HOMOTHALL1SM AND HETEROTHALLISM 



321 



dealing with rusts and smuts from Stakman and his students. It 

 should be kept clearly in mind that the Hymenomycetes do not 

 possess sexual organs, although they occur in the Uredinales, nor 



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Fig. 55. Schematic representation of sex segregation at the third nuclear 

 division in Neitrospora sitophila. Each of the pair at C, the first division, 

 and each of the pair at F, the second division, is H , but at the third divi- 

 sion + and - segregate. There result 16 possible arrangements of the 

 ascospores with regard to sex potentialities, as shown by the series of rings 



below. 



(After Dodge.) 



do they produce definitive sex cells. Instead sexual functions are 

 carried out by paired nuclei. Nevertheless most of the species 

 are heterothallic and exhibit a definite sexual process. The basi- 

 diospores, whether of a homothallic or a heterothallic species, are 

 haploid; that is, their nuclei contain n chromosomes. When two 

 haploid mycelia of opposite sex of a heterothallic species grow 



