1916] Goodspced-Ayres : Sterility of Nicotiana Hybrids 275 



maturing normal fruits and parthenogenetic or phenospermic seeds 

 (Goodspeed, 1915). The F^ hybrid between this N. Tahacum variety 

 and N. sylvestris makes apparently no viable seed under bag and its 

 flowers produce no functional pollen ; thus corresponding to all the 

 other species hybrids involving N. sylvestris as a parent. Despite the 

 absence of pollination and fertilization, however, all the flowers remain 

 attached to the plant and mature normal fruits containing pheno- 

 spermic seeds. 



To sum up what precedes, the F^ hybrids produce no good pollen 

 or so very little that the chance of a normally matured pollen-grain 

 fertilizing one of the few normally matured ovules is small. A few 

 good ovules are matured and after fertilization with normal foreign or 

 parental pollen yield fruits and viable seeds. Failing pollination with 

 functional pollen and fertilization of the few good ovules, the hybrid 

 flowers fall after anthesis. The parthenocarpic tendency of N. 

 Tahacum "Cuba" is dominant and appears intensified in F^ of the 

 hybrid with N. sylvestris. 



The above is a brief statement of what seems a rather complicated 

 situation. It includes the various phases of what constitutes the gen- 

 eral problem of partial sterility in the F, species hybrids of Nicotiana. 

 We have chosen to attack this general problem under the following 

 headings. 



(1) A cytological problem to be concerned with (a) the nature 

 and condition of the maternal and paternal chromatin in somatic 

 mitoses, as contrasted with that of the hybrids, and (&) an examin- 

 ation of the maturation divisions of the sex cells in the hybrid with 

 a view to determining, first, the stage at which the degeneration of 

 the majority of the reproductive tissues takes place and, second, the 

 chromatic condition of the nuclei in those embryo-sacs which are 

 capable of fertilization. 



(2) A general breeding problem involving a genetic analysis of 

 the generations which can be grown from (a) the open pollinated 

 seed of the F, hybrids and (h) the seed resulting from crossing the 

 parents Iiack on the hybrids. 



(8) A histological investigation dealing with the origin and natui'e 

 of the cutting-off or absciss-layer responsible for the falling of unpol- 

 linated and unfertilized tobacco flowers. 



(A-) A general physiological problem which up to the present time 

 has been taken up under the following somewhat unrelated headings: 

 (a) comparison of the rate of growth of parents and hybrids; (6) 



