SEXUAL REPRODUCTION 93 



are isolated by septa, and each of the cells produces exo- 

 genous spores, or sporidia, as they are usually called. In 

 some genera the sexual nucleus undergoes three suc- 

 cessive bipartitions in the non-septate promycelium, and 

 the eight nuclei formed pass in order into the sporidia 

 that are formed by budding, at the apex of the promy- 

 celium. 



The promycelium is none other than an ordinary sporo- 

 gonium ; it is in its interior that the sexual nucleus having 

 2n chromosomes resulting from fusion in the gametangium, 

 reverts to its normal structure, represented by n chromosomes. 

 This necessary reduction, observed by Sappin-TroufTy in 

 Uredines, is without doubt produced in the same organ in 

 all the Basidiomycetes. 



The mode of formation of the oospore in the Ustilagineae 

 does not imply changes in the protoplasm ; this act is, 

 however, not much retarded, but takes place on the ger- 

 mination of the oospore, through the frequent anastomosing 

 of the cells of the promycelium and of the sporidia. 



In the Uredineae the cells bearing ordinary conidia or 

 spermatia are uninucleate, all other cells of the mycelium 

 are binucleate ; these nuclei divide simultaneously at the 

 same level, and the spindles are parallel to the axis ; thus it 

 results that the median septum isolates for each new cell 

 two nuclei whose relationship to each other becomes more 

 and more distant. From this method, passing on from 

 the stroma of the aecidium, it can be conceived how dif- 

 ferent in origin must be the pairs of sexual nuclei present 

 in the teleutospore. The gametangia are rarely isolated, 

 usually grouped in pairs, sometimes three, according to the 

 genera, at the apex of a pedicel, and collectively known as 

 teleutospores. In each gametangium the oospore is formed 



