356 INTRODUCTION TO CYTOLOGY 



Both sexes are represented in the sporophyte (diploid) generation, as 

 shown particularly by the mosses, but the spores, though morphologically 

 similar, are of two distinct kinds: male-producing and female-producing. 

 Since it is precisely at sporogenesis that reduction occurs, the natural 

 inference is that a separation of qualitatively different sex-factors of some 

 kind occurs in the heterotypic division, the sexes of the future gameto- 

 phytes thus being automatically determined. 



That a somewhat similar qualitative difference may exist in the 

 microspores of many angiosperms, resulting in the frequent dioecious 

 condition of the sporophyte, was concluded by Correns (1907) from his 

 researches on Bryonia hybrids. He was best able to interpret the 

 phenomena observed on the hypothesis that the eggs are all similar in 

 having the female sex "tendency;" that there are two kinds of micro- 

 spores and hence two kinds of male gametes, with male and female 

 "tendencies" respectively; and that in the sporophyte the male tendency 

 dominates the female. Darling (1909) was inclined toward a similar 

 conclusion for Acer N eg undo. 



Strasburger (1910) in a general discussion of the subject growing out 

 of his researches on Elodea, Mercurialis, and other plants, summarized 

 the situation in plants as follows. In monoecious mosses the separation 

 of the sexes occurs in the somatic divisions at the time the sex organs 

 are formed. The separation has been secondarily joined with reduction, 

 so that in the derived dioecious mosses it occurs at sporogenesis. In the 

 homosporous pteridophytes it takes place at some stage in the game- 

 tophyte before the formation of the sex cells, as in monoecious mosses, 

 though in some cases (Equisetum, Onoclea, and others) a marked physio- 

 logical dioecism is present. In heterosporous pteridophytes and all 

 seed plants the gametophytes are dioecious but the sporophytes may be 

 either monoecious (hermaphroditic) or dioecious. In monoecious forms the 

 sexes are separated at some stage prior to the development of megaspores 

 and microspores, whereas in dioecious forms it must take place at some 

 other point in the life cycle, since the two kinds of sporophytes (mega- 

 spore-bearing and microspore-bearing) are distinct from their initial 

 stages. There is some evidence to show that such dioecism is due to a 

 differentiation among the pollen grains and hence among the male 

 gametes: some grains have a strong male tendency which dominates the 

 female tendency of the egg, male progeny resulting; while other grains 

 have a weak male tendency dominated by the female tendency of the egg, 

 female offspring being produced. In brief, as sex separation became 

 joined with reduction in forms with monoecious gametophytes, the 

 dioecism of the gametophyte and heterospory (first physiological and then 

 morphological) followed; and this in turn led to the dioecism of the 

 sporophyte also. Finally, in such advanced forms there appears to be a 

 differentiation among the spores of one sex, the microspores, giving male 

 gametes of two types. The sex of the resulting offspring therefore 



