PERONOSPORALES 2 1 1 



in form, branched, arising from endophj^tic mycelium, protruding 

 through the stomates, and bearing the sporangia in the air; 

 sporangia deciduous, disseminated by the wind, germinating by 

 swarmspores or germ tubes; sexual organs developed in the 

 intercellular spaces; oogonia monosporic; oospore germinating 

 by a germ tube. 



The sporangiophores, protruding in great numbers from 

 the stomates, form definite, whitish, grayish, or violet, mouldy 

 patches on the surface of the host. The same thing occurs in 

 some species of Phytophthora of the preceding family. The 

 common name downy mildews was applied to these forms to 

 distinguish them from the powdery mildews, an ascomycetous 

 group (Erysiphaceae). 



The sexual process in the family has been studied cytologically 

 in several species representing the genera Sclerospora, Plasmopara, 

 and Peronospora. In general, it does not differ strikingly from 

 that described above for Albugo Candida. A receptive papilla 

 is formed, a coenocentrum is differentiated, and a true fertiliza- 

 tion, terminated by a single nuclear fusion, occurs. A critical 

 consideration of the details of the process in the various forms 

 studied indicates a close relationship between the Albuginaceae 

 and Peronosporaceae, and suggests the probabihty that the latter 

 family is the more recently developed (Wager, 1900; Stevens, 

 1902; Rosenberg, 1903; Kriiger, 1910). 



Key to Genera of Peronosporaceae 



I. Oospore wall confluent with that of the oogonium; sporangiophore 

 typically stout with heavy branches clustered at its apex. 



1. Sclerospora, p. 212 



II. Oospore wall not confluent with that of the oogonium; sporangiophore 

 typically more slender. 



A. Sporangiophore consisting of a clavate or cylindrical trunk pro- 



vided with a swollen apex from which short sterigma-like 

 branches arise, each bearing a sporangium. 



2. Basidiophora, p. 214 



B. Sporangiophore provided with prominent branches. 



1. Branching monopodial to subdichotomous. 



a. Tips obtuse; branches arising more or less at right 



angles. 



3. Plasmopara, p. 215 



b. Tips more acute; branches forming more or less acute 



angles. 



4. Peronoplasmopara, p. 218 



