Monoecism and Intersexuality 453 



plant might be called a female, because it has no functional male 

 organs, but this would be an incorrect name, because there is still a 

 tassel, only it is modified in the female direction. This means that the 

 plant is a female as much as a normal plant after cutting away the 

 tassel. The plant is actually a monoecious plant with a shift of expres- 

 sivity in one group of flowers by a homozygous modifier. It can, of 

 course, breed true to this condition, being homozygous for the 

 modifier, thus mimicking a dioecious female condition (of a type un- 

 known in nature) without really being a female. 



A second mutant in another chromosome, silkless (sk), specifi- 

 cally suppresses the gynoecium in the lateral flowers without affecting 

 the tassel. Thus it produces what functionally is a male plant but 

 genetically still a monoecious one with sterilization of the gynoecium 

 in the female flowers. It would be comparable to a normal monoecious 

 plant which has been castrated for all ovaries in the lateral flowers. 

 This is certainly only an imitation female, though with both modifiers 

 combined a line can be established with "female" and "male" plants, 

 but only in regard to function, not to sex determination. This is due 

 to the fact that tS2 homozygous is epistatic to sk sk, and therefore in 

 both locations "female" flowers are formed. As tso does not act so in 

 heterozygous condition, Ts2 tS2 sk sk is male, like sk sk alone. Thus a 

 true breeding dioecious line is established when Ts2/tS2 sk/sk is crossed 

 with tS2/tS2 sk/sk. It is not surprising that these facts, which are easily 

 understood when put in their proper place in the general theory of sex 

 determination, have led to the most confusing interpretations when the 

 general setting has been disregarded: confusing functional sex with 

 genetic sex, sex determiners and modifiers of expressivity, monoecic 

 F/M balance (where F = M) with epistatic effects of modifiers. This, 

 then, is clearly something completely different genetically from real 

 intersexuality. 



c. Monoecism and intersexuality in Streptocarpus 



In view of this, special importance is attached to flowering plants 

 in which androecia in situ are transformed into gynoecia (or vice 

 versa) in a series of real intersexual transformation. The fact that such 

 cases seem to result frequently from species crosses, like diploid 

 intersexuality in Lymantria and a few cases in Drosophila (see II 2 

 A; discussion of crosses in the repleta group), suggests that a similar 

 method of sex determination is working within the setting of monoe- 

 cism. This might permit us to assign each mechanism to its proper 



