156 METABOLISM 



such an excretion of diastase does take place from cells, and although it has 

 been shown that it is a phenomenon specially characteristic of the grass 

 embryo, e.g. by BROWN and MORRIS (1890), it cannot be said that the evidence 

 for it is altogether above criticism (Lraz, 1896 ; GRUSS, 1897). 



As noted above, a reducing sugar was always found in the fluid used in 

 the experiments where the endosperms of grasses were allowed to undergo 

 transformation in absence of an embryo ; in addition, however, cane sugar also 

 appears in not inconsiderable quantity, that is to say, another disaccharide 

 capable of reducing only after treatment with hot acids. This sugar, according 

 to our present knowledge, cannot be produced by hydrolysis of starch, and so its 

 appearance requires to be explained. At present we must be content to believe 

 that it is present as such in the seed, a view not entirely improbable, seeing that 

 SCHULZE (1899) has found cane sugar in seeds of the Gramineae. Again PURIE- 

 WITSCH has shown, from other investigations, that the emptying of the endo- 

 sperm is not due simply to diffusion. He agrees with HANSTEEN in believing that 

 the emptying takes place more rapidly if large quantities of water be supplied 

 with the view of dissolving the sugar. The explanation seems simple enough : 

 diffusion can take place only so long as the fluid outside is less concentrated than 

 that in the cells ; for if the sugar formed in the cells cannot diffuse out then the 

 hydrolysis of starch ceases. Thus PURIEWITSCH found that far less starch was 

 dissolved from the endosperm when he used in place of water a 1-3 per cent, 

 solution of cane sugar or dextrose. Although it has not been definitely proved 

 to be the case that dextrose is actually formed from starch, one may still regard 

 these two sugars as the direct cause of the inhibition of diffusion. The situa- 

 tion is quite altered when glycerine, potassium nitrate, and sodium chloride 

 are used, for PURIEWITSCH found that these substances vigorously inhibited 

 the dissolution of starch. It is possible to explain this result physically, 

 but one may doubt the purely physical action of cane sugar and dextrose, and 

 we must remember, further, that the cells of the endosperm are living, and as 

 such are affected in a variety of ways by the environment. Sufficient evidence 

 is not forthcoming to show how it is that organisms are able to inhibit the 

 action of an enzyme, but we do know of many facts which prove to us that 

 they have this power. PURIEWITSCH, for example, found that the removal of 

 starch ceased when he supplied the endosperm (in the absence of the seedling) 

 with air free from oxygen, or with air containing chloroform ; and yet we 

 know that, apart from the cell, a solution of diastase acts equally well on 

 starch whether oxygen or chloroform be present or not. Experiments such as 

 these are full of lessons for us. It is only right that great stress should be laid 

 on the study of enzymes in modern physiological chemistry, since these bodies 

 obviously have important functions to perform in the organism ; but we must 

 not hope for too great results from such studies. We may learn in this way 

 what reagents the living cell uses in dealing with these substances, and so 

 imitate in many cases what goes on in the cell, by reproducing in test-tubes 

 the chemical transformations that go on in the organism, with the aid of acids 

 or enzymes ; but these transformations are not those which are characteristic 

 of the organism ; the secret lies in controlling the enzyme so that at one 

 moment it is active, at another it is quiescent. Further, we can give no 

 general answer to the question how an accumulation of the products of the 

 reaction interferes with the activity of the enzyme. Probably substances 

 acting as inhibiting agents or poisons to the enzyme play a principal part 

 in retarding enzymic activity (compare CZAPEK, 1903), but as yet we are 

 unable to understand how the organism governs the production at the same 

 time of an enzyme and an anti-enzyme. It cannot be doubted, however, that 

 such a co-ordination or appropriate production of these bodies really takes 

 place. Similar cases of regulation of secretions occur everywhere in organisms, 

 as we shall find later on. 



