14 STRUCTURE AND CLASSIFICATION OF FUNGI 



connections. It is much more difficult to establish sexual relation in 

 these species. 



In addition to the large, fleshy mushrooms and their allies (the 

 subclass Homobasidiomycetes) certain more minute forms are in- 

 cluded in the Basidiomycetes because of the mode of formation of 

 their basidiospores. These Heterobasidiomycetes include largely 

 plant parasites, the smuts (Ustilaginales) and the rusts (Uredinales) . 

 Smuts. As an example of the Ustilaginales we may consider the 

 causative agent of the corn (maize) smut, Ustilago Zeae. When a 

 smut sporidium, which is uninucleate and haploid, comes in contact 



with the corn plant only a super- 

 gJIg ^ "" ' .^il^^i ^^^^^ infection of fine haploid 



IdP^*;;. M^F^^ mycelium takes place. If myce- 



lia from two sporidia of opposite 

 mating type come in contact, fu- 

 sion of cells may take place and 

 a heavy and extensive mycelium 

 with paired nuclei which divide 

 conjugatcly results. This dikary- 

 otic mycelium eventually matures 

 in special galls. Many of the cells 

 become rounded, rough and dark 

 in color, and in these cells, 

 morphologically chlamydospores 

 (each at first with a single pair 

 of nuclei), the nuclei unite and 

 the cells become uninucleate and 

 diploid. These are the smut spores. They are disseminated in 

 great numbers and are capable of considerable dormancy. On 

 germination, reduction division takes place. Then septa are formed, 

 dividing the promycelium, which is a short germination hypha 

 growing from the smut spore, into three or four cells. From each 

 of these cells of the promycelium, daughter cells repeatedly bud 

 off. These are the haploid uninucleate sporidia mentioned above. 

 (See Fig. 6.) The promycelium may be considered a basidium and 

 the sporidia basidiospores. These sporidia are capable of multiply- 

 ing by budding and can be cultured on nutrient media. They have a 

 superficial resemblance to yeasts. 



It will be noted that both the mushroom and the corn smut have 

 haploid uninucleate basidiospores which germinate to form a haploid 

 mycelium. In both, mycelium of opposite mating type may fuse 

 to produce a mycelium with paired nuclei, the dikaryon. After ex- 



FiG. 5. An ear of corn affected with 



smut. Photographed by Dr. N. F. 



Huff. 



