SIGNIFICANCE OF CONJUGATE NUCLEI 



269 



in the diploid mycelium of the Hymenomycetes, the Uredineae, the 

 Ustilaginaceae, the Tilletiaceae, and the Exoascaceae and in the 

 ascogenous hyphae of the Pyrenomycetes and the Discomycetes can 

 be represented by the symbol {n)-{-{n), while the single nucleus in 

 the zygote and somatic or sporophytic cells of animals, Phanerogamia, 

 Pteridophyta, and Bryophyta can be represented by the symbol (2w). 



What biological advant- 

 age, if any, is there in the 

 diploid mycelium of the 

 Basidiomycetes and the Ex- 

 oascaceae and of the asco- 

 genous hyphae of the 

 Pyrenomycetes and the 

 Discomycetes containing 

 (n)-f(w) nuclei instead of 

 {2n) nuclei ? 



An attempt to answer 

 this question will be made 

 first for the Hymenomy- 

 cetes and subsequently for 

 the Uredineae, the Smut 

 Fungi, the Exoascaceae, the 

 Pyrenomycetes, and the 

 Discomycetes. 



The Hymenomycetes. 

 Animals and most plants 

 produce gametes. These 

 special sex cells are charac- 

 terised by being unicellular and by possessing a single nucleus 

 containing n chromosomes ; and when, during the fertilisation 

 or conjugation process, two gametes of opposite sex meet and 

 fuse, they form a single cell having a single nucleus containing 2n 

 chromosomes. 



The Hymenomycetes, in contrast with animals and most plants, 

 have no sexual organs and never produce gametes ; and yet they 

 exhibit a very definite sexual process. Most of the species are 

 heterothallic. A heterothallic species, in respect to sex factors, 



Fig. 140. — Coprinus logopus. Diagram sliowing 

 two haploid mycelia of the same sexual 

 constitution (ah), each derived from a single 

 basidiospore. Note the simple septa, the 

 wide-angled mode of brandling, the oidio- 

 phores with their groups of oidia, and tlie 

 isolated nuclei. The two mycelia, with 

 further growth , would soon fuse hyphally 

 with one another ; but, in the fusion cells, 

 the nuclei would not become associated in 

 conjugate pairs. Highly magnified. 



