222 THE LOWER FUNGI— PHYCOMYCETES 



study, dealing chiefly with the mycoplasm hypothesis, is based 

 largely on stained cytological preparations rather than on living 

 material, and is not convincing. However, since swarmspore 

 formation has been recently described in Bremia (Milbrath, 

 1923: 991) the demonstration of its occurrence in one or more 

 species of Peronos'pora would not be wholly unexpected. 



Peronospora is by far the largest genus of the order. Although 

 Wilson has not yet monographed the species of North America 

 he cites over forty of them in his host index (Wilson, 1908 b: 543). 

 Gaumann (1923) in a monograph of the species of Switzerland 

 lists one hundred and forty-two species of which nearly seventy 

 are described as new. He subdivides the genus into four groups 

 as follows : 



I. Subgenus Leiothecae Schroter — Epispore of the oospore smooth or 

 indefinitely wrinkled, never reticulate or verrucose. 



A. Group Parasiticae de Bary — Oogonial wall thick, of two layers, 



not collapsing after the spore matures. 



B. Group Effusae de Bary — Oogonial wall thin, collapsing after the 



spore matures. 

 II. Subgenus Calothecae de Bary — Epispore verrucose or covered with ridges 

 which may or may not anastomose. 



A. Group Verrucosae A. Fischer — Oospore with more or less hemi- 



spherical warts or with ridges which do not anastomose. 



B. Group Reticulatae A. Fischer — Oospore with a regular network 



of anastomosing ridges. 



The species in each group are separated primarily on the basis 

 of host range, the species in each host family then being separated 

 by differences in sporangial measurements. One old species, P. 

 parasitica (Pers.) Fr. is broken up (Gaumann, 1917) into approx- 

 imately twenty new species some of which are limited to a single 

 host. The results of cross inoculation work correlated with minor 

 differences in sporangial measurements provide the bases for 

 separation. A number of other older species are similarly spht. 

 The paper contains an extensive bibliography and a wealth of 

 information. The monographic treatises of Schroter (1889), 

 Fischer (1892), and Berlese (1903), while older, are useful. 



Two doubtful South American genera are included here for 

 reference, Chlorospora Spegazzini (Rev. Argentin. Hist. Nat., 

 1891: 29) and Drepanconis Schroter & Hennings {Hedw. 

 1896: 211). The first was based on Ch. vastatrix Speg. reported 

 from onion bulbs, the second on D. brasiliensis Schroter & Henn. 

 occurring on leaves and branches of Nectandra. 



