THALLOPHYTA: FUNGI 



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Ustilago. The life history of the corn smut (Ustilago zeae) will be 

 described. The mycelium ramifies throughout the stem and leaves of the 

 corn plant and in its vegetative condition does not seem to do much dam- 

 age. It lives in the intercellular spaces, sending short haustoria into the 

 host cells. When flowers appear, some of the ovaries become packed 

 with the mycelium and, as a consequence, become greatly swollen and 

 distorted. Swellings may also appear in other parts of the plant. Later 

 the mycelium divides up into countless numbers of black spores, called 



Fig. 116. Ustilago zeae. A, an ear of corn infected with smut, some of the grains of which 

 are greatly enlarged and filled with chlamydospores, one-half natural size; B, a germinating 

 chlamydospore, the four-celled basidium producing basidiospores, X 1,400. 



chlamydospores, which form large powdery masses (Fig. 116^4). A 

 chlamydospore is a heavy-walled cell representing merely a transformed 

 cell of the vegetative mycelium. 



A chlamydospore may germinate at once but, as a rule, falls to the 

 ground and remains dormant until the following spring. Then it sends 

 out a short filament of three or four cells that lives saprophytically on 

 organic matter in the soil (Fig. 1165). Thin-walled basidiospores are 

 bud(ied off each cell of the filament, often in great numbers. This fila- 

 ment is a basidium but, because of the large number of spores produced, 

 is not a typical one. In some smuts, however, only one spore is budded 

 off each of the four cells of the basidium. The basidiospores infect 

 young corn plants in the spring. 



The cells of the vegetative mycelium are binucleate, as are the j^oung 

 chlamydospores. But before the chlamydospore is mature the two nuclei 



