ASSIMILATION. 193 



have seen that many plants must, and all may, imbibe the whole or 

 a part of their food directly from the air into their leaves (330). 

 All leafy plants evidently 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 ex- 

 posed to full sunshine, some carbonic acid disappears, and an equal 

 bulk of oxygen gas supphes 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 returned all its oxygen to the 

 air. Plants take cai-bonic 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 vegetation 

 appropriated as tissue, can hardly be directly formed in the first in- 

 stance. The substances from which it must originate, and which 

 actually abound in the elaborated sap, are Dextrine or Vegetable 

 Mucilage (79, 83), Sugar (80), &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 

 tliree 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 unsolidified 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 would the result in any 

 respect be altered if Starch were directly produced. This substance 

 is merely dextrine, which, instead of being immediately appropriated 

 in growth, is condensed into sohd grains, and in that compact and 



* 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 circumstances : and water is 

 certainly decomposed, with the evolution of oxygen, in the formation of a class 

 of vegetable products soon to be mentioned ; besides, Edwards and Colin have 

 shown that water is directly decomposed during germination. Still, as no one 

 supposes that the residue after the liberation of oxygen is carbon and water, 

 but only the 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 tlte carbonic acid, derived partly 

 from it and partly from the water, is liberated in such cases. 



17 



