50 



PLANT PHYSIOLOGY 



ity, the free acids combine with alcohols and carbohydrates to 

 form complex esters which impart specific flavors to fruit. 

 12. Reserve Proteins and Products of Their Hydrolytic 

 Decomposition. — The nitrogenous reserve substances of plants 

 are represented almost exclusively by proteins. This group of 

 compounds is of an extremely complex composition, consisting 

 of at least five elements : carbon, oxygen, hydrogen, nitrogen, and 

 sulphur and often phosphorus in addition. Nitrogen constitutes 

 15 to 19 per cent of the protein molecule. In many respects, 

 protein substances are analogous to polysaccharides. They 

 form large molecules of a definitely colloidal character and are 



Fig. 13. — Cell from cotyledon of castor bean with aleurone grains. Individual 



aleurone grains with crystalloids at right. 



comparatively easily hydrolyzed into more simple easily crystal- 

 lizable products, which belong to the group of amino acids. 

 The protein molecule, therefore, must be considered as consisting 

 of a number of amino acids joined to each other through removal 

 of water, just as starch or cellulose is composed of molecules of 

 glucose united to each other. The essential difference, however, 

 is that in splitting polysaccharides only one or two monosac- 

 charides are formed, while at least 10 or more amino acids enter 

 into the building of most protein molecules. Owing to this 

 complex composition, the protein group is much larger than that 

 of polysaccharides. 



Reserve proteins usually are stored in seeds in the form of 

 special bodies, called ''aleurone grains." Essentially, they are 

 dried-up vacuoles, very rich in soluble proteins (Fig. 13). 



