OTHER METHODS OF SEX-DETERMINATION 249 



In a brief paper East and Mangelsclorf have given a 

 summary of work extending over several years dealing 

 with the inheritance of self-sterility in crosses between 

 two species of tobacco, viz. Nicotiana alata and N. forge- 

 tiana. Only the most general conclusions can be given here. 

 By special manipulation several selfed, homozygous 

 stxains of self-sterile individuals were carried through 

 twelve generations to obtain suitable material for testing 

 the problem. As an example the results of one type of the 

 families produced may be given. Three kinds of individuals 

 a, b and c were found. Each individual of any one of these 

 kinds is sterile with any other individual of its kind and 

 fertile with each individual of the other two kinds; but 

 the progeny resulting from reciprocal crosses is different. 

 Thus, a 2 by c 5 gives b and c individuals only, while c 

 $ by a 5 gives only a and b individuals. Two classes al- 

 ways appear in equal numbers, but the class of the mother 

 is never represented in the offspring. The explanation is 

 as follows. If three allelomorphic genes are present in such 

 a family SiS.Ss and if class a = S1S3 ; class b = S1S2 ; class 

 c = S2S3 and if the pistil of a plant affords the stimulus for 

 the growth of the pollen which bears sterility factors other 

 than its own, the results find a consistent explanation. For 

 instance, plant c (S2S3) affords a sufficient stimulus only 

 to pollen carrying factors other than S2S3. Only pollen 

 bearing the factor Si can penetrate the style and fertilize 

 the eggs. The progeny will be S1S2 (class b) and S1S3 

 (class a) in equal numbers. Reciprocally, a 9 (S1S3) by c 5 

 (S2S3) permits the S2 pollen alone to penetrate the eggs 

 giving S1S2 (b) and S2S3 (c). This result which is typical 

 of all the others, explains why the female combination is 

 absent in the progeny, why the progeny of reciprocal 

 crosses is different and why the progeny-counts of two 

 classes other than female are equal regardless of which of 

 the other two classes serves as the male parent. 



