290 RESEARCHES ON FUNGI 



the Pyrenomycetes and Discomycetes, therefore, the diploidisation 

 of one haploid myceUum by one or a few nuclei derived from 

 another mycelium of opposite sex, which is so characteristic of the 

 Basidiomycetes, never takes place. 



In certain Pyrenomycetes and in the Discomycetes, conjugate 

 nuclei, while unknown in the vegetative mycelium, have been 

 found to be present in the ascogenous hyphae which are pushed 

 out from the ascogonium. In these hyphae cell-division accom- 

 panied by conjugate nuclear division and by the formation of 

 hooks takes place and continues until the asci have been produced. 

 In each young ascus the two nuclei of opposite sex fuse together. 

 The fusion nucleus then undergoes three successive divisions thus 

 originating eight haploid nuclei, one for each of the eight spores. 

 Of the eight spores in an ascus, four are of one sex and the other 

 four of opposite sex.^ 



In some Pyrenomycetes, e.g. Gnomonia leptostyla, Venturia 

 inaequalis, Claviceps purpurea, and in some Discomycetes, e.g. 

 Pyronerna confiuens, Ascobolus magnificus, both kinds of sexual 

 organs, ascogonia and antheridia, are developed ; while in other 

 Pyrenomycetes, e.g. Polystigma rubrum, Gnomonia erythrostoma, 

 and in other Discomycetes, e.g. Ascobolus furfuraceus, Ascophanus 

 carneus, Cudonia lutea, Humaria granulaia, Rhizina undulata and 

 Thelebolus stercoreus, ascogonia are developed but no antheridia. ^ 



In the sexual process which goes on in Pyrenomycetes and 

 Discomycetes, one or more nuclei derived from an antheridium or, 

 in apandrous species, derived from some other cell form one or 

 more pairs of conjugate nuclei {n)-\-{n) in the ascogonium,^ and 



^ In some species, e.g. the Pyrenomycetes Neurospora tetrasperma investigated 

 by B. 0. Dodge {Journ. Agric. Research, vol. xxxv, 1927, pp. 289-305) and Pleurage 

 anserina investigated by Miss E. Silver Dowding {Annals of Botany, vol. xlv, 

 January, 1931), there are only four spores in each ascus, and each spore contains 

 two nuclei of opposite sex and is bisexual. There are various other exceptions to 

 the lule that in Pyrenomycetes and Discomycetes the ascus contains eight spores, 

 but it is unnecessary to discuss them here. 



^ For the literature vide H. Kniep, Die Sexualitat der niederen Pflanzen, Jena, 

 1928. 



^ For the present, with Hans Kniep, I accept the conclusion of Claussen (as 

 against that of R. A. Harper) that, in Pyronema confiuens, the male and female 

 nuclei, on meeting in the ascogonium, do not fuse but form conjugate pairs. 



