320 



ASSIMILATION. 



The following table bj- Boussingault ^ is very instructive, as it 

 shows the relation of volume between the amount of carbonic 

 acid consumed and the oxj-gen evolved in assimilation ; and 

 also the decomposing power of various kinds of plants under 

 different conditions.^ 



843. The gas emitted during the process of assimilation proper 

 is not pure oxygen. Both Daubenj-' and Draper* found varia- 

 ble amounts of nitrogen in all the cases examined b3' them. 



844. What are the products of assimilation proper? It has 

 now been shown under what conditions the green tissues of a 

 plant decompose carljonic acid and evolve oxygen.^ As the 

 chief result of this decomposition and its associated processes, 

 there is formed witliiu the cells which contain chlorophyll a carbo- 

 hydrate of some kind. This carboh3drate contains the same 

 elements as the carbonic acid and the water fi-om which it was 

 produced, but it contains less oxygen than the total amount 

 found in those substances taken together. Hence the process 

 of assimilation is essentiall}- one of reduction. There is, how- 

 ever, no substantial agreement as to the nature or constitution 

 of the primary carbohydrate formed by it. 



The difficulty wliich attends the investigation of assimilation 



1 Agronomie, iv., 1868, p. 286. 



2 A well-known relation of volume between oxygen and carbonic acid may 

 here be pointed out ; namely, that "free oxygen occupies the same bulk as the 

 carbonic acid produced by uniting it with carbon." 



* Philosophical Transactions, 1836. 



* " In every instance which I have examined, the gas evolved from leaves 

 is not pure oxygen, but a variable mixture of oxygen and nitrogen. This result 

 is of uniform occurrence" (Chemistry of Plants, 1844, p. 182). 



5 For an account of the transient evolution of oxygen under exceptional 

 circumstances where carbonic acid is not present, see Mtiller's Handbuch der 

 Botanik, 1880. 



