72 PROCEEDINGS OP THE AMERICAN ACADEMY 



about to germinate, elliptical and slightly papillate. In germination 

 they produce zoospores like P. viticola. The germination I have 

 obsei'ved in the form growing on Ambrosia at Wood's Holl, and it 

 differs in no essential respect from what takes place in all our zoo- 

 spore-producing species, to wliich reference will be made hereafter. 

 The nearest ally of the present species is P. viticola, from which 

 P. Halstedii differs but little in morphological characters ; but in 

 P. viticola the ultimate branches are distinctly more dense, so that 

 the tips almost appear denticulate at times, while in P. Halstedii the 

 ultimate branches are more or less subulate and divaricate. In 

 P. Halstedii the haustoria are numerous in the leaves, nearly glob- 

 ular, and much more easily seen than in P. viticola. The oospores 

 of the two species resemble one another, but those of P. Halstedii 

 appear to be on an average larger, and the epispore more distinctly in 

 irregular folds. As might be expected, a species appearing on so 

 many hosts presents modifications on the different hosts. The most 

 marked deviation from the type is seen in the form on Helianthus 

 tuberosus from Mr. Ellis. In the type the conidial stalks consist of a 

 main axis, from the upper part of which are given off" lateral branches 

 nearly at right angles ; the branches then give off a second and third 

 series of branchlets, which usually end in a prong-like projection, near 

 the base of which are from two to four similar prongs in a divaricating 

 tuft. The general outline of the conidial ramifications is pyramidal, 

 while in P. viticola it is more nearly linear. In the form on species 

 of Helianthus tlie lower branches are often much longer and more 

 frequently divided than in those on Ambrosia and Hidens, and the 

 branching becomes almost thyrsoitlal. In rare cases, the usually erect 

 prong becomes recurved, and the tip of the branchlet swells so that 

 there is an approach to P. gangliformis, from which, however, it can 

 easily be distinguished by the different ramification. Were it not for 

 the large series of connecting forms, one might be inclined to separate 

 the specimens of Mr. Ellis as a distinct species. The following may 

 serve as a description : — 



P. Halstedii, Farlow. Mycelium furnished with numerous oval 

 haustoria. Oogonia 30-40/x in diameter. Oospores spherical, yellowish, 

 23-30/A in diameter, epispore with ill-defined folds, endospore about 

 3/A thick. Conidial stalks fasciculate, narrowly pyramidal in outline, 

 with a percurrent axis, ll-15/x in diameter, 300-750/x long, two -four 

 times pinnate, branches given off nearly at right angles, ultimate 

 divisions approximate in clusters of three or four divaricating tips. 

 Conidial spores colorless, oval or elliptical, with a slight papilla, 

 19-30/A long by 15-26/a broad, germinating by zoospores. 



