[Chap. XXXVn HEREDITY IN PLANTS 445 



cytoplasm upon heredity. It is generally inferred, however, that cyto- 

 plasm contributes much less than the nucleus and is involved mainly in 

 the physiological effects of the genes upon cell processes. 



It is important to remember that the whole hereditary mechanism of a 

 plant is in its protoplasm, and that at one stage in the life cycle of a plant 

 this mechanism is all contained in the fertilized egg. Differences in 

 heredity must be the result of differences in this mechanism. 



We have become accustomed to thinking of protoplasm as a living sys- 

 tem composed of molecules of many kinds of substances which by them- 

 selves do not have the property of aliveness. It now becomes necessary 

 to recognize chromosomes and genes as distinct components of this liv- 

 ing system. In some way not clearly understood, they influence various 

 cell processes and thus impart certain properties to the cells which be- 

 come evident as the plant develops. If some of these components of 

 protoplasm are removed, the living system may survive, but its proper- 

 ties will be different. Even a change in the arrangement of the genes 

 within a chromosome may be the cause of a change in the development 

 of a plant. The relative location of the genes within a chromosome seems 

 to be fairly constant. 



The exact manner in which a gene may influence the development of 

 a plant is as far from being understood today as is the manner in which 

 small amounts of vitamins and hormones influence the development of 

 an organism. All these compounds influence certain complicated chemi- 

 cal processes within the cells. 



Later on we shall see that certain reproductive cells (spores, sperms, 

 eggs) and also the cells of one whole phase of the life cycle of plants 

 survive and grow with only one-half the number of chromosomes that 

 are present in the cells of the other phase of the life cycle. 



Heredity and Chromosome Behavior 



Since each chromosome contains many genes and accordingly many 

 hereditary potentialities, a knowledge of the behavior of chromosomes 

 during the life cycle of a plant is essential to the understanding of the 

 processes and consequences of heredity. Every cell that contains a par- 

 ticular chromosome also contains all its genes and their potentialities. 

 If a chromosome becomes fragmented and a part of it is lost, then certain 

 of its genes and potentialities are also lost. For the present we shall con- 

 sider onlv the usual behavior of chromosomes. In a later chapter we shall 



