i88 Plant Genetics 



50-50 ratio of males and females, but it cannot be 

 inferred that the sexes are evenly divided in every 

 tetrad; it may have been only an equal division in 

 the capsule as a whole. If, for example, the upper 

 half of a capsule produced all females and the lower 

 half all males, sex would not have been determined at 

 the reduction division, where, according to the sex 

 chromosome theory, it is determined. Sphaerocarpus, 

 therefore, gave STRASBURGER an opportunity to deter- 

 mine this situation, for he was able to isolate mature 

 individual tetrads, the four spores hanging together. 

 He sowed these tetrads in separate pots, obtaining four 

 gametophytes in each case. The experiment was con- 

 ducted upon rather a large scale, and the results were 

 strikingly constant. Two male and two female game- 

 tophytes were always produced from a tetrad. This 

 certainly established the fact that in this case sexes 

 were evenly separated at the reduction division, and 

 indicated strongly that the sex chromosome theory 

 applies in this group. What these facts established for 

 Sphaerocarpus has been taken for granted for the other 

 dioecious liverworts. 



Recently ALLEN (i) has claimed to have demon- 

 strated the x chromosome in Sphaerocarpus. He says 

 that one large chromosome, exceeding in length and 

 thickness the other chromosomes, characterizes the cells 

 of the female gametophyte, while the cells of the male 

 gametophyte are characterized by one very small 

 chromosome. His investigation seemed to show that in 

 spore formation two of the spores of the tetrad receive 

 the large chromosome, and the other two the small 

 chromosome. These chromosomes have not been defi- 



