THE FOOD OF PLANTS 129 



do not become food until a considerable amount of work 

 has been done upon them by the plant itself. 



It thus appears that, in the strict sense, the ordinary 

 green plant does not absorb its food from without. It 

 takes in various raw materials from which it manufactures 

 its food in particular parts of its own tissues. 



In connection with the nutrition of plants we have thus 

 to deal with the absorption of the crude food materials, 

 and to study the changes which they undergo after such 

 absorption. But this is not all ; the food which is manu- 

 factured from them is not merely prepared in answer to 

 the immediate requirements of the moment. A consider- 

 able excess is usually constructed, and the surplus quantity 

 is stored in various parts of the plant's body for subsequent 

 consumption. 



The food which is thus laid up in seeds, tubers, bulbs, 

 &c. is not deposited there in exactly the condition in which 

 the living substance requires it, so that there remains for 

 us to consider the processes of storage, and the changes 

 which the stored materials subsequently undergo for the 

 purpose of feeding the living protoplasm. 



The construction of food from the materials absorbed is 

 one of building up complex bodies from simple materials. 



The utilisation of the stored surplus is comparable with 

 the digestion which is so marked a feature of animal alimenta- 

 tion, and is one of breaking down of complex bodies into 

 simpler ones. 



The actual nutrition of the protoplasm shows again two 

 distinct phases : the incorporation into its substance of the 

 ultimate constituents of the food, or its assimilation, is a 

 constructive process ; it is in turn associated with a destruc- 

 tive one, an auto -decomposition of the protoplasm itself, by 

 which simpler bodies are produced from it. 



The whole round of changes which embraces all these 

 operations is called metabolism, the constructive processes 

 being grouped together under the name of andbolism, the 

 destructive ones under that of katabolism. 



9 



