Sex Dctcrminatioi 193 



sumably) one type of egg, X, result in 50 i)er cent female 

 individuals, A^.Y, and 50 per cent male individuals, AT. 



Indirect evidence of a sex chromosome mechanism in angio- 

 sperms is provided by some of the experiments of Corrk.vs and 

 Shull. Correns (12) crossed the dioecious Bryonia dioica with 

 the hermaphroditic B. alba, while Shull (21) crossed Lychnis 

 dioica with hermaphroditic mutants from the same. The sexual 

 behavior of the progenies in the two cases was not identical, but 

 both suggested a sex chromosome mechanism with the male 

 heterozygous for sex. (The theoretical explanations, however, 

 are so complex and dubious that they cannot conveniently be 

 discussed here.) 



^lore recently Correns (ii), working wiili Melatulriiim 

 {Lychnis), has uncovered an interesting phenomenon which 

 might be interpreted as indicating a sex chromosome mechanism 

 and a type of sex-linked inheritance. It is assumed that pollen 

 grains of the two types are produced in equal numbers, but that the 

 "female-determining" grains (X) contain a gene which hastens 

 pollen tube growth, while the "male-determining" grains (Y) lack 

 this gene. When a deficient amount of pollen is applied to the 

 stigmas, the resulting sex ratio is 44 per cent males: 56 per cent 

 females. When a large excess of pollen is applied, so that com- 

 petition between "male-determining" and "female-determining" 

 pollen tubes is more severe, the resulting sex ratio is 32 per cent 

 males: 68 per cent females. A moderate excess of pollen results 

 in 40 per cent males: 60 per cent females. 



At the present date there is probablx' more evidence 

 to support physiological theories of sex detemiination 

 in plants. It should be borne in mind that the majc^rit}' 

 of plants are bisexual individuals, and that such cases 

 are hardly comparable with unisexual animals. \'ery 

 often in bisexual plants the male and female g;mietes 

 are produced at slightly dilYerent stages in the life-cycle, 

 and the interpretation of such phenomena is usually 

 sought in temis of plnsiological conditions. A young 



