206 GENETICS AND EUGENICS 



size are involved and that they also are located in the chro- 

 mosomes. But it is clear that the size genes must be numer- 

 ous since the inheritance of size is blending, and they are 

 probably located in many different chromosomes or even in 

 all the different chromosomes. That genes do affect size is 

 shown by the t;yT)ical Mendelian behavior of the characters 

 tall and short in crosses of peas, and brachydactyl and nor- 

 mal in human families. The case studied by Hoshino in 

 which late flowering in peas was found to be coupled with 

 red flow^er color is important because it shows that a gene 

 which affects a quantitatively varying character, one which 

 blends in heredity, is located in the same chromosome with a 

 color gene. There is no reason to think that any genes occur 

 elsewhere in the gamete than in the chromosomes. 



An apparent exception occurs in the case of a plant, 

 Mirabilis, the cultivated four-o-clock, studied by Correns. 

 A single plant of this species arose in his cultures, which had 

 white-margined leaves, the white areas being due to an ab- 

 normal condition of the normally green plastids, which are 

 cytoplasmic (not nuclear) structures. Sometimes entire 

 branches arose on this plant (or its descendants) which were 

 white, and which contained only colorless plastids, and others 

 which were green containing normal colored plastids. White 

 branches produced only white seedlings when self-pollinated 

 and green branches produced only green seedlings. When 

 flowers on the two sorts of branches were intercrossed, the 

 seeds borne on white branches still produced only white 

 seedlings, and those borne on the green branches produced 

 only green seedlings, which thereafter bred true. The white 

 seedlings perished because without chlorophyl they could not 

 live. The case has been explained as one in which inheritance 

 is exclusively maternal, by means of the ^gg but not by 

 means of the pollen. Further it is a cytoplasmic structure, 

 the plastids of the egg, which determine the plastid character 

 of the offspring, the nucleus not being concerned in the proc- 

 ess. Here then we seem to have a case of cytoplasmic in- 

 heritance in which nuclear genes are not concerned. But a 



