THE GREAT RELAY RACE 465 



In practically all groups of plants there are certain structures 

 embedded in the cytoplasm called plastids, which are centers of meta- 

 bolic activity. They are composed of packets of various materials 

 essential to plant life, such as starch grains, chlorophyll, oil droplets, 

 and the like, having a definite chemical composition and easily visible 

 under the microscope. It is generally agreed that ])lastids are 

 derived from preceding plastids, quite as chromosomes are from 

 preceding chromosomes, and that they are not formed anew in each 

 cell. Unlike chromosomes, however, they do not undergo orderly 

 mitosis when they divide, thus securing in daughter plastids an 

 accurate halving of material as in the case of chromosomes and 

 genes, nor do they always follow the Mendelian laws in their redis- 

 tribution. A case in w^hich the chromosomes and genes do not 

 apparently play their usual equal parts, but in which it looks as if the 

 inheritance is through self-perpetuating plastids in the female cyto- 

 plasm and never through the male gametes, is found in plants with 

 variegated or striped leaves. In these plants, the cells with plastids 

 carrying chlorophyll {chloro-plasts) determine the green areas in the 

 leaf, while cells with plastids that lack the chlorophyll (leucoplasts) 

 account for the white areas. Branches and flower buds occur with 

 either chloroplasts or leucoplasts. When crosses are made between 

 flowers borne upon a green branch, and those from a white branch of 

 such variegated plants, the resulting offspring are white or green 

 according to the kind of plastids present in the maternal parental 

 branch, irrespective of the kind of pollen employed. The grand- 

 parental genes determine the character of the maternal plastids, which 

 in turn cause the new branch or plant to be white or green. 



Current opinion about the whole matter is summarized in the 

 statement of Dr. E. M. East to the effect that "though the nucleus 

 and cytoplasm co-operate in development, the only ascertained agent 

 of heredity is the nucleus." What the future may disclose still 

 remains a question unanswered, but at present it appears that 

 "cytoplasmic inheritance" is unproven. 



Sex in Heredity 



While it is quite possible for one generation to arise from another 

 by various asexual methods, yet it is evident that the whole mech- 

 anism of heredity has been revolutionized by the rise of sex. 



As previously pointed out in the section on "The Usefulness of Hy- 

 brids" (page 441), in the study of heredity so long as level uniformity 



