54 PLANT PHYSIOLOGY 



It is assumed at present that the protein molecule is very large 

 and that the molecular weight of protein substances may be 

 several hundred thousand. Such a large size for the molecule 

 explains the fact, observed long ago, that solutions of protein 

 substances always possess a colloidal character and the bound- 

 ary between molecules and micelles is effaced in them. The 

 large molecules can be thrown out of a solution by high-speed 

 centrifugation. 



A reasonable classification of protein substances must be based 

 on their composition, i.e., on the quantitative relations of the 

 amino acids composing them. Such a classification, however, 

 has not yet been perfected, and it is customary to keep to the 

 old order based chiefly on the different solubility of proteins in 

 water and in salt solutions. 



The main features of this classification may be stated thus. 

 First of all, proteins are divided into two groups, the simple 

 proteins and the complex proteins. The latter represent com- 

 bination of a protein of the structure described above with some 

 other complex molecule. If the latter is of a carbohydrate 

 character, glucoproteids are obtained, which include certain 

 mucous substances. If it is a complex of nucleic acids, nucleo- 

 proteins are obtained, which represent essential substances of 

 the cell nucleus. 



Simple proteins are divided into five groups: 



1. Albumins — soluble in water, 



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



salts. 



3. Prolamins — insoluble either in water or in salt solutions but soluble in 



70 per cent alcohol. 



4. Glutelins — insoluble in any of the foregoing solvents but soluble in 



dilute alkalies. 



5. Histones — not found in a free state but in combination with nucleo- 

 proteins and other complex proteins. 



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

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

 Of the globulins, legumin and vicilin are found in the seeds of 

 peas and some other legumes; phaseolin, in seeds of the kidney 

 bean; conglutin, in lupine; and edestin, in hemp seeds. Of 

 prolamins, the most important is gUadin, in wheat gluten; 

 hordein, in barley; and zein, in maize. Of the glutelins, the 



