Sex Chromosomes 507 



types are females, apparently the ratio of X chromosomes to 

 autosomes is not definitive. 



Since all plants lacking a Y chromosome are female, the pos- 

 sibility must be considered that simply the presence or absence 

 of the Y chromosome determines whether a plant is male or 

 female. A number of 2n, 3n, and 4n plants were examined which 



Fig. 145. Dissected flowers of Lychnis (Melandrium) dioica. Male, 

 2AXY (left), female, 2A XX (right), and male hermaphrodite, 2A XXY 

 (center) ; beneath the flowers are photomicrographs of root-tip chromo- 

 somes of each type. (Courtesy of Dr. H. E. Warmke in the American 

 Journal of Botany.) 



had one to four X chromosomes and one or two Y chromosomes. 

 Some were male, some were male with occasionally hermaph- 

 rodite flowers, and some were hermaphrodites with occasional 

 male flowers (Fig. 145) . That flowers can be produced with func- 

 tional female organs in plants with one or two Y chromosomes 

 shows that the presence of a Y chromosome is not the deciding 

 factor. 



The ratio of the X chromosome to the Y chromosome appears 

 to be a much more satisfactory explanation (Table 26). Plants 

 without an X chromosome are apparently not viable, for no 

 2A + YY individuals were found when male-hermaphrodites 



