THE FOOD OF PLANTS 123 
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 considerable 
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, 
&ce. 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 
alimentation, 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 
destructive one, by which, from the protoplasm itself, and 
by its own activity, simpler bodies are produced. 
The whole round of changes which embraces all these 
operations is called metabolism, the constructive processes 
being grouped together under the name of anabolism, the 
destructive ones under that of katabolism. 
The absence of well-differentiated organs set apart for 
the discharge of these separate functions makes it rather 
difficult at first to appreciate their independence. In most 
animal organisms such a differentiation is easily seen, but 
