Book I. PROCESS OF VEGETABLE NUTRITION. 239 



found that in the space of elx days they had acquired only half the weight of such as were Introduced at 

 the same time into a receiver containing common air. From whence it follows that oxygen, though the 

 principal agent in the process of vegetation, is not yet the only agent necessary to the health and growth of 

 the plant, and that the proportion of the constituent parts of the atmospheric air is well adapted for the 

 purposes both of vegetable and animal life. 



1532. Decomposition of water. Although the opinion was proved to be groundless, 

 by which water had been supposed to be convertible into all the different ingredients en- 

 tering into the composition of the vegetable substance by means of the action of the vital 

 energy of the plant ; yet when water was ultimately proved to be a chemical compound, 

 it was by no means absurd to suppose that plants may possess the power of decomposing 

 part, at least, of what they absorb by the root, and thus acquire the hydrogen as well as a 

 portion of the oxygen which, by analysis, they are found to contain. This opinion was 

 accordingly pretty generally adopted, but was not yet proved by any direct experiment. 

 Senebier pointed out several phenomena from which he thought it was to be inferred, but 

 particularly that of the germination of some seeds moistened mei*ely with water, and so 

 situated as to have no apparent contact with oxygen. The decomposition of water was 

 inferred also by Ingenhouz, from the amelioration of an atmosphere of common air into 

 which he had introduced some succulent plants vegetating in pure water. Saussure having 

 gathered a number of plants of the same species, as nearly alike as possible in all circum- 

 stances likely to be affected by the experiment, dried part of them to the temperature of 

 the atmosphere, and ascertained their weight ; the rest he made to vegetate in pure water, 

 and in an atmosphere of pure oxygen for a given period of time, at the end of which he 

 dried them as before, and ascertained their weight also, which it was thus only necessary 

 to compare with the weight of the former, in order to know whether the plants had in- 

 creased in solid vegetable substance or not. But after many experiments on a variety of 

 plants, the result always was, that plants when made to vegetate in pure water only, and 

 in an atmosphere of pure oxygen, or of common air deprived of its carbonic acid, scarcely 

 added any thing at all to their weight in a dried state or if they did, the quantity was too 

 small to be appreciated. But from a subsequent experiment, in which carbonic acid gas 

 was mixed with common air by the same experiment, the decomposition and fixation of 

 water by the vegetating plant is legitimately inferred. It does not appear, however, that 

 plants do in any case decompose water directly ; that is, by appropriating its hydrogen 

 and at the same time disengaging its oxygen in the form of gas, which is extricated only 

 by the decomposition of carbonic acid. 



1533. Descent of the proper juice. When the sap has been duly elaborated in the leaf 

 by means of the several processes that have just been described, it assumes the appel- 

 lation of the cambium, or proper juice of the plant. In this ultimate state of elaboration 

 it is found chiefly in the bark, or rather between the bark and wood, and may very often 

 be distinguished by a peculiar color, being sometimes white, as in the several species of 

 spurge, and sometimes yellow, as in celandine. It is said to be the principal seat of the 

 medical virtues of plants ; and was regarded by Malpighi as being to the plant what the 

 blood is to the animal body the immediate* principle of nourishment, and grand support 

 of life ; which opinions he endeavours to establish by the following analogies : if the blood 

 escapes from the vessels of the animal body, it forms neither flesh nor bone, but tumors ; 

 if the proper juices of the plant are extravasated, they form neither bark nor wood, but a 

 lump of gum, resin, or inspissated juice. The disruption of the blood-vessels and conse- 

 quent loss of blood, injures and often proves fatal to the animal. The extravasation of 

 the proper juice injures and often proves fatal to vegetables, unless the evil is prevented 

 by the skill and management of the gardener. Whatever may be the value of these re- 

 marks as tending to establish the analogy in question, it cannot be doubted that the cam- 

 bium or proper juice constitutes at least the grand principle of vegetable organisation ; 

 generating and developing in succession the several organs of the plant, or furnishing the 

 vital principle with the immediate materials of assimilation. 



1534. The proper juice is conveyed to the several parts of the plant by an appropriate set of vessels. One 

 of the earliest and most satisfactory experiments on this subject, at least as far as regards the return of 

 the proper juice through the leaf and leaf-stalk, is that of Dr. Darwin, which was conducted as follows : 

 a stalk of the euphorbia heliscopia, furnished with its leaves and seed-vessels, was placed in a decoction 

 of madder root, so as that the lower portion of the stem and two of the inferior leaves were immersed in 

 it. After remaining so for several days the color of the decoction was distinctly discerned passing along 

 the midrib of each leaf On the upper side of the leaf many of the ramifications, going from the midrib 

 towards the circumference, were observed to be tinged with red ; but on the under side there was ob- 

 served a system of branching vessels, originated in the extremities of the leaf and carrying not a red but 

 a pale milky fluid, which, after uniting in two sets, one on each side the midrib, descended along with it 

 into the leaf-stalk. These were the vessels returning the elaborated sap. The vessels observable on the 

 upper surface Darwin calls arteries, and those on the under surface he calls veins. To this may be added 

 the more recent discoveries of Knight, who in his experiments, instituted with a view to ascertain the 

 course of the sap, detected in the leaf-stalk, not onlv the vessels which he calls central tubes, through 

 which the colored infusion ascended, together with their appendages, the spiral tubes; but also another 

 set of vessels surrounding the central tubes, which he distinguishes by the appellation of external tubes, 

 and which appeared to be conveying in one direction or other a fluid that was not colored, but that 

 proved, upon further investigation, to be the descending proper juice. In tracing them upwards they 

 were found to extend to the summit of the leaf, and in tracing them downwards they were found 

 to extend to the base of the le^.talk, and to penetrate even into the inner bark. According 



