Self -Sterility 139 



among pollen grains and embryo sacs individually, and 

 not by the action of these factors on the zygotes." 



To make this suggestive situation clear the diagram 

 (fig. 32) may be considered. It enlarges a little upon 

 BELLING'S original ideas as he stated them, and empha- 

 sizes the sporophyte-gametophyte relationship. Pollen 

 grains and embryo sacs are gametophytes in the sense 

 that they inclose the male and female gametophytes, 

 so that when the diagram shows sterile gametophytes it 

 is the same as saying that both pollen grains and embryo 

 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 completely fertile races. Suppose that the parent 

 race A had a factor (X) whose absence brought steril- 

 ity in the gametophytes (pollen grains or embryo 

 sacs). Species B has a different factor (Y), with a 

 similar effect, but inherited independently. When 

 BELLING crossed these races, all of the resulting F x 

 hybrids were semi-sterile. In other words, in every 

 F z plant one-half the gametophytes were sterile. It is 

 easy to see why XO and OY are fertile, also why OO 

 is sterile (lacking both factors); but why should XY 

 be sterile when it has both factors ? BELLING explains 

 it by saying that gametophytes are unlike sporophytes 

 in that normally they have single factors instead of 

 double factors. The germinal capacity of a gameto- 

 phyte is just one-half that of a sporophyte. It is as 

 if a gametophyte were " supersaturated " by a double 

 factor. Such a situation is abnormal for a gametophyte 



