170 PLANT PHYSIOLOGY 



The peptones are further decomposition products which still 

 give the protein reactions including that of the biuret. They are 

 not precipitated by saturated solutions of ammonium sulphate 

 and are unique among proteins in this respect. The peptids are 

 the last products formed before the amino-acid stage is reached. 

 They are made up of small numbers of amino acids and may or 

 may not give a biuret reaction. 



Comparison of Plant and Animal Proteins. — Animals contain 

 a greater variety of proteins than plants. Of the simple proteins, 

 three groups — histones, protamines, and albuminoids — have not 

 yet been found in plants, while the conjugated proteins are rare. 

 This difference is partly counterbalanced by the presence of glia- 

 dins and glutelins only in plants. The fact that animals contain 

 a greater variety of proteins is associated with the difference in 

 the lives of the two groups of organisms and the wider variety in 

 the diet of animals. It should also be added that they have been 

 much more thoroughly studied. 



Of the two groups of simple proteins found in both plants and 

 animals, viz., the albumins and globulins, it is noticeable that the 

 plant proteins are often more easily precipitated by salt solutions 

 and are less easily coagulated by heat than their animal analogues. 



Plant proteins in general are a bit higher in nitrogen than ani- 

 mal proteins, with the exception of the protamines. This is prob- 

 ably due to their higher content in histidine and arginine, amino 

 acids rich in nitrogen. Of some twenty-five simple plant proteins 

 thus far investigated, about 10% are glutelins, 13% gliadins, 17% 

 albumins, and 60% globulins. 



Complete Proteins. — From the point of view of nutrition, a 

 protein is considered a complete protein if it contains the neces- 

 sary variety of amino acids not only to support life but to permit 

 growth and reproduction. Such complete proteins are rare, and 

 few plant proteins are complete for animal nutrition. Zein, e. g., 

 contains no glycine, lysine, or tryptophane and would not support 

 animal life if used as the only source of protein food. Gelatin, 

 which lacks cystine, tyrosine, and tryptophane, is another example 

 of an incomplete protein. This is only one of the reasons why a 

 wide variety of foods in the diet is beneficial. 



Although animals may be fed amino acids instead of proteins, 

 glycine is generally considered the only one which animals can 

 synthesize from the others. Recently some doubt has been thrown 



