SEXUAL REPRODUCTION 95 



oospore germinates immediately ; the sexual nucleus 

 divides indirectly, forming four new nuclei ; this division 

 is not followed by the formation of cross-walls, all trace of 

 a promycelium has disappeared, and four spores are 

 produced directly on the basidium. 



From the Uredineae to the Basidiomycetes proper the 

 progressive reduction of the promycelium to its entire dis- 

 appearance is complete, and all the other phenomena 

 correspond with absolute exactitude. 



In the Ustilagineae the mixture of different protoplasm 

 is effected by the anastomosing of the cells of the promy- 

 celium and of the sporidia. In the other groups there is 

 not the same opportunity, yet anastomosing of the hyphae 

 is of frequent occurrence, but produced at different stages 

 of development, and more or less removed from the period 

 of germination of the oospore. 



In the Ascomycetes the phenomena of reproduction are 

 simple, as in the Basidiomycetes. There is not the same 

 uniformity in the structure of the mycelium. The cells are 

 uninucleate or multinucleate ; in some species there are 

 more than thirty nuclei in a cell, and variations exist in the 

 number of nuclei present in different cells of the same 

 mycelium. When two nuclei are present in a K cell, as in 

 Exoascus, the different origin of the sexual nuclei results 

 from a simultaneous and parallel division, as in the Uredi- 

 neae. Most frequently another method is followed; the 

 extremity of a filament becomes recurved against the cell 

 below it, the nuclei of these two cells divide, then two septa 

 isolate a gametangium containing the two median nuclei, 

 which are of different origin. Sometimes the gametangium 

 is formed from two contiguous uninucleate cells whose 

 contents fuse. The gametangium contains two gametes, as 



