R. N. Salaman and J. W. Lesley 37 



Discussion. 



The two main facts to which we wish to draw attention are: 



(1) The appearance of sterile (or female) plants from the fertile 

 variety, Edzell Blue, when selfed and from crosses between it and other 

 fertiles, Myatt's Ashleaf and Edgecote Purple. 



(2) The different results obtained in F^ from a reciprocal cross. 

 The evidence certainly points to a particular one of the three varieties 



as the principal agent introducing sterility. Both Edgecote Purple and 

 Myatt's Ashleaf are themselves remarkably fertile. They have been 

 crossed reciprocally and gave a wholly fertile Fj^; there is some evidence 

 that when selfed they give fertile offspring. With Edzell Blue it is other- 

 wise. It is itself only moderately fertile. Both selfed and crossed with 

 Myatt's Ashleaf it gives a majority of sterile offspring. It is noteworthy 

 that sterihty has arisen in both cases where Edzell Blue is the mother 

 parent. It behaves as a sterile when used as female and as a fertile 

 when used as a male parent. 



In plants male sterihty arising from a cross between fertile parents 

 is, of course, a well-known occurrence, but in the present case sterility 

 arises from a cross in one direction but not in the other. 



This difference in the reciprocal cross between Edzell Blue and Edge- 

 cote Purple points to some difference in the eggs and pollen of one or 

 both of the parents. The evidence seems to point strongly to such a 

 difference in the sexual cells of Edzell Blue. It is not impossible that in 

 Edzell Blue the sterihty is attached to the cytoplasm of the egg, but 

 that the generative nucleus of the pollen grain carries the basis of fer- 

 tihty. As sterihty normally behaves as a dominant we should then ex- 

 pect all the F^ to be sterile where Edzell Blue is the mother. This, how- 

 ever, was not so. A few were fertile, but we know of no evidence sug- 

 gesting that the cytoplasm of eggs may differ in constitution. Indeed 

 the heterogeneity of the eggs suggests segregation. It is more likely that 

 a factor or factors are at work which are localised in the nucleus. 



As_sierility is dominant it would seem that the eggs of Edzell Blue 

 are of two kinds, some — possibly half — carrying male sterihty, the re- 

 mainder carrying male fertihty. The data at present available only give 

 an approximate idea of the proportions. The pollen on the other hand 

 all carries fertility. Following the same hypothesis both eggs and pollen 

 of Edgecote Purple and Myatt's Ashleaf would appear to carry fertihty. 



In order to account for the difference between the eggs and pollen 

 of Edzell Blue we suggest that at some stage of development a process 



