REPRODUCTION AND DISPERSAL 897 



determined at the time of spore formation, since two spores in each tetrad give rise 

 to male plants and two spores to female plants. In heterosporous pteridophytes 

 and in seed plants, the sex of the gametophyte is determined long before spore forma- 

 tion, since it depends upon the kind of sporangium that is produced by the preceding 

 sporophyte. On the other hand, in homosporous pteridophytes and in monoecious 

 mosses, sex -determination appears to come much later than spore formation, and 

 to depend in part, at least, upon the conditions to which the gametophyte is exposed. 

 A remarkable situation has been disclosed in the dioecious mosses, in which sex is 

 determined at spore formation, half of the spores giving rise to male plants and half 

 to female plants, as in the liverwort, Sphaerocarpus. Pieces of the sporophyte may 

 give rise vegetatively to gametophytes, and such gametophytes are bisexual, whereas 

 gametophytes that develop from spores are unisexual. Hence it appears that the 

 supposedly non-sexual sporophyte is in reality bisexual. 



In the seed plants it seems probable that the sex of the gametophyte is determined 

 far back in the history of the preceding sporophyte, at least as far back as the seed. 

 In this event, ordinary sporophytes are as characteristically sexual as are the gameto- 

 phytes to which they give rise, so that it is proper to call a staminate plant male and 

 a pistillate plant female. There is' some evidence in favor of the view that in the 

 seed plants the sex of the gametophyte is determined farthei: back than the seed, 

 perhaps as far back as the gametes of the preceding gametophyte, or even as far back 

 as the spores from which the latter gametophytes arise. In Bryonia dioica and in 

 Cannabis sativa, experiments seem to show that the eggs have a female potentiality 

 and that half of the sperms have a male potentiality and half of them a female 

 potentiality. In case a sperm with a male potentiality fuses with an egg, the de- 

 veloping sporophyte is male, because the sperm dominates over the egg. If a sperm 

 with a. female potentiality fuses with an egg, the resulting sporophyte is female. 

 An alternative hypothesis postulates that sperms have either strong male potential- 

 ities or weak male potentialities, the former dominating over the egg, and the latter 

 being subordinate to the egg. In any event it would seem that in dioecious plants 

 the sex of a given sporophyte or of the subsequent gametophyte depends upon the 

 sexual potentiality of the preceding sperm or of the still more antecedent pollen grain. 

 In all of these phenomena, external factors seem to have no part, unless, perhaps, 

 they affect in some unknown way the sex tendency of pollen grains ; in any case it 

 seems clear that external factors operating upon a sporophyte can have no influence 

 upon the sex of the subsequent gametophyte. Supplementary evidence in favor 

 of the female potentiality of the egg is afforded by the fact that in Chara crinita 

 and in Antennaria, eggs which develop parthenogenetically always give rise to 

 female plants. Further data are afforded also by Mercurialis annua, a dioecious 

 species whose pistillate plants bear occasional staminate flowers ; in the event of 

 geitonogamy, these plants nearly always have female progeny; conversely the occa- 

 sional female flowers of male plants with geitonogamous pollination have male 

 progeny. 



In animals, as in plants, sex determination appears to be unrelated to obvious 

 external factors, the sex potentiality of the gametes being predetermined. The exact 

 factors involved in such predetermination are unknown, but it has been suggested 

 that in those animals in which the female possesses one more chromosome than does 

 the male, the extra chromosome may be the sex determinant. Formerly it was 



