FORMATION OF ROOTS BY LEAVES.

23

mentioned in the Gardeners' Chronicle of 1846, p. 43, where an old Apple-tree, blown entirely out of the ground, so that all its roots were broken off, nevertheless produced roots from the hard trunk, although the accident occurred in summer when the tree was loaded with fruit. The same observer mentions an Episcia bicolor that happened to have one of its leaves injured by an accident, which cut the midrib and a portion of the leaf on both sides of it. After a certain time the wound

Fig. IV.-Leaf of Episcia bicolor, which had its midrib cut across by accident, and formed a young plant at the part that had been injured.

healed, the part next the base of the leaf remaining of the same thickness as before the injury, while the edge of the outer portion gradually thickened, and developed a small bud close to the midrib, from which a number of minute fibrous roots issued, and eventually a stem and leaves, as represented in the accompanying sketch. For several months the perfect plant continued to exist in this state, with no other nourishment than what the portion of the leaf on which it grew, and the air of a warm, damp, hothouse afforded it. As the plant increased in size, the old leaf gradually became exhausted, and perished altogether as soon as the young leaves gained the ascendancy and deprived it of the

24

FORMATION OF ROOTS BY LEAVES.

scanty means that had previously supported it. Similar instances are familiar to careful observers, not the least interesting of which is that of a broken Celery leaf which had sent out roots from the lowermost of its wounded edges.

Fig. V.-Leaf of Celery producing roots from the lower edge of a wound.

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 property, which appears to be connected with specific vitality, except the following facts, viz. that roots are most readily, if not exclusively, formed

CAUSE OF THE FORMATION OF ROOTS.

25

25

in darkness and moderate moisture; that they are not, like branches, the development 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 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. It is certain moreover that their formation is greatly facilitated by the soil being warm, as is sufficiently proved by the readiness with which they are emitted by trees transplanted in August and September; as also by the abundance of them in warm soil, and their fewness and weak condition in soil not warmed by good drainage.

It has been remarked by a translator of this work that "the young roots of some genera live for a very considerable time without the cotyledons exercising any functions. The seeds of the Peony, sown in January, will have formed roots in September, but the cotyledons will not be visible for four or five months later, viz., in January or February of the next year,"

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 development 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.

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

26

ROOTS ABSORB AT ALL SEASONS.

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 autumn advances, and leaves require a smaller quantity of food, the roots become more and more torpid.

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 circle occupied by these organs must be very much greater than that of the branches. In young Oaks this is well known to be the case, but the disproportion diminishes as such plants advance in age. 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, at that season, it is stored up till it is 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.

A critic remarks that, of the continually-absorbing power of the roots, the simile of a wick of a candle is certainly one of the most appropriate. The wick (as well as the spongioles of the root) by its hygrometric quality continually conducts fluids to the flame, only the spongioles, being continually renewed by their constant formation onwards, are permanent. Others doubt whether any winter absorption occurs, a fact however familiar to practical observers, and proved by such examples as those quoted at pages 50 to 52.

Powerful as the absorbing action of roots is found to be, those organs have little or no power of selecting their food; but appear, in most cases, to take up whatever is presented to them

ROOTS CAN SELECT THEIR FOOD.

27

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 substances 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.

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 abundance of silex in solution from the earth, and the Pea will absorb little or 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.

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.