Mar. II, 1918 Sterility in the Strawberry 663 



L and 1 form an allelomorphic pair located in a definite but different chro- 

 mosome of the three varieties and the velvet bean, respectively. Then, 

 to follow out Selling's scheme, the presence of the chromosomes bearing 

 the factors K and L in the same member of a tetrad causes abortion, and 

 likewise the presence of both chromosomes lacking the factors K and L 

 causes abortion. Belling states that abortion of the microspores takes 

 place in the vacuolate stage and that there are no intermediates between 

 the completely aborted and the most perfect grains, thus strengthening 

 the idea that in this case no more than two chromosomes have to do 

 with abortion. 



Another set of studies which point to one instead of two chromosomes 

 being the cause of pollen abortion are those of Shull {35, 36) on the 

 inheritance of sex and of a sex-linked factor in Lychinis dioica. In 

 the first of these studies Shull showed that very probably L. dioica 

 9 is homozygous for the sex determining factors, while L. dioica $ is 

 heterozygous. In crossing these forms an approximate ratio of i pis- 

 tillate to I staminate usually resulted, but with nearly always a slight 

 excess of pistillate plants, suggesting, if the females are homozygous 

 for sex, an elimination of a portion of the male gametes bearing the 

 determiner for maleness. In a later study he was able to show that the 

 determiner for maleness was linked with a factor for narrow leaves while 

 in normal plants the determiners for femaleness were linked with broad 

 leaves. In a narrow-leaved mutant male found by Baur he showed 

 that the determiner for femaleness as well as maleness was linked with 

 the narrow-leaved determiner. It was as a result of the discovery of 

 this homozygous (for leaf width only) , narrow-leaf male that the factor 

 for narrow leaves linked to maleness was able to be discovered as, being 

 a recessive character and always in a heterozygous condition, it was 

 hidden in normal males. 



When these homozygous narrow-leaved males were used in crosses 

 with either normal broad-leaved females or heterozygous females, 

 there was always produced a great excess of males, the females appear- 

 ing only in very small numbers. These results were apparently in con- 

 tradiction to those previously obtained in which females were more 

 abundant. Shull gave no explanation of these irregularities. They 

 suggest, however, that there is a fairly constant elimination of certain 

 gametes. A study of all of Shull's results, with this idea in mind, in- 

 dicates that an explanation based on the elimination of certain male 

 gametes will cover all cases of irregularity so far reported by him except 

 the nonappearance of homozygous hermaphrodites and of heterozygous 

 hermaphrodites containing male determiners. These two instances, if 

 we may draw analogies between plants and animals, are of the same 

 nature as the YY zygotes in species of Drosophila, and die (Bridges, 5).* 



' Shull (55-56) has shown that the hermaphrodites have undoubtedly been derived from males; and 

 therefore the presence of two hermaphrodites or a male and an hermaphrodite determiner would be anal- 

 ogous to the presence of two male determiners. 



