SIGNIFICANCE OF CONJUGATE NUCLEI 289 



conidia (yeast-like cells) which are able to multiply themselves by 

 further budding. Each bud is haploid and has the same sexual 

 constitution as the ascospore from which it was derived. When two 

 sprout-conidia of opposite sex lie close together, one of them sends 

 out toward the other a short process which fuses with the other 

 conidium. Thereupon the nucleus of the first conidium passes into 

 the second conidium. The second conidium then puts out a germ- 

 tube which penetrates into the host and develops there into an 

 extensive vegetative mycelium. This mycelium contains pairs of 

 nuclei which apparently multiply by conjugate nuclear divisions 

 and, finally, it gives rise to a hymenium composed entirely of asci 

 situated directly on the surface of the host. Each young ascus 

 contains a pair of conjugate nuclei which soon unite to form a 

 fusion nucleus. This fusion nucleus then divides three times and 

 so gives rise to eight haploid nuclei, one for each spore. ^ 



The life-history of a typical Exoascus, as outlined above, very 

 much resembles that of Ustilago violacea and U. avenue already 

 described. Just as in those Smut Fungi, so far as our present 

 knowledge goes, there does not seem to be any possibility of the 

 conjugate nuclei aiding the diploidisation process. If the forma- 

 tion of conjugate nuclei was of advantage for the diploidisation 

 process to the non-parasitic ancestors of the Exoascaceae, that 

 advantage may have ceased to exist when the Exoascaceae were 

 definitely evolved. 



The Pyrenomycetes and the Discomycetes. The Pyrenomycetes 

 and the Discomycetes — the two largest groups of the Ascomycetes 

 — have a vegetative mycelium which, so far as is at present known, 

 differs from that of the Basidiomycetes in being always haploid and 

 never diploid. Normally, the fruit-bodies of Ascomycetes are 

 produced on a single haploid mycelium (monoecious species) or on 

 two sexually interacting haploid myceha (dioecious species) and, 

 in a number of species, each fruit-body owes its origin to sexual 

 organs — ascogonia and antheridia. In the vegetative mycelium of 



1 M. Wieben, " Infektion, Myzeliiberwinterung und Kopulation bei Exoasceen," 

 Forschungen a.d. Geb. d. Pflanzenkrankheiten, herausgeg. v. Schaflfnit, Heft III, 

 1927, pp. 139-176 (cited from H. Kniep, " Vererbungserscheinungen bei Pilzen," 

 Bibliographia Oenetica, Bd. V, 1929, pp. 387-388). 



VOL. IV. ^ 



