EAST: INTERCROSSES BETWEEN SELF-STERILE PLANTS 145 



elusions 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, M tiller and him- 

 self; although Robertson Munro (1868), with whose work he was 

 familiar, had found cross-sterility in Passi flora alata, and even the 

 works of Hildebrand and M tiller as published leave the matter in 

 doubt. Now how much more reasonable the general induction 

 mentioned above seems if one assumes (i) that self-sterile plants breed 

 true for self-sterility but may show_a^ sliglit^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 onK wlicn 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^ertility 

 dominant. The same relation appears to hold in crosses between the 

 self-fertile species Nicotiana langsdorflii and the two self-sterile species 

 with which our work has been done, Nicotiana for getiana and Nicotiana 

 alata. Therejs 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 beWeen 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, fuUy. 

 only under environmental CQnditions which promote a normal healthy 

 growth, and during the period of intense flowering^._ Toward the 

 end of the flowerihg"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 



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