174 ANIMALS AND PLANTS vi 



of the food of the bean. But the weights of the 

 carbon, hydrogen, oxygen, nitrogen, phosphorus, 

 sulphur, and other elementary bodies contained in 

 the bean-plant, and in the seeds which it produces, 

 are exactly equivalent to the weights of the same 

 elements which have disappeared from the 

 materials supplied to the bean during its growth. 

 Whence it follows that the bean has taken in only 

 the raw materials of its fabric, and has manu- 

 factured them into bean-stuffs. 



The bean has been able to perform this great 

 chemical feat by the help of its green colouring 

 matter, or chlorophyll; for it is only the green 

 parts of the plant which, under the influence of 

 sunlight, have the marvellous power of decom- 

 posing carbonic acid, setting free the oxygen and 

 laying hold of the carbon which it contains. In 

 fact, the bean obtains two of the absolutely in- 

 dispensable elements of its substance from two 

 distinct sources ; the watery solution, in which its 

 roots are plunged, contains nitrogen but no carbon ; 

 the air, to which the leaves are exposed, contains 

 carbon, but its nitrogen is in the state of a free 

 gas, in which condition the bean can make no use 

 of it ; 1 and the chlorophyll 2 is the apparatus by 



1 I purposely assume that the air with which the bean is 

 supplied in the case stated contains no ammoniacal salts. 



- The recent researches of Pringsheim have raised a host of 

 questions as to the exact share taken by chlorophyll in the 

 chemical operations which are effected by the green parts of 

 plants. It may be that the chlorophyll is only a constant con- 

 comitant of the actual deoxidising apparatus. 



