No. 578] 



SELF-STERILITY 



to 100 per cent, of morphologically perfect pollen grains, 

 a condition about the same as was found in the pure spe- 

 cies. To this statement there is one exception. A single 

 plant was found with only about 2 per cent, of good sound 

 pollen. 



Several experiments were made in which crossing and 

 selfing was done on a large scale, using plants of the F 2 , 

 F 3 and F 4 generations which had segregated markedly in 

 size and were of at least 8 different shades of color. In 

 one of these experiments 20 plants of the F 2 generation 

 coming from 2 crosses of F x plants were used. It was 

 planned to make all possible combinations of these plants, 

 400 in all. This task proved overburdensome, however, 

 and in addition to the self-pollinations but 131 inter- 

 crosses were made with the following results. 



1. Each plant was absolutely self-sterile. 



2. Leaving out of consideration the plant with shrunken 

 imperfect pollen only two crosses failed. This failure of 

 1.5 per cent, of the crosses may have been due to im- 

 proper conditions at the time of the attempts, but as a 

 number of trials were made the possibility remains that 

 there is a small percentage of true cross-sterility. 



3. Of the 129 successful inter-crosses, 4 produced cap- 

 sules with less than 50 per cent, of the ovules fertilized. 

 The remaining crosses produced full capsules. It is 

 barely possible that this result shows a slight variability 

 in ease of cross-fertilization, but I am more inclined to 

 believe that these 4 cases where a low percentage of fer- 

 tilized ovules were obtained were accidental. 



Other crossing experiments of the same kind have cor- 

 roborated these results. Out of 120 inter-crosses, only 3 

 failed. 



Later, something over 100 inter-crosses were made be- 

 tween 12 plants of an F 3 population resulting from cross- 

 ing two sister F 2 plants. Six of the attempts at cross- 

 fertilization— 3 to 8 trials per plant being made— were 

 failures. These plants as well as others tested were com- 



