DIGESTION 259 



violet rays being apparently most powerful in effecting 

 their destruction. They are often injuriously affected by 

 neutral salts, alkalis, or acids, though in this respect there 

 exists considerable diversity throughout the group. 



The enzymes are manufactured by the protoplasm of 

 the various cells in which they occur, being produced from 

 its own substance, in a manner somewhat similar to that 

 of the formation of the cell-wall. Usually their presence 

 is accompanied by a marked granularity of the protoplasm, 

 due to the formation in it of an antecedent substance, known 

 as a zymogen, which is readily converted into the enzyme. 

 This granularity does not, however, always occur, though 

 we have reason to suppose that the secretion of the 

 enzyme always takes place by successive stages. The 

 zymogen has not, however, been definitely detected in all 

 cases. 



We find various degrees of completeness of differentia- 

 tion of the cells which produce these enzymes. In the 

 simplest cases, such as the mesophyll of the leaves of most 

 plants, or the great majority of seeds, or the tubers of the 

 potato, the enzyme is found in all the cells which contain 

 the reserve materials, so that a rapid transformation of 

 the latter is readily possible. In the Horse-radish and 

 many allied plants the cells which secrete the enzyme do 

 not themselves contain any reserve materials, but are 

 situated among those which do, so that the enzyme has to 

 pass from the seat of its formation to other cells in order 

 to discharge its function. This is a very slow and gradual 

 process, and is probably carried out through the agency of 

 the delicate filaments of protoplasm which extend through 

 the cell-walls, for enzymes are not capable of dialysing 

 through a membrane. 



The occurrence of such cells, which are apparently set 

 apart especially for the secretion of an enzyme, gives us, 

 as it were, the starting-points of the special structures 

 known as glands, whose function is similar but whose 

 structure is more complex. In some of the plants belong- 



s 2 



