NITROGENOUS ORGANIC COMPOUNDS OF PLANTS 215 



nous compounds are composed of carbon, hydrogen, 

 nitrogen, and oxygen, and many contain in addition to 

 these, phosphorus, sulphur and other elements. 



294. Classification of Nitrogenous Compounds. 



For food purposes, the nitrogenous compounds of plant 

 and animal bodies may be divided into four groups : (i) 

 Proteids, (2) albuminoids, (3) amides, and (4) alka- 

 loids. There are a few nitrogenous compounds in plants 

 that do not find a place in any of the above subdivisions. 



Proteids 



295. General Composition. Proteids are complex 

 nitrogenous compounds that contain about 16 per cent, of 

 nitrogen and less than 2 per cent, of sulfur. The per 

 cent, of the different elements in protein compounds from 

 various sources ranges as follows : 



Per cent. 



Carbon 51.2-54.7 



Hydrogen 6. 7-7.6 



Nitrogen 15.2-18.0 



Oxygen 20.2-23.5 



Sulfur 0.3-2.0 



It was believed, at one time, that all of the various 

 compounds called proteids had, in common, a nitrogenous 

 radical in their molecule to which the name protein was 

 given, but this hypothesis has not been found correct. 

 The term protein has been retained but without reference 

 to the supposed protein radical. 



296. Occurrence Proteids are found more abun.- 



dantly in the seeds of plants than in the leaves or other 

 parts, and are always present in the active living cells of 



