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, (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 
