Sex Determination 193 



therefore, depends upon which kind of pollen grain 

 functions. Such an idea, of course, would be quite in 

 keeping with a sex chromosome theory, where the male 

 is the heterozygote for sex. 



Strasburger (13) took exception to this view. He 

 thought that the phylogenetic development of the seed 

 plants had completely separated the male and female 

 tendencies through heterospory. As a consequence, he 

 would regard Correns' idea that half the pollen grains 

 have female tendencies as impossible. Strasburger's 

 idea is that the pollen mother-cell develops pollen 

 grains with stronger and weaker male tendencies, while 

 the megaspore mother-cell develops eggs with stronger 

 and weaker female tendencies. It is therefore the 

 algebraic sum of the two as they meet in fertilization 

 that determines the sex of the progeny. If a pollen 

 grain with strong male tendencies fuses with an egg 

 with weak female tendencies the progeny will be male, 

 and similarly for the other combination. This flavors 

 of Riddle's work. Just what would happen if two 

 strong male and female tendencies should meet would 

 be interesting. 



Strasbxtrger's strongest evidence for this theory is 

 his classic experiment with Mercurialis annua, with 

 which every botanist should be famihar. Mercurialis 

 may be called imperfectly dioecious. Certain plants are 

 prevailingly female but bear a few weak male flowers. 

 It would seem as though the female tendencies of such a 

 plant are stronger than the male. When such a plant is 

 inbred, using pollen from the weak male flowers on the 

 stigmas of the strong female flowers, the resulting 

 progeny is 100 per cent female, which is in accordance 



