CHAPTER XIV 

 PERISPORIALES 



The Perisporiales differ from the Plectascales in that their asci are not 

 irregularly formed in the interior of the perithecia but in a fascicle (or 

 umbel), free at maturity, attached at the base and generally still without 

 paraphyses; thus the asci all stand at the same height. Externally, 

 many Perisporiales may not be distinguished from the Aspergillaceae 

 (e.g., Thielavia group) and only an investigation of the manner of forma- 

 tion and arrangement of the asci offers a solution of the systematic posi- 

 tion of the fungi in question. 



In contrast to the Plectascales, the Perisporiales are obligate parasites 

 in which the perithecia, generally still cleistocarpous, as in the parasitic 

 Plectascales, are extramatrical, in a position to take an active part in 

 dissemination. Biologically they form a peculiar developmental series 

 from the endoparasitic to the ectoparasitic method of life, whose end 

 forms are distinguished by a special asterinoid habit (Arnaud, 1918); this 

 type includes ectoparasitic forms, chiefly in damp climates, which spread 

 radially over the surface of the host by aerial mycelia and derive their 

 nourishment by the formation of haustoria or sinkers. 



In this sense the Perisporiales are circumscribed, half morphologically, 

 half biologically. They form a transitional order from the Plectascales to 

 the Pyrenomycetes and hence to a high degree their limits are determined 

 by the subjective measurements of individual investigators. In the 

 following discussion, we will recognize three families: the Erysiphaceae, 

 the Perisporiaceae and the Englerulaceae. The Erysiphaceae include 

 the forms with white aerial mycelium ; they are cosmopolitan but are best 

 developed in the temperate zone. In addition to their hyaline extrama- 

 trical mycelium, they are distinguished by the development of imperfect 

 forms and by their unicellular ascospores. The Perisporiaceae and 

 Englerulaceae include those forms with more or less dark-colored aerial 

 mycelium. They are mostly tropical and subtropical. Imperfect forms 

 have been found with certainty only in a single genus. Their ascospores 

 are generally septate. The Englerulaceae differ from the Perisporiaceae 

 by a peculiar histolysis of their perithecial wall, whereby their mature 

 fructifications attain a half-open Discomycetous structure. 



Erysiphaceae. — The powdery mildews show most markedly the 

 character of the Perisporiales. They form a very homogeneous, sharply 

 denned group and are parasitic on Angiosperms. Together with the 



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