314 Transactions British Mycological Society. 



cates, however, that a definite physiological differentiation takes 

 place at the time the sexual organs are formed. We do not know 

 what this differentiation may be. It is not necessarily associ- 

 ated with a difference in the amount of food present in the re- 

 spective male and female organs, for the same physiological 

 differentiation obtains, as Blakeslee has shown, in the bisexual 

 morphologically isogamous mucors. We may perhaps conceive 

 it as something of the nature of a chemical difference in the 

 nuclei brought about by their reactions to cytoplasmic influence, 

 or possibly to some differentiation or segregation of hereditary 

 factors. In the subsequent fusion of the male and female nuclei 

 in the zygote, we have therefore no blending of two lines of 

 descent but simply a recombination of nuclei which had be- 

 come more or less differentiated in the individual. This re- 

 combination restores the vigour necessary for further develop- 

 ment, and is sufficient to enable the fungus to continue its 

 development by a prolonged period of vegetative reproduction 

 during which vast numbers of asexual spores are formed without 

 any further nuclear fusion, until the stage is reached when re- 

 invigoration again becomes necessary and sexual organs are 

 once more formed. 



In the higher Fungi this normal type of sexual nuclear fusion 

 has disappeared or is disappearing and is being replaced by 

 a simpler type of nuclear fusion — endokaryogamy — the 

 purpose of which is to provide for the nuclear reorganisation 

 and reinvigoration of the individual reproductive cells just 

 at the time when large numbers of spores are about to be 

 formed. 



In the Hymenomycetes and Uredineae this appears to be the 

 only type of nuclear fusion that remains. The basidium of the 

 Hymenomycetes, with its two nuclei, is the last term in a long 

 series of binucleated cells which appears to extend back to a 

 period prior to the formation of the carpophore. How this bi- 

 nucleate condition comes about we do not know, although sug- 

 gestions have been made that it may arise as the result of fusions 

 taking place between the cells of the primary mycelium by 

 which plasmogamy is effected and cells with two nuclei are pro- 

 duced. It has also been stated that the binucleate condition is 

 brought about by means of the clamp connections, but this 

 requires confirmation. On the evidence at present available the 

 most satisfactory explanation is that the binucleate condition 

 occurs simply by differentiation during the formation of the 

 cells of the primary mycelium which, at the beginning of their 

 development, may contain from one to man3/ nuclei. 



In the Uredineae the origin of the binucleate cells has been 

 more clearly determined. In those forms which possess an 



