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 pulled down till subsequent moistening straightens it again (Harper). 



Fig. 40. Perithecia of a. Erysiphe tortilis (Wallr.) Fr.; b. Mia-osphaeria sp.\ c 

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



Uncinula 



These hygroscopic movements may be repeated many times according to 

 weather conditions, even after the living protoplast has disappeared from 

 the appendage (Neger); and at last the perithecium is loosened from its 

 attachment. 



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

 may help to anchor the perithecium to its host during development; in 

 Uncinula necator their apices become mucilaginous (Salmon), and they serve, 

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

 upside down to the substratum. 



The development of the perithecium was first described in 1863 by 



6—2 



