172 VEGETABLE PHYSIOLOGY 



animal organism under the influence of the strong tryptase 

 of the pancreatic secretion, we find that among the products 

 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 amino- and amido-acids, owing to 

 their containing the group NH 3 (amidogen), replacing an 

 atom of hydrogen in some portion of the grouping of the 

 atoms in an organic acid. It is extremely probable that 

 these compounds are made use of again in the subsequent 

 reconstruction of proteins in the cells. Many of these 

 substances have been found to occur in plants, and among 

 them asparagin is extremely conspicuous. It can be 

 detected in seeds and seedlings, and in older plants it is 

 not infrequently present in the leaves. There is consider- 

 able probability that these substances occur as a stage in 

 the original construction of proteins, 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 complex acids 

 when artificially supplied to them. 



Another hypothesis of protein 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 re- 

 conversion of the nitrates into ammonia, which interacts in 

 some way with formaldehyde or one of its polymerides to form 

 one or other of these complex acids. This subsequently 

 combines with some kind of non-nitrogenous, organic sub- 

 stance, together with some compound of sulphur, to form 

 protein. 



A theory of the method of the first formation of amido- 

 compounds was put forward by Bach in 1896. He suggested 

 that the absorbed nitrates are decomposed by the organic 

 acids of the plant, and that the liberated nitric acid is 



