16 



FUNGI IMPERFECTI: THE IMPERFECT FUNGI 



THERE are a great many species of fungi of which the perfect stage is not 

 known and which therefore cannot find a place in the classes already 

 discussed. By the term ''perfect stage," as here used, is meant that stage 

 in which the ultimate sexual structures are formed, e.g., zygospores, 

 oospores, asci, basidia, and teliospores. Most of the Phycomyceteae are 

 so characteristic in their mycelial structure as well as in their modes of 

 asexual reproduction that ordinarily the genus and often even the species 

 can be determined from the asexual stage alone. Thus the Imperfect 

 Fungi are practically confined to those Higher Fungi in which the stage 

 is lacking in which the asci, basidia, or teliospores are produced. Since 

 most of the Uredinales have very characteristic asexual stages the im- 

 perfect forms of this order are readily assigned to that group and are 

 placed in one of the imperfect genera there, e.g., Aecidium, Uredo, Caeoma, 

 etc., if their host requirements and other features make it impossible to 

 assign them to described species in recognized perfect genera of that order. 

 Thus it comes about that the Fungi Imperfecti, as ordinarily considered, 

 include those fungi not otherwise referable to their natural relationship 

 (e.g., Phycomyceteae or Uredinales) whose true relationship cannot be 

 determined in the absence of the perfect stage. Judging by the rather 

 exceptional presence of clamp connections, as well as by the similarity of 

 the conidial stages to those in the Class Ascomyceteae, it is probable that 

 a great majority of species of imperfect fungi really belong to that class, 

 and the perfect stage is not present in the specimens examined. It is 

 possible that some fungi have lost entirely the power to produce a perfect 

 stage and so are truly imperfect fungi. A few assigned to this class are 

 doubtless imperfect stages of Ustilaginales or other groups of Basidio- 

 myceteae. In the first edition of Engler and Prantl, "Die Natiirlichen 

 Pflanzenfamilien," Lindau (1899, 1900) recognized about 600 genera and 

 15,000 to 20,000 species. In a more recent, as yet unpublished, work on 

 this group Dr. Harold B. Bender (1931) recognized as valid 1331 genera. 

 Since for a great many fungi the asexual and sexual stages of repro- 



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