Sterility 135 



Bej.ling (2) made a cross between two races of beans, bolli of 

 which were completely fertile. 'I'he resulting hybrids were semi- 

 sterile. Uniformly just one-half of the pollen grains ai)fK'ared 

 empty and collapsed, while one-half of the ovules had no embryo 

 sacs. The sterile pollen and ovules appeared in random distribu- 

 tion with the fertile. 



Inbreeding the semi-sterile hybrids, Belling obtained an F, 

 generation which showed the following features: one half of the 

 plants had perfect pollen; the other half had a mixture of equal 

 numbers of good and bad pollen grains in all their flowers. The 

 plants which had perfect pollen also had perfect ovules, while the 

 plants with 50 per cent sterile pollen also had 50 per cent sterile 

 ovules. In the F3 generation all the descendants from the fertile 

 plants had perfectly good pollen and ovules; but the progeny of 

 the semi-sterile plants again split up into the two classes, fertile 

 and semi-sterile, as before. 



Belling states his general conclusion as follows: "The e.x- 

 planation of the random abortion of one-half of the pollen 

 and one-half of the embryo sacs must apparently be by the segre- 

 gation of Mendelian factors among pollen grains and embr>'o 

 sacs individually, and not by the action of these factors on the 

 zygotes." 



To make this situation clear a diagram (fig. 21) may be con- 

 sidered. It enlarges a little upon Belling's original ideas as he 

 stated them, and emphasizes the sporophyte-gametophyte rela- 

 tionship. Pollen grains and embryo sacs are gametophytes in the 

 sense that they include the male and female gametophytes, so that 

 W'hen the diagram shows sterile gametophytes it is the same as 

 saying that both pollen grains and embr}'o sacs are sterile. This, 

 of course, is just what Belling found; whenever one-half the 

 pollen grains in random distribution were sterile one-half the 

 embryo sacs in random distribution were also sterile. 



It should be remembered that Belling started with two com- 

 pletely fertile races. Suppose that the parent race . 1 had a factor 

 A' whose absence brought sterility in the ganietophytes (pollen 

 grains or embryo sacs). Race B had a different factor 1', with 

 similar effect, but inheritetl independently. When Helling 

 crossed these races, all of the resulting F, hybrids were semi- 



