180 VEGETABLE PHYSIOLOGY 



constructive process. When proteids are digested in the 

 animal organism under the influence of the strong ferment 

 of the pancreatic secretion, we find that among the pro- 

 ducts of the decomposition certain nitrogenous compounds 

 occur which are crystalline and capable of diffusing through 

 animal and vegetable membranes. These substances, the 

 chief of which are tyrosin and leucin, with a little asparagin, 

 are known technically as amido-acids, owing to their con- 

 taining the group NH 2 (amidogen), replacing one atom of 

 hydrogen in the hydrocarbon group of an organic acid. It 

 is extremely probable that these compounds are made use of 

 again in the subsequent reconstruction of proteids in the 

 cells. Many of these substances have been found to occur in 

 plants, and among them asparagin is extremely conspicu- 

 ous. It can be detected in seeds and seedlings, and in older 

 plants it is not infrequently present in the leaves. There is 

 considerable probability that these substances occur as a 

 stage in the original construction of proteids, though they 

 may no doubt also be formed during its digestion in the 

 vegetable as well as in the animal organism. This 

 probability is supported by the observation that green plants 

 are able to absorb from the soil and utilise many such 

 amido-acids when artificially supplied to them. 



Another hypothesis of proteid construction has been 

 advanced which takes account of these substances as stages 

 in the process. We have seen that salts of ammonia are 

 converted into nitrates in the soil before being absorbed. 

 The first step in the construction is thought to be the recon- 

 version of the nitrates into ammonia, which interacts in 

 some way with formaldehyde or one of its polymerides to 

 form one or other of these amido-acids. This subsequently 

 combines with some kind of non-nitrogenous organic sub- 

 stance together with some compound of sulphur, to form 

 proteid. 



This view of the construction, though of course only 

 hypothetical, associates certain processes which apparently 

 occur in nature. The formation of proteid seems to involve 



