Monoecious and Hermaphroditic Plants 509 



per cent the female parts possess ovules capable of developing 

 into seeds. When there are four times as many X chromosomes, 

 nearly all the flowers have well-developed pistils, setting abun- 

 dant seeds. Warmke has pointed out that the X / Y balance is 

 in the nature of a threshold reaction. Even though there is an 

 increase in the relative number of X chromosomes in the ratios 

 from 0.5 to 1.5, there is no increase in the degree of femaleness. 

 Ratios of 2.0 and 3.0 are sufficiently high for femaleness to be 

 expressed provided conditions are suitable, but when the ratio 

 is 4.0 almost every flower contains functional female organs. 

 The change from 3.0 to 4.0 apparently crosses the threshold so 

 that one additional X chromosome results in an almost complete 

 change from maleness to hermaphroditism. Increases beyond 4.0 

 in the ratio in plants containing at least one Y chromosome have 

 not yet been obtained. 



Although plants with the formula XXY are frequently male 

 with a few hermaphrodite flowers, a type exists in which the 

 XXY plants have stronger female tendencies. When this was 

 selfed, XX and XY types were recovered which produced a ratio 

 of 1 female : 1 male-hermaphrodite when crossed. The male- 

 tendency genes in the Y chromosome are at least three, as can be 

 shown by including in 2A + XX plants fragments of the Y chro- 

 mosome whose lengths vary in different individuals. If the Y is 

 completely absent, the plant is a female; but if parts of the Y are 

 present, male structures will be found which usually do not 

 attain complete development. Of the three genes, one appears 

 to be near the centromere, and is necessary if male structures are 

 to be initiated, one carries male development to completion, and 

 one suppresses femaleness. 



Monoecious and Hermaphroditic Plants 



In a number of species some flowers are male and others female 

 on the same plant, and in some species each type of flower is 

 produced at only one particular part of the plant. In many of 

 the higher plants, however, the stamens and the pistils are 

 produced near each other in different regions of the same bi- 

 sexual flower. 



Monoecious and hermaphroditic plants do not appear to have 

 any morphologically recognizable sex chromosomes, but they 

 undoubtedly possess both male and female tendencies even 



