other part of the root, yet it is equally certain that the whole surface of young roots also possesses an absorbing property, only in a more limited degree. It is not until their tissue is solidified that roots become incapable of passing fluid through their sides; and when very young

and soft, there is probably but little difference between their action and that of the spongelets themselves; for it is to be remembered that the latter are not special organs, but are only the very youngest part of the root.

25. The absorbent power of the spongioles must be much greater than would have been supposed, if we consider that it is almost entirely through their action that the enormous waste of fluid, which takes place in plants by perspiration, is made good; and hence their importance to plants, and the danger of destroying them, become manifest.

26. The spongioles and youngests parts of roots are found to be rich in nitrogen, a principle once supposed to be unknown in the vegetable kingdom; and it seems that a supply of this gas is indispensable to their healthy condition.

27. Roots being furnished with the power of perpetually adding new living matter to their points, are thus enabled to pierce the solid earth in which they grow, to insinuate themselves between the most minute crevices, and to pass on from place to place as fast as the food in contact with them is consumed. So that plants, although not locomotive like animals, do perpetually shift their mouths in search of fresh pasturage, although their bodies remain stationary.

28. The only known exceptions to the rule that

roots do not lengthen by a general distension of their tissue, occur in parts growing in air or water, which are non-resisting media, or in certain endogenous trees, whose roots lengthen to such a degree as to hoist the trunk up into the air off the ground, with which it at first was level.

29. It is not, however, merely in length that the root increases; if such were the case, all roots would be mere threads. They also augment in diameter, simultaneously with the stem, and under the influence of exactly the same causes. Neither is it by an embryo alone that roots are formed. A plant, once in a state of growth, has the power of producing roots from various parts, especially from its stem, and from older roots.

30. The immediate cause of the formation of roots is involved in obscurity, and is one of the most important parts of vegetable physiology still to be investigated with reference to horticulture. We all know how difficult it is to cause the cuttings of some kinds of plants to produce young roots, and how rapidly they are emitted by others; it is to be supposed, that the difficulty would be diminished in all such cases, if we knew exactly under what circumstances roots are formed. Nothing, however, sufficiently certain and general to merit quotation has yet been ascertained concerning this important subject, except the following facts, viz. that roots are most readily, if not

exclusively, formed in darkness and moderate moisture; that they are not, like branches, the developement of previously formed buds, but appear fortuitously and irregularly from the woody rather than the cellular part of a plant; and that their production is in some way connected with the presence of leaves or leaf-buds, because portions of a stem having neither leaves nor leaf-buds produce roots unwillingly, if at all; and that such roots perish if their appearance be not speedily followed by the formation of leaves. Thus, although the first appearance of the root in the embryo plant, at the time of germination, precedes the expansion of the seed-leaves, yet the young root will not live unless the seed-leaves are enabled to act.

31. But, although the immediate cause of the formation of roots is unknown, the remote cause is apparently the elaboration of organisable matter by the leaves; for there can be no doubt that the developement of roots is much assisted by the descending sap. When a ring of bark is removed from a branch, if the wound is wrapped in damp moss, roots will invariably push from the upper lip of the wound, while the lower will produce none; a fact so well known, that it has been one of the causes of an opinion, that roots are bundles of wood liberated from the central perpendicular

system, and that the wood itself is nothing but a mass of roots formed by the leaves and buds.

32. The principal office of the root is to attract food from the ground. For this purpose it is furnished, as has been seen, with an extremely hygrometrical point or spongelet, which is capable of absorbing incessantly whatever matter of a suitable kind may lie in its neighbourhood. Its force of absorption is always proportioned to the quantity of food that a plant requires: when the sap is consumed rapidly by the leaves, as in the spring, the roots are in rapid action also; and as the summer advances, and leaves require a smaller quantity of food, the roots become more and more torpid.

33. The proportion borne by the root to the stem is very variable. In such plants as succulent Euphorbias, and probably in all plants whose perspiring powers are feeble, the roots are much smaller than the stem; but, in others, the surface of these organs must be very much greater than that of the stem and leaves. In young Oaks this is well known to be the case, but the disproportion diminishes as such plants advance in age.

34. There is no period of the year when the roots become altogether inactive, except when they are actually frozen. At all other times, during the winter, they are perpetually attracting food from the earth, and conveying it into the interior of the plant, where it, at that season, is stored up till it is

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