SEXUALITY IN PLANTS 255 



pal factor. Hermaphroditism in plants also exhibits the same 

 trend. The hermaphrodites of monoecious species exist as 

 definite sexual types. The hermaphrodites of polyoecious species 

 tend to propagate themselves while the hermaphrodites of 

 dioecious species are unstable and, more definitely, transitory 

 forms of maleness or femaleness. The hermaphrodites of sub- 

 dioecious forms show intermediate characteristics. They tend 

 to propagate themselves when selfed but behave as modified 

 males when crossed with females. 



It must be emphasised that the botanist associates somewhat 

 different ideas with the terms that are used by the animal 

 worker. In the animal the diploid or sporophyte generation is 

 all predominant and the gametophyte generation is practically 

 non-existent. In plants gametogenesis does not always coincide 

 with meiosis, therefore the haploid phase may be of some dura- 

 tion. I have already given examples of sexuality in the haploid 

 phase which are not paralleled in the higher animals. These 

 examples are instructive in that, although gamete formation 

 may be many haploid cell-descendants from meiosis, they indi- 

 cate that segregation of sex factors has occurred at meiosis. 



The angiosperms as a group approach most closely in their 

 life history to the higher animals. In these plants the diploid 

 phase is predominant and gamete formation is only a few cell- 

 generations from meiosis. Therefore these plants are the only 

 groups having similar conditions of sex expression to the con- 

 ditions prevailing among the higher animals. Monstrosities are 

 infrequent but sometimes the female organs become more or less 

 male-like Salicacece (Harrison, Heribert-Nilsson), AriscBma 

 (Mcekawa). 



Recent work has shown that the pollen grains of some 

 dioecious species of Angiosperms are physiologically dimorphic 

 in regard to sex. The sizes of pollen grains of Elodea (Santos), 

 Cannabis sativa (Sinoto, 1930) and Rumex acetosa (Sinoto, 

 1930) form bimodal curves. This is probably an expression of 

 the difference in chromosome complements of the female and 

 male determining pollen grains. 



Pollination with much pollen in Rumex acetosa and Melan- 

 drium increases the proportion of females in the progeny. The 

 growth rate of female-determining pollen is greater than that of 

 male determining, and this results in there being an excess of 



