CONSTITUENTS OF PLANTS 357 



Classes of Proteids. 1 The proteids are divided into three 

 groups; simple proteids, conjugated proteids, and derived 

 proteids. 



I. SIMPLE PROTEIDS. (a) Albumins. Albumins are soluble 

 in water and are not precipitated by sodium chloride, or mag- 

 nesium sulphate. They are coagulated and made insoluble by 

 heat. The best example of an albumin is the white of an egg. 



(b) Globulins. Globulins are not soluble in water, but are 

 soluble in solutions of sodium chloride, and other neutral salts. 

 They are precipitated by removing the salts, or by saturating the 

 solution with salts. 



(c) Protamins are proteids soluble in alcohol. Several have 

 been isolated. Gliadin is found in wheat ; zein, in Indian corn, 

 etc. 



(d) Glutelins. These are not soluble in water, salt solutions, 

 or alcohol. The glutenin of wheat is the only well characterized 

 representative of this group yet obtained, though there are indica- 

 tions that they may be present in other cereals. 



(e) Albuminiods, (/) Histones, and (g) Protamines have not 

 been found in plants. They are animal proteids. 



II. CONJUGATED PROTEIDS. Coagulated proteids are complex 

 proteids which can be split up into proteids and other bodies. 

 Nueclo-proteids, formed from nucleic acid and protein, have been 

 isolated from some plants. Glycoproteids may be split up into 

 proteids and carbohydrates. Phosphoproteids contain phosphorus. 



III. DERIVED PROTEIDS. Derived proteids are produced from 

 proteids by the action of acids, alkalies, alcohol, or digestive 

 juices. The three important groups are proteoses, peptones, and 

 peptides. 



Proteoses. Proteoses are soluble in water, and are not 

 coagulated by heat, differing in this respect from albumins. They 

 are diff usable. 



Peptones. Peptones are very easily soluble in water, and are 

 not precipitated by heat, by neutral salts, or by nitric acid. They 

 are precipitated by tannic acid, by absolute alcohol, and by picric 

 1 Osborne, The Vegetable Proteins. 



