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-poilina- 
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 effected 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 
