METABOLISM 275 



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 to such localities as ensure their being withdrawn from 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 nature 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 

 which certain vacuoles may be seen. Immediately before 

 secretion begins an increase of the amount of the protoplasm 

 can be observed, which is effected at the expense of various 

 nutritive products which are transported to it. During the 

 whole of the process, when this is prolonged, such a supply 



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