300 



THE FUNGI WHICH CAUSE PLANT DISEASE 



disappearing partially or wholly through gelatinization; fertile my- 

 celium compacting into masses and giving rise to numerous chlam- 

 ydospores formed from its contents. Conidia rarely develop on 

 the exterior of the host. Sori prominent, usually forming dusty or 

 agglutinated spore-masses that break out in definite places on the 

 host or more rarely remain permanently embedded in the tissues. 

 Spores (chlamydospores) light to dark colored, single, in pairs, 

 or in spore-balls, the latter often composed in part of sterile cells. 

 The Ustilaginales are all parasites on higher flowering plants. 

 The vegetative mycelium is mostly inconspicuous and is often 



■■«^s: 



Fig. 217. — Ustilago. 2, promj'celium with nucleus in mi- 

 tosis; 5, with 4 nuclei; 6, with conidia. After Harper. 



distributed very widely in the host plant without giving external 

 evidence of its presence until time of spore formation. It sends 

 variously formed botryose or spherical haustoria into the host 

 cells. At time of maturity of the fungus, the mycelium develops 

 in great abimdance at certain special places in the host, often in 

 the ovary, leading to the development of large mycelial structures 

 in ihe place of the host tissue. 



The chlamydospores develop directly from the vegetative my- 

 celium; new and nmnerous transverse cell-walls are formed; the 

 resulting short cells swell, round off and become coated with a 

 gelatinous envelope. This later disappears and the spores develop 

 a new, thick, usually dark, double wall which is variously marked. 



