268 VEGETABLE PHYSIOLOGY 



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 

 of nutritive material takes place. If during the secretion this 

 supply is stopped, the process is rapidly suspended. This 

 can be detected easily in the case of the epithelium of the 

 scutellum of the barley grain, which we have seen produces 

 considerable quantities of diastase. The first stage of the 

 process is thus evidently anabolic. As soon as the nutrition 

 of the cell has reached a certain point the appearance of 

 the protoplasm undergoes a change. Minute granules 

 begin to be formed in its substance, which increase in 

 number until the hyaline character is replaced by a 

 marked uniform granularity, the cell substance becoming 

 somewhat like ground-glass in appearance. The growth 

 of the protoplasm and this subsequent formation of granules 

 lead to the obliteration of the vacuoles, till the cell is com- 

 pletely filled. After a time as the secretion leaves the cell 

 the latter shrinks again ; the granules are passed out in 

 solution in the sap which is exuded, and the protoplasm is 

 seen to be less plentiful and to become hyaline and vacuo- 

 lated as at first. 



Following the anabolic changes we have thus the breaking 

 down of the protoplasm, attended by the appearance of 

 the granules to which it has given rise. There is reason 

 to believe that the granules consist of the zymogen rather 

 than the enzyme, and that the final transformation of 

 the former into the latter takes place just as the exuda- 

 tion of the sap occurs. 



In glands in which the process of secretion is repeated 

 more than once, similar changes may be traced. The 

 secretion of the enzyme in these cases can be shown to 

 take place by successive stages. The preliminary hyaline 



