1 88 RESEARCHES ON FUNGI 



of pairs of conjugate nuclei may come into existence. Finally, such 

 pairs of nuclei come to lie in special cells — in the Hymenomycetes 

 in the basidia, in the Uredineae in the teleutospores, and in the 

 Smut Fungi in the chlamydospores— where they fuse. The fusion 

 nucleus, which contains 2n chromosomes, then divides twice (in some 

 Tilletiaceae several times) and, during these divisions, a reduction 

 of the chromosomes accompanied by segregation of sex factors, etc., 

 takes place, so that the four nuclei (in some Tilletiaceae more than four 

 nuclei) which result from the division of the fusion nucleus each con- 

 tain n chromosomes. Of these nuclei one passes into each basidiospore 

 or its equivalent ; and thus each basidiospore is a haploid cell. 

 The basidiospores give rise to haploid mycelia ; and, when two such 

 mycelia (or their equivalents) of opposite sex meet and fuse, once 

 more two nuclei of opposite sex become associated as a conjugate pair. 



Conjugate nuclei are also present in the Exoascaceae, the 

 Pyrenomycetes, and the Discomycetes, subdivisions of the Asco- 

 mycetes, but in the Pyrenomycetes and the Discomycetes they are 

 confined to the ascogonium and to the ascogenous hyphae and 

 young asci of the fruit-bodies, and they are unknown in the 

 vegetative mycelium. 



In all animals, in Algae, the Bryophyta, the Pteridophyta, and 

 the Phanerogamia, when two nuclei of opposite sex meet in a cell 

 (usually the egg-cell), they fuse and form a single nucleus containing 

 2n chromosomes, and in all these organisms the phenomenon of 

 conjugate nuclear division is entirely unknown. 



In view of the fact that conjugate nuclei are known only in 

 the Basidiomycetes and in certain Ascomycetes, we may ask the 

 question : of what biological advantage, if any, are conjugate nuclei 

 in these two groups of fungi ? 



In this chapter an attempt will be made to throw light on the 

 significance of conjugate nuclei and conjugate nuclear division, with 

 special reference to the Hymenomycetes. 



While reflecting on the problem of the significance of conjugate 

 nuclear division in the Hymenomycetes, it occurred to me that the 

 non-fusion of nuclei of opposite sex in the diploid mycelium of 

 Mushrooms and Toadstools and their association as conjugate pairs is 

 not only an interesting cytological fact, but also may be something 



