336 THE BIOLOGY OF FLOWERING PLANTS 



obtained the same result for the cross between the dicEcious 

 Lychnis alba and the hermaphrodite Silene viscosa. Shull 

 (1910, 191 1), using hermaphrodite individuals of Lychnis 

 dioica, of which he found 6 among 8000 individuals, 

 obtained similar but rather more complex results. 



In these three cases, and perhaps in the majority of 

 dioecious plants, the appearance of the male sex is determined 

 by the presence of a single dominant Mendelian factor, which 

 suppresses the female potentialities. This gives a reason- 

 able explanation of how the potentialities of both sexes 

 may be present though only one appears : in the absence 

 of the special factor the female potentiaHty is most powerful. 

 As the determining factor is, of course, subject to the influence 

 of external conditions, its action may be modified by these, 

 as when the pistillate flower of the red campion produces 

 stamens. 



Dioecious plants do not, however, all behave alike in this 

 respect. Strasburger (1909, 1910) obtained results with Mer- 

 curialis annua, confirmed by Bitter (1909) and Yampolsky 

 (1919), which do not admit of this explanation. The female 

 plants of this species occasionally bear male flowers, and 

 the male plants female. If the female flower is fertilised 

 by pollen from male flowers on the same plant, the off- 

 spring are all, or nearly all, female. If the occasional 

 females on a male plant are fertilised by pollen of 

 that plant, the offspring are all, or nearly all, male. The 

 normal cross fertilisation gives equal numbers of male and 

 female. The male plant does not therefore behave as a 

 heterozygote ; all its germ cells appear to bear male deter- 

 minants. Strasburger supposed that these were of two 

 *' strengths." Half the male plants bear a " weak " male 

 determinant = M I, the other half a " strong " = M HI. 

 All the female plants have a female determinant expressed 

 as F II. When an M I sperm cell meets an F II ovule the 

 female tendency predominates and a female results ; when 

 a strong male sperm, M HI, meets an F II egg cell the male 

 tendency predominates and a male results. Something of 

 this sort may occur in cases of complex distribution. 



