as the absorption of the sap may principally, if not entirely, be owing to the operation of mechanical causes, dependent, however, for their lengthened continuance upon the existence of the vital energy by which those conditions are perpetually renewed, and without which the endosmosis would of necessity soon cease." (Henslow's Principles of Botany, pp. 181-2.)

40. It would seem natural here to observe what course the sap takes in its rise in the plant, but the question of the channels through which it is propelled is by no means one to which an indisputable answer can be given. "The great difficulty," says Professor Henslow, "in determining the precise channel through which the progression of the sap takes place, must be ascribed to the perfect transparency of the vegetable membrane, and the extreme minuteness of these organs themselves. By placing a branch in colored fluids, such as a decoction of Brazil wood or cochineal, they are absorbed and the course of the sap through its whole passage into the leaf may be regularly traced; but on examining microscopically the stains which have been left, it is scarcely possible to feel satisfied whether they are on the outer or inner surface of the vessels and cells which they have discolored. . . . . . Since there are many plants which possess no vascular structure, in them at least we must allow the cellular tissue to be the true channel through which the sap is conveyed.

The probability seems to be, that the crude sap rises, at least in woody stems, through the intercellular passages, where it bathes the surface of the cells and vessels, all of which are so many distinct organs destined to act upon it." (Henslow's Principles of Botany, p. 179.) Many excellent observers, however, deny the general system of intercellu

lar passages, or of consequence the passing of the sap by these means; the question must therefore be considered as undecided.

41. Heat and light exercise great influence on the ascent of the sap. A plant exposed to the light takes up a sensibly larger quantity of water than one kept in darkness. The leaves exhaling moisture in great abundance (to the amount of about two-thirds of the water taken up), and consequently requiring and receiving a proportionate supply, tend largely to promote the direct ascent of the sap, and a terminal bunch, such as is always left by mulberry growers when the leaves are picked, determines the rise of the sap to the top of the tree, whereas if the summit be left bare, the juices will scarcely be active enough to reach it, and in addition to this vertical action, the cellular envelop which surrounds the branches, and which communicates with all the woody and cortical layers by the medullary rays, draws the sap, by the action of the living cellules, in a transverse direction. In Endogenous plants, in which there are no medullary prolongations, the sap is necessarily drawn to the summit by the leaves, and it is only in youth that the cellular envelop of the branches can receive a small quantity of moisture: as soon as the action becomes hardened, further lateral growth is impossible. The powerful action of the leaves, &c., as here described, in determining the ascent of the sap, is a much more probable account of that phenomenon than any propulsive vis à tergo like that supposed in the extract from Professor Henslow in paragraph 39, to be resident in the lowest extremities of the roots.

42. It is well known that fresh plants exposed to the air part with a considerable portion of their moistThis exhalation is not performed equally all

ure.

over the plant, but is in exact proportion to the quantity of stomata on any given part, and it is curious that this fact was established by the experiments of Guillard, Saint-Martin, Bonnet, and Senebier, before the existence of stomata was known. Light has great influence in increasing the transpiration of plants. This exhalation may sometimes be observed in the form of drops of water resting on the leaves, &c., when circumstances preclude the possibility of their arising from rain or dew. "The manner in which the stomata act is unknown; and consequently we are compelled to ascribe the function which they perform to the immediate operation of the vital force." (Henslow.)

43. The influence of the atmosphere on the nourishment of plants, or in other words, their respiration, is the most complicated and perhaps the most important of all the processes of vegetable economy. Animal respiration, which is in effect, that process by which the blood is exposed to the action of the air, may show us by analogy how necessary it must be to consider the relations of the nutritious juices of this class also of organized beings with atmospheric action in order to comprehend their physiology, Thirty years after Bonnet (then occupied in researches on the uses of the foliage of plants), had first observed that air was given out by living green leaves, Priestley's attention was turned to the subject; and he submitted the air thus obtained to analysis: it proved to be either pure oxygen, or to contain that gas in a much larger proportion than atmospheric air does: other chemists confirmed the details of Priestley's experiments. The phenomenon is evidently connected with the life of the plant, since leaves, though still green but no longer living, give out no gas at all until the commencement of decom

position. The direct rays of the sun are necessary to the effect: no other light, however strong, will suffice. The course of the phenomena connected with the respiration of plants appears to be the following. The water which enters the plant by the roots contains carbonic acid, which is carried with it into the green parts; it is there decomposed under the influence of the sun's rays-the carbon is fixed in the plant, and the oxygen escapes. The carbonic acid which is formed from the oxygen of the air, in all those portions of the plant which are not green, is partly dispersed in the atmosphere, partly dissolved in water, which water at last reaches the plant again, and thus is ultimately absorbed by the roots, drawn up to the leafy parts and there decomposed. The water taken up by the roots holds, besides its carbonic acid, a certain quantity of soluble matter containing carbon: this carbon is also carried with the sap into the green parts; it combines during the night with the oxygen which had been previously absorbed by them, and the following day such of this carbonic acid thus formed in the leaves as has not been given out during the night, is decomposed by the solar light, as if the carbon could not be usefully deposited in the nutritive juices unless it proceed from the decomposition of carbonic acid gas. Thus the whole of this important function, i. e., vegetable respiration, appears to have for its object the fixing carbon in the plant, while the result of animal respiration is to diminish its quantity in the body, or, in other words, to supply animal heat by its combustion.* It is well remarked by Mr. Hunt, that "the animal kingdom is constantly producing carbonic acid, water in the state of vapor, nitrogen, and, in combina

* See Introduction to Practical Organic Chemistry, p. 61.

tion with hydrogen, ammonia. The vegetable kingdom continually consumes ammonia, nitrogen, water, and carbonic acid. The one is constantly pouring into the air what the other is as constantly drawing from it, and thus is the equilibrium of the elements maintained.

"Plants may be regarded as compounds of carbon, vapor, oxygen, hydrogen, and nitrogen gases, consolidated by the all-powerful, all-pervading influences of the solar ray; and all these elements are the produce of the living animal, the conditions of whose existence are also greatly under the influence of those beams, which are poured in unceasing flow from the centre of our system. Can anything more completely display a system of the loftiest design and most perfect order than these phenomena ?"*

44. It has been shown that the watery juices, pumped up as it were, by the roots, have been drawn to the leafy parts; a large part of the water is there evaporated, green matter is formed, and the decomposition of carbonic acid, ammonia, and water, fixes carbon, nitrogen, and hydrogen in the residuum. From these changes, to which the term assimilation has been given, results the formation of a new and descending juice whose existence is perhaps less palpable than that of the ascending sap, but concerning which there can be no doubt. If a circular incision be made in the bark of an exogenous tree, a tumor will in a short time appear above the wound; this tumor increases, and if the cut be very narrow, it soon reaches the lower lip of the wound, the communication is restored, and the tree lives on as usual, but if the wound be too wide to admit of this junction, the tumor continues to increase till the branch

* Researches on Light, p. 200.