146 BROOKLYN BOTANIC GARDEN MEMOIRS 



is rare, but it is a possibility. Three cases of seed production out of 

 over three hundred plants tested have been observed in Nicotiana 

 forgetiana. A considerably higher percentage of fertility has been 

 marked in Nicotiana alata. Self-sterility can be restored in such 

 plants, however, if they are allowed to go through a period of rest and 

 are then, by proper treatment, brought into vigorous flower again. 



This is not the whole evidence that this occasional end-season 

 fertility is a pseudo-fertility brought about by external conditions — 

 a fluctuation. Three generations of Nicotiana alata plants have been 

 grown from selfed seed produced by end-season fertility without the 

 occurrence of a single plant which behaved in every way like a truly 

 self-fertile individual. This phenomenon, therefore, while teaching 

 us to test self-sterility only during the main part of the flowering 

 season, has shown that there is no reason why fusion between gametes 

 produced by a self-sterile plant may not occur provided the male 

 generative nucleus enters the embryo sac. Such unions may take 

 place without affecting the self-sterility of the progeny. 



What is then the difference in behavior that makes a cross-pollina- 

 tion effect fertilization while a self-pollination produces nothing? 

 What occurs is this: After a self-pollination the pollen grains germinate 

 and the tubes pass down the style at such a slow even rate that they 

 reach only about half way to the ovary before the flower wilts and 

 falls off; while the pollen tubes after a cross-pollination, though 

 starting at the same rate as the others, grow faster and faster until 

 fertilization is eft'ected in four days or less. The curve of distance 

 traversed plotted against time is in the case of the self-pollination 

 nearly a straight line, while in the case of the cross-pollination it 

 simulates that of an autocatalytic reaction. 



From these facts it seems reasonable to suppose that the secre- 

 tions in the style offer a stimulus to pollen tubes from other plants 

 rather than an impediment to the development of tubes from pollen 

 of the same plant. And we believe that this stimulus is in some way 

 caused by certain effective differences in the factorial composition 

 characterizing two compatible plants and that if two plants do not 

 have these effective differences in factorial composition they are by 

 the same token cross-sterile with each other. It is clear that this 

 assumption presumes that the pollen grains matured by a given plant 

 behave as if they are sporophytic as regards that part of their con- 

 stitution that affects self-sterility and cross-sterility. The pollen 

 grains of any plant may carry many different hereditary factors, they 

 may even carry .several different factors which function in controlling 

 the success or failure of particular cross-matings in the next generation, 

 but in their own action on the stigmas of other plants they behave 



