ERYSIPHACEAE US 



point of contact of the oogonium and antheridium open by a 

 broad pore, after wliich the protoplasts intermingle. In Sphae- 

 rotheca, bearingr a single ascus, there are two cells formed from 

 the oogonial structure, the upper of which becomes the ascus 

 and the lower skives rise to the nurse tissue and wall of the 

 cleistocarp. 



Harper (1895, 1905) verified these observations of de Bary and 

 contributed many additional cytologic facts in studies based on 

 the same species and on Phyllactinia cor y lea and species of Erv- 

 siphe. He traced the development of antheridia and oogonia, 

 the fusion of nuclei, one from each ors^an, and the cytologic fea- 

 tures that accompany nuclear division ^\•ithin the ascus and de- 

 limitation of ascospores. The development in powdery mildews 

 producing more than a single ascus differs mainly in that the 

 oogonium becomes transformed into a row of cells. Each of 

 these cells becomes an ascus or else gives rise to short branches 

 (ascogenous hyphae), whose penultimate cells become asci. 



Certain mycologists, among them Eftimiu (1929), maintain 

 that there is no migration of antheridial protoplast into the 

 oogonium in powdery mildews. In Eftimiu's observations were 

 included Erysiphe galeopsidis, E. tortilis. Uncimila chvidestiua, 

 and Microsphaera herberidis.. Colson (1938) concluded that the 

 cleistothecia of Phyllactmia corylea, growing on Cory his avel- 

 lana, develop apogamously. The antheridial nucleus degenerates 

 within the antheridium. The oog^onium becomes four-nucleate 

 by division of its single nucleus. These four nuclei become 

 separated by walls, with two nuclei in the middle cell. After 

 further nuclear divisions the ascogenous hyphae grow out from 

 this middle cell. By septation these hyphae become rows of bi- 

 nucleate cells, the terminals being uninucleate. The asci then 

 arise directly from these binucleate cells, nuclear fusion follows, 

 and eventually several uninucleate ascospores are delimited within 

 each ascus. 



Distribution of pou'dery viildeii's. Most of this group of 

 fungi are known to occur in the North Temperate Zone, perhaps 

 because more collectors of fungi live in this region. Approxi- 

 mately one-third of all known species is confined to Europe, and 

 one-third is common to both Europe and North America. Thir- 

 teen species and 5 varieties of those listed by Salmon (1900) 

 occur in North America. Phyllactinia corylea is essentially 



