11 



CLASS ASCOMYCETEAE: ERYSIPHALES, 



ASPERGILLALES, MYRIANGIALES, 



SACCHAROMYCETALES 



AS WAS true for several other orders of the Class Ascomyceteae the 

 l\ investigations of the last three decades have necessitated numerous 

 changes in the limits of the orders to be taken up in this chapter. Even as 

 presented in this edition much must be recognized as only tentative, for a 

 great deal more very intensive study will be necessary before a satis- 

 factory classification can be attained. 



Order Erysiphales (Perisporiales of most authors i). Lindau, in Engler 

 and Prantl (1897), placed in this order almost all of the plant-inhabiting 

 Ascomyceteae with external mycelium and with the perithecia also ex- 

 ternal. Ostioles are mostly lacking except in the Family Microthyriaceae. 

 The latter family has now been transferred to the Order Hemisphaeriales 

 (see Chapter 10) and the remaining two families have been increased to 

 five or six, mainly by the discovery of new forms and by the division 

 of the Perisporiaceae. 



In general the Erysiphales carry on a parasitic, less often saprophytic, 

 existence on the surface of the host plant. Exceptions are found in the 

 species feeding on "honey dew" which occur wherever the latter accumu- 

 lates in sufficient quantity. As parasites the mycelium may not even 

 penetrate through the epidermis but in many cases the epidermal cells 

 of the host are penetrated by haustoria. The mycelium is septate and 

 branched, mostly with uninucleate cells. Rarely no mycelium is visible 

 except that making up the perithecium. 



1 The Order Perisporiales and Family Perisporiaceae are based upon the genus 

 Perisporium. In view of the fact that the type species of this genus, P. gramineum Fr., 

 has been shown not to belong to this order as customarily limited, it is necessary to 

 base the order upon a generally recognized genus whose connection with the order 

 is beyond doubt. Following the proposal of Gwynne-Vaughan the name Erysiphales 

 has been selected, based upon the genus Erysiphe. 



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