CHROMOSOMES AND SEX 



389 



division is equational, so that half of the microspores (and hence male 

 gametes) receive a F-chromosome and 23 autosomes while the other half 

 receive 23 autosomes and an X-chromosome. In the nuclei of the 



Fig. 222. — Diagrams of two types of sex-chromosome beha\'ior in angiosperms. In 

 each case the upper row represents the staminate plant and the lower row the carpellate 

 plant. Number of autosomes arbitrarily shown as four. 



carpellate plant there are two X-chromosomes, every egg receiving one 

 of them. It is therefore evident that the union of an egg with a male 

 gamete carrying a F-chromosome results in the production of a staminate 



b c d 



Fig. 223. — Sex-chromosomes in heterophytic angiosperms. a, division I in micro- 

 sporocyte of Melandrium album, showing unequal XY pair about to disjoin, b, Populus 

 balsamifera. c, Valeriana dioica. d, Rumex acetoselia, showing Mm disjoining from m. 

 (c after Winge, 19236; b-d after Meurman, 1925.) 



plant (46 + XY), while a union with a male gamete with an Z-chromo- 

 some leads to the production of a carpellate plant (46 + XX) (Santos, 

 1923, 1924). 



