PERONOSPORALES 215 



cylindrical, delicate sterigmata protrude bearing sporangia 

 (Fig. 76). 



The sporangium is almost globose, possesses a definite apical 

 papilla, and germinates by swarmspores. Two species occur in 

 North America, B. entospora Roze & Cornu on Aster, Solidago, 

 Erigeron, and Leptilon, and B. kellermanii (Ellis & Halsted) 

 Wilson on Iva xanthifolia. 



3. Plasmopara Schroter (1889: 236). 



Mycelium intercellular; haustoria unbranched and knob-like; 

 sporangiophores (Fig. 78) protruding from the stomates, erect, 

 solitary to densely fasciculate, monopodially branched, the 

 branches arising more or less definitely at right angles to the main 

 axis; secondary branches also at right angles; the terminal 

 branches apically obtuse; sporangia small, hyaUne, papillate, 

 germination sometimes by germ tube, but in the majority of cases 

 by swarmspores; oospores yellowish brown; the epispore wrinkled 

 and sometimes somewhat reticulate; the oogonial wall persistent, 

 but not fused with the epispore as in Sclerospora (Wartenweiler, 

 1917; 1918 a, b). 



Oospore germination has been observed in but few species. 

 Gregory (1912) pictures it for P. viticola (B. & C.) Berl. & 

 De Toni (Fig. 77, a, b). In this species the oospore wall cracks 

 open and a germ tube protrudes. At its end a sporangium 

 similar to the summer sporangium is cut off, and germinates by 

 swarmspores. A similar method of germination is to be expected 

 in other species. 



The genus is spht by Wilson (1907 b) into two, Rhysotheca 

 Wilson and Plasmopara (sensu strictu). The majority of the 

 species are incorporated in Rhysotheca. One, P. pygmaea (Unger) 

 Schroter, is retained in Plasmopara. In it, according to Wilson, 

 swarmspores are not produced. Instead the sporangial wall 

 ruptures, and the entire protoplasmic content escapes in a mass, 

 forming a non-motile protoplast (perhaps enveloped in the endo- 

 spore) which later germinates by a germ tube. Since the tips 

 of the branches of the sporangiophore are obtuse, the branching 

 not definitely dichotomous, and the sporangia hyaline, this 

 pecuharity of germination is the only essential point in which 

 this species differs from those assigned to Rhysotheca. 



The genus Pseudoplasmopara Sawada (1922: 77) based on 

 Pseud, jusliciae Sawada on Justicia procumbens in P'orniosa is 



