VII] 



USTILAGINALES 



189 



formation of clamp-connections (fig. I53«), or between unrelated cells 

 through a conjugation tube (fig. IS3(^). Where basidiospores are formed 

 they conjugate in a similar manner. In every case the nucleus of one 

 of the paired cells passes over into the other, and the two nuclei lie close 

 together, though without fusion. The mycelium throughout the develop- 

 ment of the host plant consists of binucleate cells and breaks up in spore- 

 formation into binucleate segments (fig. 152 a). Each young spore has 

 thus two nuclei which fuse during development so that the mature brand- 

 spore is uninucleate. 



In Ustilago Avenae, U. Hoi-dei a^nd U. Tritici, sub-species of U. Carbo, 

 Lutman observed that on conjugation some of the cytoplasm of one of the 

 cells passed over with the nucleus, the empty cell becoming shrivelled 

 (fig. 154). In U. Avenae a long fusion tube is frequently formed and both 



F'g' i.'iS- Ustilago Carbo: a. fovmation of basidiiim; 

 b. conjugation ; c. binucleate mycelium ; after 

 Rawitscher. 



Fig. 154. Ustilago Hordei\ 

 conjugation; after Lut- 

 man. 



nuclei, as well as the greater part of the cytoplasm, pass into it, leaving the 

 conjugating cells comparatively empty. In these varieties of U. Carbo Lutman 

 found that, after conjugation, the two nuclei lie closely pressed together so 

 that it was sometimes impossible to differentiate them. 



Ustilago Tragopogonis pratensis is parasitic on Tragopogon pratensis, in 

 the flower heads of which it produces a mass of dark violet spores. In the 

 young flower buds hyphae are abundant only in the anthers and ovary. 

 Later they spread to the surface of these organs and form a dense mycelium 

 of'delicate filaments. According to Dangeard and to Rawitscher they divide, 

 with the onset of spore-formation, into small binucleate cells; nuclear fusion 

 takes place and the spore acquires a thick reticulate wall. In germination 

 a three or four celled basidium is produced, each cell containing a single 

 nucleus, and gives rise to uninucleate basidiospores, which increase by 

 budding. 



