AGRICUI/niRAI, CHEMISTRY- 



both plants and animals. The proteids take an important 

 part in life processes, protoplasm being largely of a 

 proteid nature. In the growing plant r the proteids are 

 found most abundantly in the leaves ; at maturity, they 

 are stored up in the seed for the future use of the embryo. 

 The proteids occur either in a soluble form in the liquids- 

 of plant and animal tissues or in a semisolid, insoluble 

 condition as a part of the tissues. The proteids from 

 animal and plant sources are closely related, but are not 

 in every respect identical. For food purposes, however,, 

 they may be jointly considered. 



297. Physical Properties. While the members of 

 the proteid group differ materially,, they all have certain* 

 physical properties in common. All are optically active 

 and turn polarized light to the left. The soluble proteids,. 

 with the exception of peptones and proteoses, are coagu- 

 lated by heat. The proteids show a wide range in solur 

 bility, but all are soluble in either acid or alkaline solu- 

 tions. As a class, they do not crystallize, and are 

 diffusible with the exception o the peptones and pro- 

 teoses. 



298. Chemical Properties. In structure the proteid 

 molecule is an exceedingly complex and unstable body. It 

 is readily acted upon by ferments and chemicals. Nitro- 

 gen seems to form a weak link in the chain of elements. 

 Proteids unite with acids and alkalies to form acid and 

 alkali proteids. In plants, the proteids are generally united 

 with small amounts of organic acids and mineral com- 

 pounds, particularly phosphorus and potassium. They 

 all respond to certain reactions : ( i ) Nitric acid gives a 



