196 Outline of Genetics 



lished ; a regular sex chromosome mechanism has not as 

 yet been perfected. 



It is small wonder that bewildering sex conditions appear 

 in these "imperfectly dioecious" angiosperms. Schaffner and 

 others have pointed out how extrinsic factors may operate to 

 determine sex in such forms. It is evident, though, that intrinsic 

 hereditary factors may also play their part in such cases. This 

 may be illustrated by some of the experiments of Strasburger 

 (22) on Mercurialis (later confirmed by Yampolsky 23). Stras- 

 burger had the idea that the pollen mother-cell develops pollen 

 grains with stronger and weaker male tendencies, while the mega- 

 spore 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 mates with an egg with weak 

 female tendencies the resulting individual will be male, and simi- 

 larly for the other combination. These assumptions are supported 

 by the behavior of Mercurialis. This form has for the most part 

 pure male and pure female individuals, but at times it throws 

 intersexes of various grades. Certain plants are prevailingly 

 female, but bear a few "weak" male flowers. In a plant of this 

 sort, it would seem that the female tendencies 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 all female, which is in accordance with Stras- 

 burger's theory. Other plants are prevailingly male, but bear 

 a few weak female flowers, and inbreeding these results in all 

 males. Finally, there are some plants which are evenly monoe- 

 cious, half their flowers being strong males and the other half 

 strong females. Inbreeding such plants yields a progeny which is 

 50 per cent male and 50 per cent female. It is obvious that from 

 such results Strasburger would be convinced of his theory of 

 male and female tendencies. 



Of considerable interest to botanists is some work that has 

 been done on the sexual condition of Mucor and related genera of 

 fungi. Blakeslee (2, 3) found three different sexual types of 

 mycelia, two of which he called "plus" and "minus" strains. 



