THE CONSTBUCTION OF PEOTEIDS 181 



the simultaneous presence in the cells of some amido-acid, 

 frequently asparagin, and some carbohydrate such as sugar. 

 If shoots of plants which exhibit no accumulation of 

 asparagin during normal growth are cut off and kept in 

 darkness for some time, a gradual accumulation of the 

 amido-acid can be observed. This in all probability is the 

 expression of the decomposition of proteid taking place during 

 the life of the shoot, and is presumably a normal occur- 

 rence. The reconstruction which would explain its non- 

 accumulation during illumination is prevented by the non- 

 formation of the needed carbohydrate in the darkness. 



The probability of a combination or interaction of these 

 two classes of substance in the synthesis of proteids is 

 supported by the fact that at the active growing points, where 

 protoplasm is energetically formed, and where consequently 

 abundant supplies of proteids are needed, neither sugar nor 

 amido-acids can be detected, though they can be traced quite 

 readily up to a short distance below the place where this 

 active growth is proceeding. This fact is easily understood 

 if we admit that proteid is constructed there at the expense 

 of these two constituents, supplemented, of course, by the 

 necessary compound or compounds of sulphur. If either of 

 these supplies ceases to be available, the growth of the plant 

 at that point stops. 



Though we have seen reasons for thinking that nitrates 

 and amido-acids form two stages in the normal process of 

 proteid construction, we must not conclude that they in- 

 variably do so. In one plant, Pangium edule, which was 

 examined by Treub in 1894, the nitrogen needed for pro- 

 teid construction appears to be supplied in the form of 

 hydrocyanic acid. In the shoots of this plant, cells occur 

 in the cortex which contain this acid. In those nearest the 

 apex the latter occurs alone, but as they grow older, a little 

 proteid is found to be mixed with it. In still older ones 

 the proteid preponderates, and at some distance behind the 

 seat of growth it occurs alone, the acid having disappeared. 

 Certain fungi can utilise nitrogen-containing derivatives of 



