SEX DETERMINATION 333 



shows that so simple an explanation is not sufficient. 

 It does not explain the occurrence of the intergrading 

 individuals. 



An illuminating example is that of Lychnis diotca, the red 

 campion. It is strictly dioecious and intermediates are rare, 

 having been observed by one investigator only (G. H. Shull, 

 1910). It is subject, like many other Caryophyllaceous plants, 

 to the attack of a smut fungus, Usiilago violacea, which 

 invades the anthers and fills them with violet spores. If 

 the smut infects a female plant, which bears no stamens, its 

 presence acts as a stimulus to the production of these organs 

 and the spores are formed in the anthers as usual. Here is 

 an individual, normally pure female, which can yet produce 

 male organs under suitable conditions. It must in reality 

 be potentially bi-sexual. It is difficult to avoid the con- 

 clusion that if this is so in an extreme case of unisexuality 

 like Lychnis, then it must be true of all individuals of all 

 flowering angiosperms. Maleness and femaleness as such 

 cannot be inherited alternatively ; they must exist as 

 potentialities side by side in the same individual and flower, 

 their expression determined by other factors. 



These determining factors may in some cases be external 

 to the plant. In monoecious species it is frequently possible, 

 by change in the external conditions, to modify profoundly 

 the types of flower borne by the individual. Thus Riede 

 (1922) found that conditions favouring assimilation promote 

 the development of female flowers in the monoecious 

 maize, while increased absorption of mineral salts favours 

 the male flowers. In Ariscema triphyllum, a dioecious 

 aroid with many intermediate individuals, Schaffner (1922) 

 found that plants cut back and kept dry tend to produce only 

 male flowers in the following year, while plants well watered 

 and dunged produce only female. Stout (1923) has shown 

 that in Cleome spinosa, which goes on flowering for two to 

 three months, production of male flowers alternates regularly 

 with production of female and hermaphrodite flowers. 

 This looks like a nutritive effect comparable to the two 

 other cases cited. 



