ASCOMYCETES 



209 



differentiation takes place in the sclerotial areas, so that there is 

 finally developed by the next spring rather unlimited, complex 

 apothecia, often 1.5 cm. broad, which rupture by irregular fissures 

 along the ridges of the wrinkled surface. The asci are club-shaped, 

 and bear eight needle-shaped spores. Numerous paraphyses with 

 incurved or hooked tips are present. The asci are 120-130 x 9- 

 10 /a. At maturity the large spores (65-80 x 1 .5—3 /a) are ejected 

 forcibly from the ascus, doubtless distributed by the wind, and 

 they are provided with a mucilaginous membrane which, accord- 

 ing to Klebahn, serves for adherence to the host. Artificial infec- 

 tion with ascospores has been effected, and after such infection 

 the pycnidial stage may be produced within about eight weeks. 



Among other common and conspicuous species of Rhytisma 

 of wide distribution are Rhytisma Salicinum (Pers.) Fr. occur- 

 ring on various species of Salix ; Rhytisma Vaccinii (Schw.) Fr. 

 on species of Ericaceae, notably Vaccinium arboreum in the 

 Appalachians. 



XVII. PERISPORIALES 



This order includes a few families well distinguished from the 

 preceding Ascomycetes by the presence of a more or less mem- 

 branous, generally spherical, closed fruit body, or perithecium, 

 produced directly on the mycelium. In the two families which may 

 here be considered, Perisporiaceae and Erysiphaceae, there is no 

 mouth or ostiolum. The families may be distinguished as follows : 



Perisporiaceae. Mycelium generally dark in color ; perithecium 

 without differentiated appendages, and conidial stages not com- 

 parable to the form genus Oidium. 



Erysiphaceae. Mycelium generally hyaline ; perithecium with 

 appendages, often highly modified ; and conidial stage, when 

 present, invariably an Oidium. 



XVIII. PERISPORIACEAE 1 



This is a small family although some authors may include 

 in it as many as twenty genera. The genera, as a rule, comprise 



1 The two genera which are here discussed have been included by Fischer 

 (Engler and Prantl, /. c.) in the Plectascineae, and there is considerable diversity 

 of opinion as to their true position. 



