No. f>25] 



SEXUAL FEPRODC( TWX IX I TXGl 



541 



fuse, but are paired and divide by conjugate division in 

 the ascogenous hyphae, fusing only after they reach the 

 young ascus. This is a direct transference to the ascomy- 

 cetes of the conditions described by Blackman and Christ- 

 man for the rusts, and it is certainly an obvious and easy 

 suggestion. It was made in a letter to the writer by Eaci- 

 borski in 1895 and later published by him. 1 In my opin- 

 ion, as noted above, judging from the size of the nuclei 

 in Claussen's figures of the young ascogenous hyphae, the 

 nuclear fusion has already occurred at this stage. 



It is evident that such profound changes as have led 

 to the wide-spread occurrence of endokaryogamy among 

 so many and such diverse groups of the fungi can hardly 

 have come about suddenly. Even on the mutation theory 

 it would hardly be supposed that in the rusts, for ex- 

 ample, the disappearance of function in the spermatia, 

 the new fusion in the aecidium, the long series of conju- 

 gate divisions in the destructive uredo stage of the 

 fungus and the endokaryogamy in the teleutospore 

 should all have appeared by a single step simultaneously. 

 There is a degree of correlation in all these changes with- 

 out doubt, but they must fairly be assumed to have 

 worked themselves out gradually in connection with the 

 development of the complicated life history with heterce- 

 cism and the numerous spore forms which characterize 

 the group. 



It is evident that at present the fusion of nuclei in the 

 teleutospore is simply the delayed union of the gametic 

 nuclei that came together in the cell fusion at the base 

 of the row of a?cidiospores. In my opinion, however, as I 

 have argued at length elsewhere, this fusion in the spore 

 mother-cell originated as a purely vegetative union as- 

 sociated in the ascus and basidium with the development 

 of the relatively gigantic size of these cells and the main- 

 tenance of the nucleo-cytoplasmic relation, the factors 

 involved being the abundance of nutritive material con- 

 centrated in the ascus and basidium as sporebearing or- 



