iQo Outline of Genetics 



This mechanism of sex determination suggests the 

 following conclusions, which contrast sharply with the 

 corresponding conclusions that were drawn from the 

 sex chromosome mechanism: 



a) The sex ratio has no fixed value, but may be 

 modified artificially through manipulation of the effec- 

 tive physiological conditions. 



h) Sex is a quantitative matter, '^ strictly male" 

 and ''strictly female'' being merely the tw^o extremes, 

 between which there may occur various grades of "inter- 

 sexes" or ''sex intergrades." 



Having as a background these two contrasting the- 

 ories on sex determination in animals, w^e may consider 

 briefly some of the situations that have been uncovered 

 in the plant kingdom. A few meager bits of evidence 

 suggest a sex chromosome mechanism in plants. 



Strasburger (22) has described some experiments 

 with the liverwort SphaerocarpHS, which is peculiarly 

 favorable material for such work. It is "dioecious," 

 like many liverworts, but a remarkable feature is that 

 the spores hang together in the tetrad. Ordinarily 

 when spores mature the tetrads are no longer distinguish- 

 able. Sowing such free spores, one may get the 50-50 

 ratio of male and female gametophytes, but this is no 

 sure indication that the sexes are evenly divided in every 

 tetrad; it may have been only an equal division in the 

 capsule as a whole. Sphaerocarpus, howxver, provided 

 an opportunity to test this matter, for one could isolate 

 mature individual tetrads, the four spores hanging 

 together. When such tetrads were sown in separate 

 pots, four gametophytes were obtained in most cases, 

 and practically always two of the gametophytes were 



