THE CONSTRUCTION OF PROTEINS 169 
Neither peptones nor proteoses occur very plentifully 
in plants, and they are probably formed in them only from 
the decomposition of the more stable forms of globulin and 
albumin. There is no evidence at present that they are 
stages in the constructive process. 
Some of the proteoses occur in certain seeds in associa- 
tion with some of the globulins. Both the albumins and 
the globulins, and probably the meta-proteins as well, are 
transformed into proteoses and peptones by the action of 
hydrolysing agents, such as dilute mineral acids and certain 
secretions of the protoplasm known as enzymes, whose 
action will be treated of in a subsequent chapter. 
Besides these classes of proteins, another occurs, the 
members of which present the curious peculiarity of being 
soluble in alcohol. Proteins of this group have been extracted 
from the endosperm of some of the cereal grasses. Examples 
of them are found in the zein of maize, and the gliadin and 
glutenin of wheat flour. Zein differs from the crystallised 
protein of the hemp in its comparatively low content of 
nitrogen, which amounts to only 16°18 per cent. It dissolves 
easily in alcohol of about -820 specific gravity, but is 
insoluble in absolute alcohol. It is insoluble also in water, 
but in mixtures of alcohol and water it dissolves to a greater 
or less extent, being most easily soluble in a mixture con- 
taining about 90 per cent. of the spirit. In one of a lower 
concentration than 50 per cent. it is very sparingly soluble. 
Zein can be dissolved by glycerine if heated to 150° C.; 
also by glacial acetic acid and by dilute solutions of caustic 
potash. Like other proteins, it is converted into peptone 
by peptase and hydrochloric acid. 
The original construction of protein matter, like that 
of carbohydrates, seems to be carried out only by 
vegetable protoplasm. It does not, however, appear to be 
dependent upon the same conditions as the process already 
described. It cannot be classed with the latter as a process 
of photosynthesis, and it is only indirectly dependent 
upon the action of the chlorophyll apparatus. Unlike the 
