PLECTOMYCETES 



[CH. 



Fig. 39. Erysiphe Polygoni; young perithecium containing 

 uninucleate asci ; after Harper. 



82 



which undergo a change apparently analogous to lignification. In Phyllactinia 

 the outermost layer, from which the secondary mycelium and the character- 

 istic appendages are derived, 

 consists of thin-walled cells, 

 but in other genera it is not 

 differentiated from the pro- 

 tective zone. 



A single ascus or several 

 may be formed in the peri- 

 thecium ; the ascospores, 

 numbering two to eight in 

 each ascus, begin to develop 

 during the summer or au- 

 tumn, but they remain in 

 the perithecium under nor- 

 mal conditions and do not 

 germinate till the beginning 

 of the following year, when 

 they are set free by the rupture of the perithecium wall, and produce the 

 first infections of the season. In Erysiphe Graminis and Sphaerotheca mors- 

 uvae some of the.perithecia separate readily from the mycelium in August, 

 and, if supplied with moisture, will eject their spores after a few hours. They 

 may in this way prove a source of infection in the season in which they 

 were produced, and this is probably true of other species also, and should 

 be borne in mind if ripe ascospores are being searched for. 



The family includes six genera, all of which are British, and are readily 

 distinguished (fig. 40). 



In Sphaerotheca (one ascus), and in Erysiphe (several asci), the perithecial 

 appendages are filamentous, unbranched or branched irregularly, and very 

 like the ordinary hyphae of the mycelium; in Podcsphaera (one ascus), and 

 in Microsghaera (several asci), they are usually dichotomously branched ; in 

 Uncinnla the apices of the appendages are spirally coiled, and in PJfyllactinia 

 the perithecium bears stiff, pointed hairs with swollen bases. In this genus 

 also the apex of the perithecium is furnished with a ring of richly branched 

 penicillate cells. At about the time of spore-formation these break down, 

 forming a sticky gelatinous cap, by means of which the perithecium, after 

 it is first set free, adheres upside down to its original host plant or to other 

 objects. In view of this peculiarity the ascription of Phyllactinia to any 

 host in contact with which its perithecia may be found demands careful veri- 

 fication. 



The function of the true appendages in the Erysiphaceae is somewhat 

 variable. In Phyllactinia the bulb at the base of the appendage is thick- 



