Ill] 



ERYSIPHALES 



83 



walled over its upper surface, but an oval region remains thin on the lower 

 side. As the ripening perithecium loses water so do the appendages; the 

 thin area below the bulb is pushed in by atmospheric pressure, the under 

 surface becomes consequently shorter than the upper and the end of the 



spine is [lulled down till subsequent moistening straightens it again ( Harper). 



Fig. 40. Perithecia of a. Erysiphe tortilii (Wa.llr.) IV.: b. Microsphaeria sp.\ c. Uncinate 

 Aceris (DC.) Sacc. ; d. Phyllactinia Corylca (Pers.) Karst. ; x 120. 



These hygroscopic movements may be repeated many times according- to 

 weather conditions, even after the living protoplast has disappeared from 

 the appendage (Xeger); and af last the perithecium is loosened from its 

 attachment. 



In other cases, such as Erysiphe and Sphaerotheca, the appendages 

 ma}' help to anchor the perithecium to its host during development; in 

 Uncinula necator their apices become mucilagini >us < Salmi >n ), and they serve, 

 much as do the penicillate cells of Phyllactinia, to attach the free perithecium 

 upside down to the substratum. 



The development of the perithecium was first described in 1863 by 



6—2 . 



