382 CLASS BASIDIOMYCETEAE 



Sexual reproduction occurs in two usually separated steps : the union 

 of two monocaryon cells to initiate the dicaryon mycelial phase and 

 eventually the union of the nuclei in the teliospores to form diploid nuclei. 

 The points at which the dicaryon phase may be initiated are quite variable 

 in the Smuts but much more definite in the Rusts. The details will be 

 taken up in the discussion of the respective orders. 



Order Uredinales (The Rusts). These form a group which manifests 

 a very high degree of evolutionary development in many directions. The 

 7,000 or more species are all strict parasites of Ferns (Pteridophyta), 

 Conifers (Strobilophyta), and Flowering Plants (Anthophy ta) . Not only 

 are the Rusts strict parasites but in many cases they are highly specialized 

 into biologic races which are confined to certain species of a host genus or 

 even to special agricultural varieties of a host species much as in the case 

 of Erysiphe graminis. In Europe Eriksson (1894, 1902) was the first to 

 make extensive studies into physiologic races confined to related species. 

 He was followed by numerous other investigators on that continent. In 

 the United States Carleton (1899, 1904) was the first to follow up Eriks- 

 son's work, followed by Freeman and Johnson (1911), and by Stakman 

 (1914), and others. The study of the physiologic forms on special agri- 

 cultural varieties of these species has been carried out in this country 

 very extensively by Stakman and various collaborators (1922). For 

 example in Puccinia graminis tritici Erikss. & Henn., the physiologic race 

 that attacks common wheat (Triticum aestivum L.) and durum wheat 

 {T. durum Desf.), over 150 physiologic forms have been distinguished in 

 North America and many additional forms, mostly distinct from these, in 

 Europe. These distinctions were made by means of a study of their power 

 of infecting 10 or more differential varieties of wheat. Similar physiologic 

 forms have been found in leaf rust of wheat (P. ruhigo-vera iritici (Erikss. 

 & Henn.) Carl.) by Mains and Jackson (1926) in the United States, 

 Scheibe (1929) in Germany, and Sibelia (1936) in Italy. Stakman and 

 others (1928) have also found them in maize rust (P. sorghi Schw.). 

 Physiologic forms have also been distinguished in flax rust (Melampsora 

 lini (Pers.) Lev.) by Flor (1935), in Puccinia helianthi Schw. b}^ Brown 

 (1936), in P. iridis (DC.) Wallr., by Mains (1938), in Uromyces phaseoli 

 typica Arthur, by Harter and Zaumeyer (1941), etc. 



With one or two exceptions the teliospores are produced within the 

 tissues of the host and remain internal or break out through the epidermis 

 separate and free or united together in a waxy mass or attached by stalks. 

 The sporidia are capable of wind distribution but are short-lived and 

 delicate so that they can be carried only short distances in a living condi- 

 tion. Two other spore forms are usually produced, the aeciospores and 

 the urediospores, both of which are relati\ ely thick-walled and capable 

 of remaining alive while being carried many miles by the wind (hundreds 



