The Mode of Inheritance of Semi-Sterility in the Offspring of Certain Hybrid Plants. 317 



well-known studies of Baue, Nilsson-Ehle and Emersox, to a se- 

 gregation of genetic factors affecting the zygotes. And these studies 

 were made on well-cared-for plants, with a minimum of the competitive 

 struggle for life. I have found that certain segregates of my crosses 

 do not survive a struggle with crowding sorghum (Florida Report for 

 1912). The competitive struggle for life between developing micro- 

 spores iu an auther must be somewhat severe, judging by tiie way in 



Fig. 9. Camera drawing of pollen from the Velvet by China F, hybrid. 

 Scale in hundredths of a millimeter. 



which the abortive grains are stripped of apparently every particle of 

 their organic contents. I therefore assume that segregable genetic 

 factors affect the ontogeny of the haijloid generation also. Then, in 

 these hybrids, there is a segregation of microspores into viable and 

 non-liable under the environment of the anther loculus; and we have 

 here visibly the segregation among pollen-grains which Mendel stated 

 as a hypothesis. The two simplest haploid segregations are, of course, 

 1:1 for one factor, and 1:1:1:1 for two factors. 



To sum up: — Fifty per cent of the pollen-grains of the hybrids 

 (Fl) lose their \i\ing contents while in the vacuolated stage. The other 



