32 OF THE ASSIMILATION OF CARBON. 



respiration in plants, similar to that of animals, and 

 like it, having for its result the separation of car- 

 bon from some of their constituents. This opinion 

 has a very weak and unstable foundation. 



The carbonic acid, which has been absorbed by 

 the leaves and by the roots, together with water, 

 ceases to be decomposed on the departure of day- 

 light ; it is dissolved in the juices, which pervade 

 all parts of the plant, and escapes every moment 

 through the leaves, in quantity corresponding to 

 that of the water, which evaporates. 



A soil, in which plants vegetate vigorously, con- 

 tains a certain quantity of moisture, which is indis- 

 pensably necessary to their existence. Carbonic 

 acid, likewise, is always present in such a soil, whe- 

 ther it has been abstracted from the air, or has been 

 generated by the decay of vegetable matter. Rain 

 and well water, as well as that from other sources, 

 invariably contain carbonic acid. Plants during 

 their life constantly possess the power of absorbing 

 by their roots moisture, and, along with it, air and 

 carbonic acid. Is it, therefore, surprising, that the 

 carbonic acid should be returned, unchanged, to 

 the atmosphere, along with water, when light (the 

 cause of the fixation of its carbon) is absent ? 



Neither this emission of carbonic acid nor the 

 absorption of oxygen has any connexion with the 

 process of assimilation ; nor have they the slightest 

 relation to one another ; the one is a purely me- 

 chanical, the other a purely chemical process. A 



