i88 HEMIBASIDIOMYCETES [CH. 



pairs, and, the nuclei not being identified, there was some question as to 

 whether the process was to be regarded as sexual (de Bary), or as a merely 

 vegetative phenomenon (Brefeld), like the formation of H-pieces and clamp- 

 connections. 



In 1894 Dangeard described the fusion of nuclei in the young brand- 

 spore, but it was not till several years later that this was correlated with 

 the union of the sporidia and the nuclear life-history made clear. The 

 salient points of this history are (i) nuclear association, (2) nuclear fusion, 

 (3) germination of the brand-spore, and formation of the basidium. At this 

 stage the fusion nucleus divides twice or oftener, and uninucleate cells are 

 formed. This sequence of events indicates that the basidium and its spores 

 are the starting point of a brief haploid phase, which gives way to the diploid 

 generation when conjugation takes place. 



The life-history of the Ustilaginales would appear to be reduced rather 

 than primitive, the conjugation of the spores replacing some ordinary 

 sexual process ; but the present state of our knowledge scarcely permits 

 speculation as to what the earlier alternation of generations may have been. 

 Tubiircinia primulicola, which has X.\yo parasitic phases, respectively uni- 

 nucleate and binucleate, suggests closer comparison with the Uredinales 

 than with any other investigated form. 



The Ustilaginales are divided into two groups, distinguished by the 

 character of the basidium, which is septate in the Ustilaginaceae and con- 

 tinuous in the Tilletiaceae. The two families are of about equal size, in- 

 cluding together over 400 species. 



The difficulty at first experienced in classifying these fungi is indicated 

 by the occurrence of such names as Uredo, Caeoma, Erysiphe, Ascomyces 

 (= Exoascus), and Lycoperdon among the older synonymy of the species. 



Ustilaginaceae 



Ustilago, with nearly 200 species, is the most important genus of the 

 Ustilaginaceae. It is cosmopolitan, occurring on all sorts of host plants, and 

 is characterized by the fact that its brand-spores are produced singly. 



Ustilago Carbo infects species oi Avena, Triticum and Hordeuni, the forms 

 on the different hosts being biologically distinct. Rawitscher observed that 

 the spores germinate readily in dilute nutritive solutions, forming a three 

 or four celled basidium from which basidiospores may be abstricted in the 

 usual way. More commonly, however, the basidia develop without spore- 

 formation into branched mycelia, between the cells of which conjugation 

 may take place. Union is accomplished between neighbouring cells of the 

 same filament by means of short outgrowths, which meet and fuse as in the 



