THE HIGHER PLANTS 49 



small Y. Since then (Allen, 1936) some thirty other bryo- 

 phytes have been shown to have distinguishable sex- 

 chromosomes. 



Mackay and Allen (1936) have found X:2A and 2X:2A 

 female gametophytes and 2Y:2A male gametophytes in 

 Sphaerocarpus. But gametophytes with the constitution 

 XY:2A were found to be intersexes. Similar polyploids 

 described by Knapp (1936) indicated that the male-deter- 

 mining genes preponderate in the autosomes and that the 

 Y was neutral. 



The Higher Plants. Sex-chromosomes have been dis- 

 covered also in the angiosperms. Santos (1923) found 24 

 matched pairs of chromosomes in the female and 23 matched 

 pairs and an unequal XY pair in the male of the dioecious 

 Elodea gigantea. Since then some fifty dioecious angiosperms 

 have been shown to have distinguishable sex-chromosomes 

 and in some twenty other dioecious species no such discern- 

 ible difference could be detected. It would appear that, as 

 a rule, in these dioecious species the male is the hetero- 

 gametic sex. Only in one of them was the female the hetero- 

 gametic sex and only in one, Dioscorea siniiata, was the male 

 found to be XO, 



Sex-linkage has been encountered in Melandrium (Baur, 

 1912; Shull, 1914). In this form Winge (1923) found a visibly 

 distinct XY pair of sex-chromosomes in the male, the female 

 being XX. Other sex-linked characters in Melandrium have 

 been described by Winge (193 1), who showed that crossing- 

 over between the X and the Y occurred. 



The monoecious (haemophroditic) Bryonia alba is closely 

 related to the dioecious (bisexual) B. dioica. Correns (1907) 

 found that the cross B. dioica 2 by the pollen of B. alba 

 gave only females, occasionally with a few^ male flowers. 

 The reciprocal cross B. dioica (^ x B. alba pistillate flowers 

 gave females (with an occasional male flower) and males in 

 equal numbers. 



There are no visibly distinguishable sex-chromosomes in 

 either of these species (Meurman, 1925). The hybrid pro- 

 geny are all sterile. 



