Sex Determination 185 



higher plants exhibit alternation of generations, a phe- 

 nomenon practically unrepresented in animals. In at- 

 tempting to apply the sex chromosome theory through 

 the hfe-history of such a plant, with its alternation of 

 generations and heterospory, great theoretical difficulties 

 are encoimtered. In general, therefore, we reach the 

 conclusion that the sex chromosome theory has not been 

 established in plants and that certain obstacles seem 

 to stand in the way of its establishment. 



On the other hand, no very general physiological 

 theory has been established ; in short, sex determination 

 in plants is not yet well understood. A few suggestive 

 facts are known, however, and when more facts are 

 available some general theories doubtless will be devel- 

 oped. A general survey of the plant kingdom will show 

 the suggestive facts that are available. 



Algae. — No conclusive work has been done with this 

 group, but some of the experimental work of Klebs (8) 

 suggests that sex may be controlled in algae. 



Fungi. — Mention will be made only of the work 

 of Blakeslee and Burgeff. In his experimental 

 work with Mucor, Blakeslee (2) found in general 

 three different sexual types of mycelia, two of which he 

 called plus and minus strains. A.lthough they looked 

 ahke in every particular, he concluded that they were 

 sexually different for the following reason. Neither 

 strain by itself is capable of producing zygotes, but 

 when plus and minus strains are brought together sexual 

 brandies from the one meet sexual branches from the 

 other and produce abundant zygotes. The natural 

 conclusion is that Blakeslee's plus and minus strains 

 represent the male and female conditions, although the 



