EAST: INTERCROSSES BETWEEN SELF-STERILE PLANTS 145 
clusions as the outgrowth of work on heterozygosis and did not refer 
to Darwin’s view until recently. Be this as it may, a short com- 
parison of Darwin’s main induction with the facts from which it came 
will, I think, show a real reason for wonderment. He believed in 
universal cross-fertility of self-sterile plants, his basis being the small 
number of cross-fertilizations made by Hildebrand, Miiller and him- 
self; although Robertson Munro (1868), with whose work he was 
familiar, had found cross-sterility in Passiflora alata, and even the 
works of Hildebrand and Miiller as published leave the matter in 
doubt. Now how much more reasonable the general induction 
mentioned above seems if one assumes (1) that self-sterile plants breed 
true for self-sterility but may show a slight degree of self-fertility as a 
fluctuation under certain conditions, (2) that a variable but limited 
number of germinal “‘factors’’ influence the success of matings, cross- 
fertilization being possible only when two plants differ in these effective 
factors, and (3) that when two plants have the same effective factorial 
composition, cross-sterility of the same type as self-sterility exists. 
This is what we believe our own work has shown, as we shall try to 
demonstrate. 
Emphasis must first be laid upon the fact that the behavior of 
self-sterile plants among themselves and the relation between self- 
fertile and self-sterile plants are distinct problems. Compton (1913) 
found the relation between self-fertile and self-sterile plants of Reseda 
odorata to be that of a simple Mendelian monohybrid with self-fertility 
dominant. The same relation appears to hold in crosses between the 
self-fertile species Nicotiana langsdorffii and the two self-sterile species 
with which our work has been done, Nicotiana forgetiana and Nicotiana 
alata. There is some single differential between self-fertility and self- 
sterility. Given the proper composition a plant breeds true for self- 
sterility. The behavior of self-sterile plants among themselves 
therefore must be considered separately. 
Our work, as stated before, has been done with the two self-sterile 
species, Nicotiana forgetiana and Nicotiana alata, and largely with 
crosses between these species. Both of these species are affected in 
their manifestation of self-sterility by certain environmental changes, 
Nicotiana alata much more than Nicotiana forgetiana. Self-sterility 
is determined by the inheritance received, but it can develop fully 
only under environmental conditions which promote a normal healthy 
growth, and during the period of intense flowering. Toward the 
end of the flowering period, especially under conditions adverse to 
vegetative growth, self-sterility sometimes shows a marked and rather 
sudden decline. A few seeds, or even a well-developed seed capsule 
may then be obtained. This is not a common occurrence; indeed, it 
