48 OF THE ASSIMILATION OF CARBON. 



experienced neither increase nor diminution in any- 

 appreciable quantity. For if it received an addi- 

 tional quantity to its usual proportion, an increased 

 vegetation would be the natural consequence, and 

 the excess would thus be speedily removed. And, 

 on the other hand, if the gas was less than the 

 normal quantity, the progress of vegetation would 

 be retarded, and the proportion would soon attain 

 its proper standard. 



The most important function in the life of plants,, 

 or, in other words, in their assimilation of carbon, 

 is the separation, we might almost say, the genera- 

 tion of oxygen. No matter can be considered as 

 nutritious, or as necessary to the growth of plants, 

 which possesses a composition either similar to or 

 identical with theirs, and the assimilation of which, 

 therefore, could take place without exercising this 

 function. The reverse is the case in the nutrition 

 of animals. Hence such substances as sugar, starch, 

 and gum, which are themselves products of plants, 

 cannot be adapted for assimilation. And this is 

 rendered certain by the experiments of vegetable 

 physiologists, who have shown that aqueous solutions 

 of these bodies are imbibed by the roots of plants, 

 and carried to all parts of their structure, but are 

 not assimilated ; they cannot therefore be employed 

 in their nutrition. We could scarcely conceive a 

 form more convenient for assimilation than that of 

 gum, starch, and sugar, for they all contain the 

 elements of woody fibre, and nearly in the same pro- 

 portions. 



In the second part of the work we shall adduce 

 satisfactory proofs that decayed woody fibre (Jiumus) 

 contains carbon and the elements of water, without 

 an excess of oxygen ; its composition differing from 

 that of woody fibre in its being richer in carbon. 



Misled by this simplicity in its constitution, phy- 

 siologists found no difficulty in discovering the mode, 

 of the formation of woody fibre ; for they say,* hu- 



* Meyen, PJlanzenphysiologief II. S. 141. 



