THALLOPHVTES 81 



Among the higher Basidiomycetes the basidia form a definite layer 

 (hymenium), whose structure and position are important in classifi- 

 cation. As yet. the classification of this great group is very uncertain, 

 but for our purpose two great series may be recognized. 



I. Protobasidiomycetes, in which the basidium is four-celled, each cell 

 bearing a spore; and II. Autobasidiomycetes, in which the basidium 

 is one-celled and bears four spores (or at least produces four nuclei). 



I. PROTOBASTDIOMYCETES 



(a) Ustilaginales 



General character. These are the smuts or brand fungi, destructive 

 parasites that attack the floral and other organs, notably the ovaries 

 of grasses, and are of course best known in connection with their 

 ravages among cereals. Ustilago Maydis, the corn smut, may be taken 

 as a representative. 



Corn smut. The mycelium may infect any part of the host, even 

 the roots, and may give external indications of its presence in any re- 

 gion. At the time of flowering, for example, the ovary may become 

 packed with mycelium, which causes a distorted, swollen, tumor-like 

 growth. These tumor-like swellings may be observed also in other 

 parts of the plant, including the tassels. Later this mycelium forms 

 additional cross walls; the short cells become rounded off and thick- 

 walled, and the mycelium is thus transformed into a mass of black 

 spores, which are the so-called brand spores, the whole mass being the 

 so-called smut. This kind of heavy-walled spore, which is a trans- 

 formed vegetative cell of a septate mycelium, is called a chlamydospore, 

 the name referring to the heavy, protective wall. These spores fall to 

 the ground and pass the winter. Upon germination in the spring, the 

 spore develops a short filament of three or four cells. This filament is 

 saprophytic and each cell buds out spores laterally and the end cell ter- 

 minally, suggesting conidium-formation. If abundant food supply is 

 available, spores continue to be abstricted in great numbers, and may 

 be multiplied further by the yeastlike budding of the spores (see p. 70). 

 This filament of three or four cells is thought to represent the basidium, 

 but in this case the very indefinite number of spores produced obscures 

 the resemblance. The spores produced, therefore, are probably basidi- 

 ospores, and the brand spores hold the same place in the life history 

 of smuts as that held by the teleutospores in the life history of rusts 



