METABOLISM 261 



substances which are useless to them or which may even 

 be deleterious. There are numerous products which 

 come under this category, but from the mode of construc- 

 tion of the body of the plant they are not cast off as they 

 would be from the animal organism under similar conditions. 

 Instead of being eliminated entirely they are only removed 

 to such localities as ensure their being withdrawn from 

 the spheres of vital activity. They are generally deposited 

 in such regions as leaves which are about to be shed, or 

 the bark of trees, which is a collection mainly of dead 

 matter ; or they may be stored away in special cells, or in 

 cell-walls, or intercellular passages, or elsewhere. These 

 bodies really correspond to excreta, and the processes of 

 their formation and deposition are called processes of 

 excretion. 



Most of the katabolic constructive processes are directly 

 applied to the production of substances which are of great 

 use to the plant. Emanating as these do directly from the 

 protoplasm, their formation is generally termed secretion. 



Though they originate, however, directly in and from 

 the living substance, the latter does not always present 

 them in the form in which they are found in the adult 

 plant-body, for various changes both of the mature of oxida- 

 tion and reduction may take place in them after they have 

 been secreted. 



The processes included under the general term katabolism 

 are thus seen to be very varied. During the course of such 

 changes many substances are frequently formed which 

 seem to have no direct bearing on the vital processes, and 

 whose meaning is still obscure. These are often spoken of 

 as the bye-products of metabolism. 



We may now pass to consider in some detail some of 

 the more prominent processes of secretion. 



For many reasons the formation of such enzymes as are 

 used during digestion may be regarded as the most typical 

 of these. A cell which is about to secrete is generally 

 found to be filled with colourless hyaline protoplasm in 



