BOOK II.

PHYSIOLOGY; OR, PLANTS CONSIDERED IN A STATE OF ACTION.

GENERAL CONSIDERATIONS.

WE have thus far considered plants as inert bodies, having certain modifications of structure, and formed upon a plan, the simplicity and uniformity of which is among the most beautiful proofs of the boundless power and skill of the Deity.

Our next business is to enquire into the nature of their vital actions, and to consider those phenomena in which the analogy that undoubtedly exists between plants and animals is most striking; in a word, to make ourselves acquainted with the exact nature of the laws of vegetable life.

In explaining these things, it is not my purpose to notice all the different speculations that ingenious men have from time to time brought forward: for this would be incompatible with the plan of my work, and would be far more curious than useful. On the contrary, I propose, in the first place, to give a summary exposition of the principal phenomena of vegetation, and then to support the statement by a detailed account of the more important proofs of all doubtful points.

I am the more anxious that this should be understood, because I know how prone the world is to misconstruction: I therefore beg it to be remembered, that when particular opinions are here passed over in silence, it is because I do not think them sufficiently proved to be recorded consistently with the plan I have prescribed to myself.

If we place a seed, that of an apple, for instance, in earth at the temperature of 32° Fahr., it will remain inactive till it finally decays. But if it is placed in moist earth above

the temperature of 32°, and screened from the action of light, its integument gradually imbibes moisture and swells, oxygen is absorbed, carbonic acid expelled, and the vital action of the embryo commences. It elongates downwards by the radicle, and upwards by the cotyledons; the former penetrating the soil, the latter elevating themselves above it, acquiring a green colour by the deposition of carbon absorbed from the atmosphere in the light, and unfolding in the form of two opposite roundish leaves. This is the first stage of vegetation: the young plant consists of little more than cellular tissue; only an imperfect developement of vascular and fibrous tissue being discoverable, in the form of a sort of cylinder of bundles, lying just in the centre. The part within the cylinder, at its upper end, is now the medulla, without it the bark; while the cylinder itself is the preparation for the medullary sheath, and consists of vertical fibres passing through cellular tissue, which separates them horizontally in every direction.

The young root is now absorbing from the earth its nutriment, which passes up to the summit of the plant by the cellular substance of the medulla, and is thence impelled into the cotyledons, where it is aërated and evaporated: such of it as is not fixed in the cotyledons passes down through the bark into the root.

Forced onwards by the current of sap, which is continually impelled upwards from the root, the plumula next ascends in the form of a little twig, at the same time sending roots in the form of fibres downwards in the centre of the radicle, which become the earliest portion of wood that is deposited: these fibres, by their action, now compel the root to emit little ramifications. Previously to the elongation of the plumula its apex has acquired the rudimentary state of a leaf: this continues to develope as the plumula elongates, until, when the first internodium of the latter ceases to lengthen, the leaf has actually arrived at its complete formation. When fully grown it repeats in a much more perfect manner the functions previously performed by the cotyledons: it aërates the sap that it receives, and returns the superfluous portion of it downwards through

the bark to the root; it also sends fibres down between the medullary sheath and the bark, thus forming the first stratum of wood in the new stem. During these operations, while the plumula is ascending, its leaf forming and acting, and the woody matter created by it descending, the cellular tissue of the stem is forming, and expanding horizontally to make room for the new matter forced into it; so that developement is going on simultaneously both in a horizontal and perpendicular direction. This process may not inaptly be compared to that of weaving, the warp being the perpendicular, and the weft the horizontal, formation. In order to enable the leaf to perform its functions of aeration completely it is traversed by veins originating in the medulla, and has delicate evaporating pores (stomata), which communicate with a highly complex pneumatic system that extends to almost every part of the plant.

After the production of its first leaf by the plumula, others are successively produced around the axis at its elongating point, all constructed alike, connected with the stem or axis in the same manner, and performing precisely the same functions as have been just described. At last the axis ceases to elongate; the old leaves gradually fall off; the new leaves, instead of expanding after their formation, retain their rudimentary condition, harden, and fold over one another, so as to be a protection to the delicate point of elongation; or, in other words, become the scales of a bud. We have now a shoot with a woody axis, and a distinct pith and bark; and of a more or less conical figure. At the axilla of every leaf a bud had been generated during the growth of the axis; so that the shoot, when deprived of its leaves, is covered from end to end with little, 'symmetrically arranged, projecting points, which are the buds. The cause of the figure of the perfect shoot being conical is, that, as the wood originates from the base of the leaves, the lower end of the shoot, which has the greatest number of strata, because it has the greatest number of leaves above it, will be the thickest; and the upper end, which has had the fewest leaves to distend it by their deposit, will have the least diameter. Thus that part of the stem which has two leaves above it will have wood formed by two

successive deposits; that which has nine leaves above it will have wood formed by nine successive deposits; and so on: while the extreme vital point, as it can have no deposit of matter from above, will have no wood, the extremity being merely covered by the rudiments of leaves hereafter to be developed.

If at this time a cross section be examined, it will be found that the interior is no longer imperfectly divided into two portions, namely, medulla and skin, as it was when first examined in the same way, but that it has distinctly two, internal, perfect, concentric lines, the outer indicating a separation of the bark from wood; and the inner, a separation of the wood from the medulla: the latter too, which in the first observation was fleshy, and saturated with humidity, is become distinctly cellular, and altogether or nearly dry.

With the spring of the second year and the return of warm weather vegetation recommences.

The uppermost, and perhaps some other, buds which were formed the previous year gradually unfold, and pump up sap from the stock remaining in store about them; the place of the sap so removed is instantly supplied by that which is next it; an impulse is thus given to the fluids from the summit to the roots; new sap is absorbed from the earth, and sent upwards through the wood of last year; and the phenomenon called the flow of the sap is fully completed, to continue with greater or less velocity till the return of winter. The axis of the buds elongates upwards, forming leaves and buds in the same way as the parent shoot: in like manner also each bud sends down its roots, in the form of fibres within the bark and above the wood of the shoot from which it sprang; thus forming on the one hand a new layer of wood, and on the other a fresh deposit of bark. In order to facilitate this last operation, the old bark and wood are separated in the spring by the exudation from both of them of the glutinous, slimy substance called cambium; which appears to be expressly intended, in the first instance, to facilitate the descent of the subcortical fibres of the growing buds; and, in the second place, to generate the cellular tissue by which the horizontal dilatation of the axis is caused, and which maintains a communication between the

bark and the centre of the axis. These lines of communication have, by the second year, become sufficiently developed to be readily discovered, and are in fact the medullary rays spoken of in the last book. It will be remembered that there was a time when that which is now bark constituted a homogeneous body with the medulla; and that it was after the leaves began to come into action that the separation which now exists between the bark and medulla took place. At the time when they were indissolubly united they both consisted of cellular tissue, with a few spiral vessels upon the line indicative of future separation. When a deposit of wood was formed from above between them they were not wholly divided the one from the other, but the deposit was effected in such a way as to leave a communication by means of cellular tissue between the bark and the medulla; and, as this formation is at all times coætaneous with that of the wood, the communication so effected between the medulla and bark is quite as perfect at the end of the third year as it is at the beginning of the first; and so it will continue to be to the end of the growth of the plant. The sap which has been sucked into circulation by the unfolding leaves is exposed, as in the previous year, to the effect of air and light; is then returned through the petiole to the stem, and sent downwards through the bark, to be from it either conveyed to the root, or distributed horizontally by the medullary rays to the centre of the stem. At the end of the year the same phenomena occur as took place the first season wood is gradually deposited by slower degrees, whence the last portion is denser than the first, and gives rise to the appearance called the annual zones: the new shoot or shoots are prepared for winter, and are again elongated cones, as was the first; and this latter has acquired an increase in diameter proportioned to the quantity of new shoots which it produced, new shoots being to it now what young leaves were to it before.

The third year all that took place the year before is repeated sap is absorbed by the unfolding leaves; and its loss is made good by new fluids introduced by the roots and transmitted through the alburnum or wood of the year before; new wood and liber are deposited by matter sent