14 MISCELLANEOUS PUBLICATION 955, U.S. DEPT. OF AGRICULTURE 



bodies, which may range from microscopic to structures weighing 20 

 or 30 pounds. 



Some species are strictly saprophytic, others are parasitic, but can 

 be grown free of their host, and still others are obligate parasites. 

 Many Basidiomycetes cause the decay of wood, whereas many others 

 are symbiotically associated with the roots of plants. 



The rusts (order Uredinales) are all obligate parasites on higher 

 plants. They have not been grown in pure culture. There are about 

 5,000 known species. Some rusts have as complex a life history as 

 any fungi known. For example, wheat rust (Puccinia graminis 

 Pers.) produces uredospores on wheat. These spores may germinate 

 to infect additional wheat plants and again produce more uredospores. 

 Eventually the mycelium-producing uredospores will produce a second 

 type of spore, teliospore, which germinates to produce a basidium and 

 basidiospores. The basidiospores then infect the barberry, forming 

 spermatia, and after fertilization the dikaryotic phase is established, 

 which then forms aeciospores on the barberry. Aeciospores are car- 

 ried by the wind to wheat plants, and the cycle is repeated. Both 

 hosts, which are unrelated, are required to complete the life cycle of 

 the wheat rust. Studies of plant tissue culture and the rust growing 

 on it offer a promising field for research. 



The smuts (order Ustilaginales) are parasitic fungi and are prac- 

 tically always found on flowering plants. This group consists of about 

 40 genera and 700 species. Often smuts produce large tumorlike 

 growths in the host, as in corn smut. Unlike the rusts, smuts can be 

 grown in pure culture on both solid and liquid media without much 

 difficulty. Often the mycelium fragments or even appears yeastlike 

 in culture. Pure cultures can be maintained by lyophilization. For 

 purposes of identification, it is important that the smut fungus be 

 studied from the host material. Work on smuts in culture has shown 

 that they produce a large number of interesting compounds, ranging 

 from lysine to ustilagic acid. 



Fungi in the order Tremellales are especially abundant in the Trop- 

 ics. Most of them are saprophytic on wood, and their fruiting bodies 

 are generally gelatinous when wet, hence the name "trembling fungi." 

 When dry, they become inconspicuous and hornlike. Although little 

 studied in pure culture as yet, their growth in pure culture is not 

 difficult. Carotenes have been identified already in some forms. 



The Polyporales, or bracket fungi, are easily grown in culture on 

 such media as malt agar. Almost all genera grow in dead or living 

 trees and are the most important group of fungi responsible for timber 

 decay. As far as we know, all genera and species can be grown in 

 pure culture, but it is difficult to retain viable cultures because they 

 lack spores. Identification depends on the nature of the fruiting 

 bodies, which are producing basidiospores. In some, spores are simply 

 borne on the smooth surface of the fruiting structure, others on a 

 wrinkled or roughened surface, and still others inside pores or tubes, 

 which have openings to the air on the lower side of the sporocarp. In 

 one group, spores are borne on the surface of toothlike outgrowths. 



All fungi that produce gills and are macroscopic are placed in the 

 Agaricales. This group is commonly called mushrooms or gill fungi. 

 In the more restricted sense there are about 4,000 species in approxi- 



