PARTHENOGENESIS 173 



dioica is homozygous for the sex factor. The recip- 

 rocal cross is explained on the basis that maleness 

 dominates femaleness. It is difficult to bring this view 

 into line with other hypotheses of sex determination. 

 Shull obtained as a mutant a hermaphroditic plant 

 of Lychnis dioica. The next diagram (Fig. 88) gives 

 the principal facts of his crosses. When a female 

 plant is fertihzed by the pollen of the hermaphrodite, 

 two kinds of offspring are produced — females and 

 hermaphrodites. When the hermaphrodite is self- 

 fertilized, the same two classes are produced. When 

 the ovule of the hermaphrodite is fertilized by the 

 pollen from the male plant, two kinds of offspring 

 are again produced — female and male. Shull's inter- 

 pretation is too involved to give here. In the diagram 

 the scheme is worked out on the purely arbitrary 

 scheme that the hermaphrodite is FH, in which F 

 is a female factor, and H a modification of it which 

 gives hermaphroditism. This leads to the further 

 assumption that ovule and pollen, bearing the H 

 factor, cannot produce a plant nor can the combination 

 / H. This scheme is only intended as a shorthand way 

 of indicating the results, and not as an interpretation 

 of actual conditions. 



PARTHENOGENESIS 



A third important condition in which the heredity 

 of sex is involved is found in parthenogenesis. 



It has long been known to biologists, that in many 

 different species of animals and plants eggs develop 

 without being fertilized. This is recognized as a 

 regular method of propagation in some species. The 



