VIl] 



USTILAGINALES 



187 



In the regions where the formation of brand-spores is to take place, the 

 mycehum becomes richly branched and often swollen and gelatinous. In 

 Ustilago and Sphacelotheca the sporogenous hyphae are divided into a 

 number of short segments in each of which the contents form a spore 

 surrounded by an independent membrane. The spores are enclosed at first 

 within the gelatinous parent walls, but later these disappear so that the 

 whole mycelium is transformed into a pulverulent mass of spores. In 

 Tilletia and Entyloma the sporogenous cells are budded off laterally from 

 the mycelium. 



In Tuburcinia a number of richly septate hyphal branches become inter- 

 woven, forming a knot or spore-ball in which spores to the number of 50 

 or 100 are developed from within outwards. In Urocystis the spore-ball is 

 small and the outer cells remain pale or colourless and do not function as 

 spores though they resemble them in form (fig. 151). In Doassansia the 

 outer sterile cells are wedge-shaped, and in Sorosporium they form a 

 gelatinous investment in which their individual boundaries are no longer 

 recognizable. 



Fig. 151. Urocystis Fischeri; spore- 

 ball, one spore germinating, x 500 ; 

 after Plowright. 



Fig. 152. Ustilago Carbo\ u, young, binucleate 

 brand-spores ; l>. older spores after nuclear 

 fusion ; after Rawitscher. 



The young spore, like the cells of the mycelium from which it is derived, 

 contains two nuclei (fig. 1^20). These undergo fusion, so that the mature 

 spore is uninucleate (fig. 152(5). The pairing of the nuclei, which begins 

 with the association of the basidiospores (or their conidia), is thus completed 

 in the brand-spore. 



The minute investigation of the group may be said to have begun in 

 1807 when Provost recorded the germination of the spores of Tilletia 

 Tritici. His work was continued by Berkeley, Tulasne, de Bary, the latter's 

 pupil Fischer von Waldheim, and Brefeld. 



Many of these early investigators observed the union of the sporidia in 



