The Mode of Inheritance of Semi-Sterility iu the Offspring of Certain Hybrid Plants. 33] 



with half the pollen-grains aborted, the number of aborted ovules is 

 decreased from a theoretical 50 per cent to^an average of 42 per cent, 

 doubtless by the falling of flowers with many aborted embryo-sacs. 

 Theoretically, if it were not for the falling of such flowers, the semi- 

 sterile plants should give a series for 18 ovules correspontling to the 

 binomial (1 -\- 1)^*. The series they do show is reniarkal)ly regular, 

 though the mode is 7"5, instead of the theoretical 9. 



(Only 9 plants with Idack tomcntum had pods available at the time. 

 Number of ovules aborted in six pods (first three places). 



Little can be inferred from so few plants.) 



Another difference between the normal and the semi-sterile plants 

 consists in the numbers of pods they bear with no undeveloped ovules 

 in the whole pod. Theoretically the normal plants should have all pods 

 full of seeds; and the semi-sterile plants only 1 full pod out of 31 (for 

 a flower with all ovules aborted drops) if we reckon with five ovules 

 to a pod, and 1 full pod out of 63 if there are six ovules to every pod. 



Number of perfect pods out of six. 



The average is 39 per cent for the normal plants, and 1 percent 

 for the semi-sterile plants. Here we see, what was to be expected, that 

 most plants with normal pollen have fewer than six pods out of six 

 full of seeds, and even that 20 of the plants showed no such pods out 

 of the sample of six, though the average was 1 full out of every 2 or 

 3 pods. On the other hand, the plants with semi-sterile pollen had in 

 no case more than one full pod out of six pods, with an average of 

 1 full pod out of 84 pods. (Some ovules doubtless accidentally abort 

 even in the semi-sterile plants where there is usually plenty of room 

 in the pod.) Hence, allowing for zygotic modifications, we may con- 

 clude that the plants with fertih; pollen liav<! fertile ovules, and that 



