Sex Chromosomes 505 



does not contain the organs of both sexes. Because of the separa- 

 tion of the sexes in this type, it might be thought that it would 

 be in this type that plants might be found which have sex chro- 

 mosomes that operate like the XY or the ZW mechanism in the 

 Animal Kingdom. A study of the chromosomes in many dioe- 

 cious species has shown that sex chromosomes certainly are to 

 be found in many of these forms but that dioecism is not neces- 

 sarily determined by sex chromosomes that are distinguishable 

 in their morphology. In a very extensive review of sex expres- 

 sion in the flowering plants, Allen has listed fifty-five species and 

 two varieties of angiosperms in which the female is definitely XX 

 and the male XY. This simple "Drosophila type" of mechanism 

 is not the only sex chromosome mechanism that is operating in 

 the dioecious angiosperms. In Dioscorea sinuata we find another 

 familiar type in which the female is XX and the male XO, 

 whereas in Fragaria elatior the female is the heterozygous sex 

 and is ZW and the male is ZZ. 



In some species we find sex chromosomal types that we do not 

 encounter in animals. In Humulus japonicus and in eight species 

 and two varieties of the dock Rumex, the female is XX and the 

 male has one X chromosome but two Y's, designated Yi and Y2. 

 The mechanism in Humulus Lwpulus and in a variety of these 

 species is much more complicated for, in addition to the two 

 sets of autosomes, the female has two pairs of X chromosomes, 

 designated Xi and X2, and the male has one member of each 

 of these pairs plus two nonhomologous Y chromosomes, desig- 

 nated Yi and Yo. Thus the complete formula for the female 

 would be 2A + 2Xi + 2X2 and for the male, 2A + Xi + Xo + 

 Yi + Y2. Almost the same formula could be assigned to Atri- 

 plex hymenelytra, which differs only in the presence of but one 

 Y chromosome in the male. The most unusual sex chromosome 

 mechanism is found in Phoradendron fiavescens var. macrophyl- 

 lum and in Ph. villosum. In these plants the female has two 

 sets of autosomes but no sex chromosomes and the male has the 

 two sets of autosomes and a Y chromosome. At the first meiotic 

 division this chromosome is unpaired and passes as a univalent 

 to one pole. The female has twenty chromosomes and the male 

 twenty-one. 



In his tabulation of dioecious species, Allen has listed forty-six 

 reported as having no chromosomes which can be identified as 



