1916] Goodspeed-Ayres: Sterility of Nicotiana Hybrids 28<J 



such as to give evidence against the view that specific chemical sub- 

 stances play the important role in determining whether or not pollen 

 will germinate, and the results obtained are to be explained as the 

 effect of the reagents upon the swelling of cell colloids. 



4. The formation of an absciss-layer in the pedicel of tobacco flowers 

 is the cause of flower and fruit-fall. Non-fertilization is the stimulus 

 which is responsible for the formation of this absciss-layer. 



5. From a considerable series of pot cultures it was determined 

 that flower and fruit-fall in tobacco could be at least retarded by 

 lowering the total concentration of available mineral nutrients, while 

 variations in the amount of individual constituents of a nutrient 

 solution were without effect in this connection. 



6. A few normalh^ matured ovules capable of fertilization are 

 prociuced in the F^ flowers and a little viable seed is formed after 

 pollination with the normal pollen of the parents. The back-crosses 

 are difficult to make in the field, since it is hard to anticipate the 

 activation of the absciss-layer. The plants growing under conditions 

 of low nutrient content retain their flowers so much longer that the 

 back-crosses were almost uniformly successful. It must be emphasized, 

 how^ever, that no increase in the number of ovules capable of fertiliza- 

 tion accompanied this more favorable condition for successful back- 

 crossing. 



7. Grafts between the F^ hybrid and its parent N. Tab. var. 

 macrophylla produced flowers containing that proportion of pollen 

 of normal appearance and couvstitution and functionless pollen which 

 is characteristic of the same plants growing on their own roots. The 

 retention of the flowers and fruits of the hybrid was, however, favored 

 when it was grown on the parent as a stock. 



We take pleasure in acknowledging our indebtedness to Professor 

 C. B. Lipman for continued interest in and many suggestions con- 

 cerning the experiments reported upon and in progress. We are, 

 further, under obligations to Professor Francis E. Lloyd for many 

 suggestions as to the nature of abscission in tobacco. 



Transmitted February 3, 1916 



