TIIALLOPHYTES 8l 



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

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

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



I. Protobasidiomycetes 



(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. 



Com smut. — The mycelium ranges widely through the host, even 

 in the roots, and becomes externally visible only upon flowering. At 

 that time the ovary, for example, becomes 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-waUed, 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 transformed 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 terminally, 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 re- 

 semblance. The spores produced, therefore, are probably basidiospores, 

 and the l^rand spores hold the same place in the life history of smuts as 

 that held by the teleutospores in the life history of rusts (see p. 82). 



