ORGANIC SUBSTANCES AS PLANT FOOD 223 



else than very complex polypeptides constructed in the same 

 manner as his artificial polypeptides. 



The investigations of Emil Fischer have made an epoch in the 

 study of proteins, and the above scheme for the structure of a pro- 

 tein molecule was for a long time generally accepted. 



Recent researches, however, have shown that besides such pep- 

 tide bonds in the protein molecule, as well as in the molecule of 

 polysaccharides, there also exist bonds of an anhydrous character, 

 leading to the formation of ring, not open chain, compounds. This 

 forces one to conclude that the protein molecule is of a more com- 

 plex structure than it seemed immediately after the brilliant work 

 of Emil Fischer. This does not mean, however, that the protein 

 molecule is extremely large. On the contrary, there is reason to 

 believe that in colloidal solutions proteins, as well as polysacchar- 

 ides, form aggregates of many molecules, and that the size of each 

 molecule consists not of thousands, as was thought until recently, 

 but only of several, or at most of a few hundreds of atoms. 



A rational classification of protein substances should be based 

 upon their composition, that is, upon the quantitative relationship 

 of the amino acids entering into their structure. This classifica- 

 tion has not yet been worked out, and customarily the old classi- 

 fication, based primarily upon differential solubility of proteins in 

 water and salt solutions, is used. 



The main features of this classification are as follows: First 

 they are separated into simple and complex proteins, or proteids. 

 The latter are combinations of a strictly protein molecule, the 

 structure of which we have just learned, with another complex 

 molecule. If this complex is of a carbohydrate character, we obtain 

 glucoproteids, to which belong many of the mucilages; if it is a 

 complex of nucleic acids, we obtain nucleoproteids. 



Simple proteins are divided into the following four groups: 



1. Albumins — soluble in water. 



2. Globulins— insoluble in water, but soluble in weak solutions of 

 neutral salts. 



3. Prolamins — insoluble both in water and in salt solutions, but soluble 

 in 70 per cent alcohol. 



4. Glutelins — -insoluble in all of the above solvents, but soluble in 

 weak alkalies. 



Reserve proteins of seeds belong to the group of simple pro- 

 teins. Of these, albumins are practically unknown in plants. Of 



