INFLUENCE OF OXYGEN GAS UPON NUTRITION. 83 



oxygen which they can absorb. The oxygen absorbed by 

 leaves enters into a state of combination in them ; the oxy- 

 gen which can be disengaged from them in a vacuum, by 

 means of heat, amounts to only ^^ of the volume absorbed ; 

 the gas thus extracted is not pure, but consists of azote, 

 carbonic acid, and oxygen. 



It is very probable that the oxygen absorbed by plants 

 growing in darkness, combines with their carbon to form 

 carbonic acid ; this remains in solution in their juices, till 

 the sun effects its decomposition, when the oxygen is 

 thrown out into the air by the transpiration of the leaves, 

 whilst the carbon enters into the composition of the 

 plants. 



Plants can unfold only in an atmosphere containing 

 oxygen ; nevertheless, they thrive less in the shade in pure 

 oxygen, than if it be combined with other gases, as azote 

 and carbonic acid. 



The leaves of different plants do not consume in the 

 shade the same quantity of oxygen. Those of fleshy plants 

 absorb but little, which they retain obstinately ; and dis- 

 engage a still less quantity of carbonic acid. As these 

 plants preserve better than others their carbon, and require 

 but a small quantity of oxygen, they can live in soils of 

 but little fertility : they will •flourish upon heights where 

 the air is much rarefied, and upon arid sands. 



The leaves of those trees which are naked during the 

 winter, are, in general, those which absorb the most oxy- 

 gen, and contain the most carbon. Not only do these 

 plants prepare all the juices which are essential to vegeta- 

 tion, and to the formation of fruits ; but after having ful- 

 filled these functions, they continue to extract, from the 

 earth and air, the principles of their nourishment ; these 

 they elaborate and deposit between the bark and the wood, 

 to serve for their first aliment at the return of spring, till 

 the developement of the leaves and the excitement of the 

 roots by heat, can provide for their nourishment by the 

 absorption of foreign substances. The experiments of 

 Mr. Knight have established this theory. 



This phenomenon in vegetation bears a close resem- 

 blance to that which we observe to take place in the 

 greatest number of insects, in some birds, and in many 

 quadrupeds ; which become torpid during the winter, and 

 are nourished, whilst in that state, by the fat deposited in 

 their cellular membranes during the autumn. 



