20 4 RESEARCHES ON FUNGI 



interior, so that continued spore-production is correlated with 

 continued constructive metabolism in the mother-cell. 1 If Sporo- 

 bolomyces is phylogenetically a reduced Hymenomycete and if the 

 yeast cells are in some degree equivalent to basidia, we may suppose 

 that the change from the production of several spores on several 

 sterigmata simultaneously to the production of several spores on 

 one and the same sterigma in succession took place when the 

 mycelium became aquatic and developed its habit of budding. 



Granted that the facts concerned with the drop-excretion 

 mechanism support the view that Sporobolomyces is of basidio- 

 mycetous origin, let us now enquire whether or not this conclusion 

 is in harmony with the cytological investigations made by Guillier- 

 mond and by Miss Macrae and myself, as already recorded. 



It has been ascertained that the yeast cells and also the conidia 

 of Sporobolomyces each contain only one nucleus and that, through- 

 out the life-history, there is no trace of either conjugate nuclei or 

 karyogamy. This absence of sexual phenomena of the kind usually 

 found in typical Basidiomycetes, it must be frankly admitted, does 

 not support the idea that Sporobolomyces belongs to the Basidio- 

 mycetes ; but, as Kluyver and van Niel 2 have pointed out, it does 

 not prove that Sporobolomyces does not belong to the Basidiomycetes, 

 but rather leaves the question open. 



If Sporobolomyces is of basidiomycetous origin, how comes it 

 that the yeast cells show no signs of conjugate nuclei or of karyo- 

 gamy ? There are two possible answers to this question. One is 

 concerned with the phenomenon of heterothallism and the other 

 with a possible loss of sexuality. 



Sporobolomyces may be heterothallic, and it may be that only 

 one sexual strain of each species, i.e. a ( + ) or a ( — ) strain, has so 

 far been isolated and grown. If this is so, the cytological investi- 

 gations have been made on haploid unisexual strains of Sporobolo- 

 myces only, and the absence of conjugate nuclei and karyogamy 

 can readily be understood. Attempts should be made to isolate 

 new wild strains of Sporobolomyces and to mate them with our 

 present strains in the hope that, when strains of opposite sex have 

 been brought together, conjugation may be observed. 



1 Vide supra, p. 184. 2 A. J. Kluyver and C. B. van Niel, loc. cit., p. 393. 



