ASSIMILATION. 197 



330). All leafy plants doubtless obtain a part of their carbonic 

 acid in this way. It is accordingly found, that when a current of 

 carbonic acid is made slowly to traverse a glass globe containing 

 a leafy plant exposed to the full sunshine, the carbonic acid disap- 

 pears, and an equal bulk of oxygen gas supplies its place. Now, 

 since carbonic acid gas contains just its own bulk of oxygen, it is 

 evident that what has thus been decomposed in the leaves has re- 

 turned all its oxygen to the air. Plants take carbonic acid from 

 the atmosphere, therefore (directly or indirectly) ; they retain its 

 carbon ; they give back its oxygen.* 



349. But cellulose, being the final, insoluble product of vegeta- 

 tion appropriated as tissue, cannot itself be formed in the first in- 

 stance. The materials from which it is deposited, and which we 

 actually find in the elaborated sap, are Dextrine pr Vegetable Mu- 

 cilage (81, 83), sugar (84), &c. The first, of these is probably 

 directly produced in assimilation. Its chemical composition is the 

 same as that of pure cellulose : it consists, not only of the same 

 three elements, but of the same elements in exactly the same pro- 

 portion. Dextrine, vegetable mucilage, &c., are the primary, as yet 

 unappropriated materials of vegetable tissue, or usolidified cellu- 

 lose, and their production from the crude sap is attended with the 

 evolution of the oxygen which was contained in the carbonic acid 

 of the plant's food, as already stated.! Nor is the result in any 



* At least, the result is as if the oxygen exhaled were all thus detached 

 from the carbon of the carbonic acid. Just this amount is liberated, and the 

 facts obviously point to the carbonic acid as its real source. But, on the other 

 hand, it appears unlikely that a substance which holds oxygen with such 

 strong affinity as carbon should yield the whole of it under these circumstan- 

 ces : and water is certainly decomposed, with the evolution of oxygen, in the 

 formation off a class of vegetable products soon to be mentioned; besides, Ed- 

 wards and Colin have shown that water is directly decomposed during germi- 

 nation. Still, as no one supposes that the residue after the liberation of oxy- 

 gen is carbon and water, but onlylhe three elements in the proportions which 

 would constitute them, it amounts to nearly the same thing whether we say 

 that the oxygen of the carbonic acid, or an amount of oxygen equivalent to that 

 of the carbonic acid, derived partly from it and partly from the water, is liber- 

 ated in such cases. That Schleiden should assert that the oxygen liberated 

 comes from the decomposition of water alone, shows gross carelessness, or an 

 ignorance of the elements of arithmetic as well as chemistry, which is the less 

 excusable in one who, in a scientific treatise, habitually applies opprobrious 

 epithets to a great part of his fellow-laborers. 



t The result is just the same, if, with Henfrey, we suppose that the mat- 

 17* 



