288 RESEARCHES ON FUNGI 



In Ustilago violacea and U. avenae, according to Kniep,i it is 

 not possible to infect the host plant with unisexual cultures of 

 sporidia, whilst infection is readily effected with cultures containing 

 sporidia of both sexes. Apparently, two sporidia of opposite sex 

 unite on the surface of the host and then send a diploid mycelium 

 into the host's interior. It would therefore appear that, in U. 

 violacea and U. avenae, a diploidisation cf a multinucleate haploid 

 mycelium of one sex by one or a few nuclei derived from another 

 haploid mycelium of opposite sex does not occur. 



On the other hand, in Ustilago zeae, according to Hanna,' the 

 host plant becomes infected with haploid mycelia, and two haploid 

 mycelia of opposite sex, on meeting inside the host, fuse together 

 and give rise to a diploid mycelium. It may be that, in this species, 

 as in the Hymenomycetes, owing to the formation of conjugate 

 nuclei {n)-{-{n) instead of (2w) nuclei, one or a few nuclei of a 

 haploid mycehum of one sex are able to diploidise all the rapidly- 

 growing hyphae of another haploid mycelium of opposite. sex. If 

 diploidisation is able to proceed in this way, the avoidance of the 

 formation of (2w) nuclei and the formation of conjugate nuclei 

 {n)-\-{n) is as prime a factor in the working of the diploidisation 

 process in Ustilago zeae as it is in the Hymenomycetes. 



The formation of conjugate nuclei in the diploid phase of the 

 mycehum in the Smut Fungi has doubtless been inherited from 

 basidiomycetous ancestors in which the conjugate nuclear arrange- 

 ment was of high importance for the diploidisation process ; and 

 it may be that, in the Smut Fungi, the arrangement of the nuclei 

 in the diploid mycehum in the form of conjugate pairs {n)-{-{n) 

 instead of as isolated nuclei (2w), while still advantageous to some 

 species, e.g. Ustilago zeae, in other species, e.g. U. violacea and U . 

 avenae, has lost its primary significance. 



The Exoascaceae. The Exoascaceae are a small group of highly 

 specialised ascomycetous parasites. In Taphrina epiphijlla and 

 T. Klebahnii, each ascus contains eight haploid ascospores. Under 

 favourable conditions, these ascospores bud and give ofE sprout- 



1 Hans Kniep, Die Sexualitdt der niederen Pflanzen, Jena, 1928, pp. 1-544. 



2 W. F. Hanna, " Studies in the Physiology and Cytology of Ustilago zeae and 

 Sorosporium reilianum,'" Phytopathology, vol. xix, 1929, pp. 415-442. 



