SIGNIFICANCE OF CONJUGATE NUCLEI 287 



If Hypothesis No. 2 is a sound one, then the biological advantage 

 which accrues to the Uredineae in having the nuclei in their diploid 

 myceUum organised as conjugate pairs of nuclei (w)+(w) rather 

 than as isolated nuclei {2n) lies in this : that the nuclear arrange- 

 ment just indicated is correlated with an inhibition which prevents 

 two nuclei of opposite sex fusing with one another, which inhibition 

 makes possible the diploidisation of an extensive multicellular and 

 multinuclear haploid mycelium by means of one or a few nuclei 

 derived either from another haploid myceHum of opposite sex or 

 from one or more pycnidiospores of opposite sex. 



The U stilaginaceae and the Tilletiaceae. The Smut Fungi, like 

 the Rust Fungi, are a group of highly specialised parasites ; and 

 it may be that they have been evolved from some branch of the 

 Hymenomycetes. In any case, there can be but little doubt that 

 the Hymenomycetes, the Rust Fungi, and the Smut Fungi have all 

 been derived from a common basidiomycetous stock. 



The Smut Fungi, like all other Basidiomycetes, are devoid of 

 sexual organs and of true gametes ; and their sexual process, in 

 its general features, resembles that of the Hymenomycetes and the 

 Rust Fungi. Union takes jjlace between two morphologically 

 indistinguishable sporidia outside the host [Ustilago violacea, U. 

 avenae), or between two haploid mycelia inside the host {Ustilago 

 zeae), and a diploid mycelium containing conjugate nuclei is then 

 formed. This mycelium develops in the host plant ; and, as it 

 grows and forms new cells, conjugate nuclear division is accom- 

 panied by the formation of clamp-connexions ( Ustilago, Doassansia, 

 Urocystis, Tilletia, Entyloma, Tubercinia).^ A pair of conjugate 

 nuclei is cut off in each young chlamydospore and then conjugate 

 nuclear division comes to an end. The two nuclei of the pair soon 

 unite, so that the mature chlamydospore is uninucleate. When a 

 chlamydospore germinates, the fusion nucleus {2n) divides twice, 

 or in some cases more times, and these nuclear divisions are accom- 

 panied by a segregation of the sex factors and doubtless also by a 

 reduction in the number of chromosomes to one-half. Each haploid 

 nucleus {n) finds its way into a sporidium, and so haploid cells are 

 once more brought into existence. 



1 R. Seyfert, " tjber Schnallenbildung im Paarkernmyzel der Brandpilze," 

 Zeitschriftf. Botanik, Bd. XIX, 1927, pp. 577-601. 



