1916] Goodspeed-Ayres : Sterility of Nicotiana Hybrids 281 



abnormal nuclear constitution germination of the pollen-grains that 

 mature can be brought about. To this end it may be necessary to 

 supply some external conditions which are absent, such as oxygen or 

 water-supply. Mechanical resistance to enlargement may enter in 

 as a factor and it is possible that the histology and microchemistry 

 of the pollen coats may give important evidence concerning the failure 

 of the hybrid pollen to germinate. We are indebted to Dr. William 

 Crocker of the University of Chicago for his interest in the work and 

 for a number of suggestions which have been embodied in the above 

 discussion. 



In summing up the experiments on pollen germination detailed 

 above we are concerned almost entirely with negative results so far as 

 the primary objective of the experiments is concerned. It has been 

 shown, however, that the stigmatic secretion produced in the flowers 

 of the Fi hybrid is a favorable medium for the germination of tobacco 

 pollen, the pollen of normal appearance shed in these same hybrid 

 flowers being the only exception to this statement which we have 

 found. Further it has been shown that the pollen of N. Tahacum var. 

 macrophylla differs from the pollen of N. sylvestris with respect to 

 growth response under the influence of 10 per cent solutions of 

 dextrose, levulose, and maltose and in tap water. At the time this 

 seemed to indicate that these or other specific substances might be 

 required as a growth stimulus for the germination of the hybrid pollen. 

 Nothing was discovered, however, which would provide this stimulus 

 and bring about germination. 



III. Experiments upon the Relation op Nutrition to Sterility 



AND FlOV^ER-FALL 



Our purpose in the experiments detailed below was to bring about 

 such a condition of nutritive balance wnthin the partially sterile F^ 

 hybrids under discussion that a greater proportion of normally con- 

 stituted pollen-grains and ovules would be matured. As has been 

 noted above, the flowers in the Fj hybrids fall a short time after 

 anthesis. Tliis falling of the flower is due to the presence in the 

 pedicel of an absciss-layer which is formed when pollination and 

 fertilization do not take place. Castralion of self- fertile species 

 simil;ii'l\- results in the fall of the flower after anthesis while, at hvist 

 in the hybrid, the formation of ;iii absciss-layer is inhibited by the 

 fertilization of a relatively few ovules. The partially sterile character 



