SEX- CHROMOSOMES 2 1 9 



found in certain mosses that two kinds of spores are produced in 

 equal numbers in the sporogonium, and that these spores develop 

 into male and female plants respectively. These workers were 

 quite unable to alter experimentally the sexes of the gametophytes. 

 Very similar results to these were obtained by Schweizer for 

 Sphichnum sphcericum, and by Fleicher for some other genera of 

 mosses. Here again the spores were of tw^o sizes, large and small. The 

 large apparently gave rise to female plants and the small to male. 



The suggestion that sex-chromosomes were present in unisexual 

 Angiosperms was first made by Blackburn and Harrison for 

 Populus at the Hull meeting of the British Association. Only 

 the chromosome complement of the pollen was investigated and the 

 XY condition was suggested 



to be present. Later, however, V ^ (j 



Blackburn investigated both 

 the male and female side in ^ 

 Melandrium, and found the 

 male to have the XY com- 

 position and the female XX. Fig. 78. — Chromosome sets from 

 rpi . • . , male and female gametophyte of 

 ine most important case Sphcerocarpus. (After Allen, 



among Bryophytes, is that of from Sharp, Introduction to 



Sphcerocarpus. It was first ^ ^ ^^^^'' 



shown by Strasburger that the spores of a single quartet give rise 

 to tw^o male and two female plants. Ten years later Allen demon- 

 strated the presence of the first sex-chromosomes in plants. In 

 S. Donnellii the sporophyte has eight pairs of chromosomes, 

 including an unequal XY pair At the heterotypic division of 

 meiosis, this XY pair separate and then divide longitudinally 

 at the homotypic division. The two spores receiving the X- 

 chromosome develop into female plants, while those receiving the 

 Y-chromosome give rise to male plants. Schacke has demonstrated 

 the same situation in S. taxanus. 



The problem of sex-chromosomes in plants has now been 

 much investigated to the Angiosperms, and their presence has been 

 demonstrated in a number of heterophytic species. 



Santos has investigated the case of the unisexual plant, Elodea. 

 The nuclei of the somatic tissues contain 48 chromosomes. At 



