Book [. PROCESS OF VEGETABLE NUTRITION. 239 



plant, and the contrary, the following is the sum of the results: — The green parts of plants, hut especially 

 the leaves, when exposed in atmospheric air to the successive influence of light ami shade, inhale and 

 evolve alternately a portion of oxygen gas mixed with carbonic acid. But the oxygen is not immediately 

 assimilated to the vegetable substance ; it is first converted into carbonic acid by means of combining with 

 the carbon of the plant, which withers if this process is prevented by the application of lime or potass. 

 The leaves of aquatics, succulent plants, and evergreens consume, in equal circumstances, less oxygen 

 than the leaves of other plants. The roots, wood, and petals, and in short all parts not green, with the 

 exception of some coloured leaves, do not effect the successive and alternate inhalation and extrication of 

 oxygen ; the inhale it indeed, though they do not again give it out, or assimilate it immediately, but con. 

 vev it under the form of carbonic acid to the leaves, where it is decomposed. Oxygen is indeed assimilated 

 to the plant but not directly, and only by means of the decomposition of carbonic acid ; when part of it, 

 though in a very small proportion, is retained also and assimilated along with the carbon. Hence the most 

 obvious influence of oxygen, as applied to the leaves, is that of forming carbonic acid gas, and thus pre- 

 senting to the plants elements which it may assimilate ; and perhaps the carbon of the extractive juices 

 absorbed even by the root, is not assimilated to the plant till it is converted by means of oxygen into car- 

 bonic acid. But as an atmosphere composed of nitrogen and carbonic acid gas only is not favourable to 

 vegetation, it is probable that oxygen performs also some other function beyond that of merely presenting 

 to the plant, under the modification of carbonic acid, elements which it may assimilate. It may affect also 

 the disengagement of calorie by its union with the carbon of the vegetable, which is the necessary result 

 of such union. But oxygen is also beneficial to the plant from its action on the soil ; for when the ex- 

 tractive juices contained in the soil have become exhausted, the oxygen of the atmosphere, by penetrating 

 into the earth and abstracting from it a portion of its carbon, forms a new extract to replace the first. 

 Hence we mav account for a number of facts observed by the earlier phytologists, but not well explained. 

 Du Hamel remarked that the lateral roots of plants are always the more vigorous the nearer they are 

 to the surface ; but it now appears that they are the most vigorous at the surface because they have there 

 the easiest access to the oxygen of the atmosphere, or to the extract which it may form. It was observed, 

 also, by the same phvtologist, that perpendicular roots do not thrive so well, other circumstances being the 

 same, in a stiffand wet soil as in a friable and dry soil ; while plants with slender and divided roots thrive 

 equally well in both : but this is, no doubt, owing to the obstacles that present themselves to the passage of 

 the oxygen in the former case, on account of the greater depth and smaller surface of the root. It was 

 further observed, that roots which penetrate into dung or into pipes conducting water, divide into immense 

 numbers of fibres, and form what is called the fox-tail root ; but it is because they cannot continue to 

 vegetate, except by increasing their points of contact, with the small quantity of oxygen found in such 

 mediums. Lastly, it was observed that plants, whose roots are suddenly overflowed with water remaining 

 afterwards stagnant, sutler sooner than if the accident had happened by means of a continued current. It 

 is because in the former case the oxygen contained in the water is soon exhausted, while in the latter it is 

 not exhausted at all. Hence also we may account for the phenomenon exhibited by plants vegetating 

 in distilled water under a receiver filled with atmospheric air, which, having no proper soil to supply the 

 root with nourishment, effect the developement of their parts only at the expense of their own prcper 

 substance ; the interior of the stem, or a portion of the root, or the lower leaves, decaying and giving up 

 their extractive juices to the other parts. — Thus it appears that oxygen gas, or that constituent part of the 

 atmospheric air which has been found to be indispensable to the life of animals, is also indispensable to the 

 life of vegetables. But, although the presence and action of oxygen are absolutely necessary to the process 

 of vegetation, plants do not thrive so well in an atmosphere of pure oxygen, as in an atmosphere of pure or 

 common air. This was proved by an experiment of Saussure's, who, having introduced some plants of 

 Pisum sativum, that were but just issuing from the seed, into a receiver containing pure oxygen gas, 

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

 the same time into a receiver containing common air. 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. 



1560. 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 merely 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 inlo 

 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 expeiiments 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 similar experiment, in which carbonic acid gas was 

 mixed with common air, the decomposition and fixation of water by the vegetating plant 

 are 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. 



1561. Descent of the proper juice. When the sap has been duly elaborated in the leaf 



