40 Elementary Biology. 



the chlorophyll body. The significance of these changes in 

 constitution will be discussed later on. 



The percentage chemical composition of chlorophyll has 

 been lately ascertained, and is as follows : 



Carbon 73-34 



Hydrogen . . . . . .972 



Isitrogen . . . . . .5-68 



Oxygen 9-54 



Phosphorus . . . . . I -38 

 Magnesium ..... 0-34 



lOO'OO 



It has already been noted that in consequence of meta- 

 bolic changes constantly taking place in organisms, large 

 quantities of carbonic acid were continually being produced. 

 Notwithstanding this fact, the quantity of carbonic acid in 

 the atmosphere is tolerably nearly a constant quantity. The 

 question immediately arises, What becomes of it? In brief 

 terms it may be said that it forms the main constituent of 

 the food of plants, although they themselves, in common with 

 animals, excrete considerable quantities of it. Yet, gene- 

 rally speaking, the quantity they absorb is not only largely in 

 excess of their own production, but balances the production 

 of the same gas by the animal, on which it acts as a poison. 

 Plants are able to make use of this gas as food in virtue ot 

 their possessing chlorophyll, which in presence of sunlight 

 is capable of bringing about the assimilation of carbonic 

 acid by vegetal protoplasm. It is evident, therefore, that 

 chlorophyll is a substance of the very highest importance, 

 not only in the vegetal economy but indirectly in the animal 

 economy also. 



Further, it is manifest that the absorption by the vegetal 

 of large quantities of carbonic acid must mean the reception 

 into the vegetal organism of an enormous amount of oxygen 

 far more than is necessary for the carrying on of the 

 metabolic changes taking place in vegetal cells. As a 

 matter of fact the oxygen is in great part given back into the 



