[Chap. XXXVII HEREDITY IN PLANTS 457 



In the chapter on fat and protein synthesis one other amazing fact 

 about chromosomes and genes was mentioned. Since the properties of 

 chromosomes and genes remain constant through bilhons of cell di- 

 visions in each plant and from generation to generation, their chemical 

 composition and internal organization must also remain constant. They 

 are composed largely of protein substances. Since the chromosomes and 

 genes are duplicated in number every time cells divide, the maintenance 

 of their size must depend upon growth by the synthesis of new chromo- 

 somal substances from foods in the cell. The constancy of chromosomal 

 synthesis peculiar to each species of plant and animal, and the constancy 

 of chromosome behavior in cell division constitute an important part of 

 the hereditary mechanism of cells. The persistence of species and races 

 is dependent upon this constancy. A high degree of constancy accom- 

 panied by some irregularity results in a gradual evolution of varieties 

 and of species of plants. 



REFERENCES 



Schrader, F. The present status of mitosis. Biological Symposia. Vol. 1, pp. 



87-95. The Jacques Cattail Press. 1940. 

 Sharp, L. W. Introduction to Cytology. McCraw-Hill Book Company, Inc. 



1934. 

 Sinnott, E. W., and L. C. Dunn. Principles of Genetics. 3rd ed. McGraw-Hill 



Book Company, Inc. 1939. 

 Snyder, L. H. The Principles of Heredity. 2nd ed. D. C. Heath and Company. 



1940. 



