508 



The Determination of Sex 



were selfed even though a certain percentage would be expected 

 among the male offspring. Plants in which the ratio is 0.5, 1.0, 

 and 1.5 are male, but when the ratio is 2.0 or 3.0 the plants are 

 males but have a few hermaphrodite flowers. AVhen the ratio is 

 4.0, the plants are hermaphrodites with occasional male flowers. 



TABLE 26 



Ratio of X Chromosomes to Y Chromosomes in Diploids and Polyploids 

 OF Lychnis (Melandrium) and Its Bearing on Sex 



(Modified from Warmke in the American Journal of Botany.) 



When only X chromosomes are present, the plants are females. 

 The results of these different ratios indicate that the Y chromo- 

 some produces a male tendency and the X chromosome a female 

 tendency. Since plants with a ratio of 1.0 and 1.5 are male, the 

 male tendency in the Y chromosome must be stronger than the 

 female tendency in the X chromosome. AVhen there are two or 

 three times as many X chromosomes as Y chromosomes, female- 

 ness can be expressed when the internal and external environ- 

 mental conditions are favorable. In such plants, less than 10 

 per cent of the flowers have female parts, and in less than 1 



