ITS SOURCE, THE ATMOSPHERE. 25 



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. 



In the second part of this work, we shall adduce 

 satisfactory proofs that decayed woody fibre (humus) 

 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. 



Vegetable physiologists consider the formation 

 of woody fibre from humus as very simple ; they 

 say, humus has only to enter into chemical com- 

 bination with water, in order to effect the formation 

 of woody fibre, starch, or sugar *. 



But the same philosophers have informed us, 

 that aqueous solutions of sugar, starch and gum, 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 proportions. 



All the erroneous opinions concerning the mo- 



* Mcyen, Pflanzenphysiologie, ii. s. 141. 



