SEXUALITY IN PLANTS 249 



may be carried further, in certain cases, to the inflorescence, 

 giving a monoecious species, or even to the whole plant, giving a 

 true dioecious species. Thus we have in higher plants complete 

 differentiation of the gametes and degrees of differentiation of 

 the plant body. 



The differentiation of the plant body in regard to sexuality 

 will be considered first. 



Heterochromosomes have been found in a number of plants 

 distributed irregularly throughout the Orders and Phyla of the 

 plant kingdom (cf. Sinoto). Generally, except Bryophytes, 

 the plants with a sex chromosome mechanism are sexually 

 differentiated throughout the whole sporophytic generation. 

 Secondary sex dimorphism occasionally occurs, but it is prob- 

 ably always of a secondary genotypic nature and is not due to 

 gonadic activity. 



The males of Mercurialis annua are lighter green, with 

 longer internodes than the female and earlier in flowering. In 

 hemp, the difference between males and females can be detected 

 in the seedling stage by the longer internodes and darker green 

 colour of the males. The length of pedicel of the flower of 

 Silene Roemeri, S. Otites, Antennaria dioica and Petasites is 

 greater in the female. The female starts to flower on the 

 average before the male in Cannabis sativa, Humulus japonica, 

 H. lupulus, Mercurialis annua and in Salix species. The 

 modification of the flower alone, according to sex, is of frequent 

 occurrence, and extreme cases of floral dimorphism such as in 

 several of the Orchidacea and Restiacece are known (cf. Goebel). 



Sex-linked factors are present in Melandrium, where the 

 male has a heterochromosome pair. The narrow-leaved char- 

 acter is controlled by a factor on the X chromosome, and there 

 is probably a pollen lethal carried on the Y chromosome 

 (Shull, Winge, Baur, Ubisch). 



In these heterophytic plants where sex determination is 

 relatively stable, hermaphrodites do occur, but there are suffi- 

 cient genetic data to show that they are a direct result of 

 either {a) genetic change in the sex chromosome, {B) modifica- 

 tion of the gene complex of a male or female, or {c) environ- 

 mental influence. 



Hermaphrodites in Melandrium may be either of genetic 

 origin or the result of one individual, genetically a female, be- 



