1 86 



HEMIBASIDIOMYCETES 



[CH. 



Fig. 149. Ustilago antherarum Fr.; a. and b. conju- 

 gating basidiospores; c. conjugation between a cell 

 of the basidium and a basidiospore ; after Harper. 



fields. As a rule the conidia are of the same oblong form as the basidio- 

 spores, but, in the genus Tilletia and some of its allies, they may be stout or 

 sickle-shaped, whereas the basidiospores are long and narrow. 



In Entyloma the brand-spores are capable of germination on the tissues 

 of the host leaf, where they give rise to hyphae which penetrate through 

 the stomata and form basidia from which basidiospores are produced. 



During their development the cells of the basidium, the basidiospores, 



or the conidia budded out from 

 them, may become united in 

 pairs (figs. 148 , 149), by means 

 of a tube of variable length put 

 out by one or both participants 

 and recalling somewhat the con- 

 jugation tube of the Zygnema- 

 ceae. The growth of these tubes 

 is very accurately directed and 

 appears to depend on a chemo- 

 tropic stimulus. 



In the majority of cases the 

 nucleus of one of the associated 

 cells passes down the tube into 

 the other, but does not fuse with its nucleus (fig. 150). Later both nuclei 

 divide, and a mycelium of binucleate cells is produced. It is on this 

 mycelium that the infection of the host depends ; it penetrates the tissues 

 usually of the seedling, but sometimes of the developing parts of the mature 

 plant, being in most cases derived from spores which adhered to the seed 

 coat. These may be destroyed by dipping the seed into hot water or formalin 

 solution before sowing. 



Once in the tissues of the host the binucleate mycelium penetrates in all 



directions, ramifying between 

 the cells of the host and send- 

 ing haustoria into them. The 

 internodes of the stem are tra- 

 versed by long, unbranched 

 hyphae, but in the nodes branch- 

 ing is frequent, and here also 

 the majority of the haustoria are to be found. Where the host is perennial 

 the mycelium perennates in it, and, if the host dies down during the winter, 

 remains alive but quiescent in the upper part of the root stock till the growth 

 of new shoots in spring gives it a fresh opportunity of development. 



Conidia have been recorded on the parasitic mycelium of Tuburcinia 

 and Entyloma but are not of common occurrence at this stage. 



Fig. 150. Ustilago Hordei; conjugation; after 

 Lutman. 



