160 ANIMALS AND PLANTS. [LECT. 



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 manufactured 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 decomposing carbonic acid, 

 setting free the oxygen and laying hold of the carbon 

 which it contains. In fact, the bean obtains two of the 

 absolutely indispensable 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 which the 

 carbon is extracted from the atmospheric carbonic 

 acid the leaves being the chief laboratories in which 

 this operation is effected. 



The great majority of conspicuous plants are, as 

 everybody knows, green ; and this arises from the 



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

 in the case stated contains no ammoniacal salts. 



2 The recent researches of Pringsheim have raised a host of ques- 

 tions as to the exact share taken by chlorophyll in the chemical opera- 

 tions which are effected by the green parts of plants. It may be that 

 the chlorophyll is only a constant concomitant of the actual deoxidising 

 apparatus. 



