202 COMPARATIVE MORPHOLOGY OF FUNGI 



by the special structure of the perithecial rind. Before the rind has 

 completed its peridial structure, some of its cells develop to long append- 

 ages; these are generally characteristic and offer one of the most conven- 

 ient points for the separation of genera. In Erysiphe, Leveillula (Fig. 127) 

 and Sphaerotheca, they are simple, hyphal-like and intertwined with the 

 mycelium; in Podosphaera (Fig. 128, 4) and Microsphaera (Fig. 128, 1 to 

 3), they are repeatedly dichotomously branched at the tips; in Uncinula 

 (Fig. 128, 5 to 7), they are coiled more or less spirally at the tips; in 

 Typhulochaeta, they are clavate and arranged in a ring consisting of 



Fig. 127. — Leveillula taurica. Section through the lower surface of a leaf and the 

 peridium of a perithecium, showing extra- and intramatrical mycelium and the hyphal 

 appendages. (X 400; after Arnaud, 1921.) 



two or three rows around the top of the perithecium ; and in Phyllactinia, 

 unbranched, but setiform and rigid, with a saccate base (Fig. 129, 1). 



As regards the structure of perithecial wall, there are two groups. In 

 Sphaerotheca, Erysiphe (Fig. 125, 2) and Leveillula, the rind (as in the 

 Plectascales) is spread approximately evenly over the whole fructifica- 

 tion; in Microsphaera and Uncinula, it possesses a dorsi ventral structure 

 consisting generally at the base of wide-lumened, thin-walled cells and 

 at the top of narrow-lumened, thick- walled cells. Neger (1901, 1902) 

 attempts to interpret these observations biologically as follows: in the 

 first group, with the peridium symmetrical on all sides, the perithecia are 

 generally sessile. Their appendages are interwoven with mycelium and 



