Witson: NortH AMERICAN PERONOSPORALES 389 
Conidiophores persistent, slender, usually freely branching ; odspores free 
rom the walls of the odgone 
Branches of the condiophore slcaity obtuse. 4. Rhysotheca. 
Bra of the conidiophore apically acute. 5. Pseudoperonospora. 
Condiophores fAduside branched, the branches arising at right angles to the 
main axis, successively shorter; conidia germinating by a germ-tube. 
PERONOSPOREAF. 
1. PHYTOPHTHORA de Bary, Jour. Roy. Agr. Soc. 
England II. 12: 240. 1876 
Mycelium much-branched, hyaline; conidiophores arising 
singly or in groups from the stomata, or breaking through the 
epidermis, branched or apparently simple, with irregular thicken- 
ings below the conidia, which are borne apically in a scorpoid 
cyme ; conidia oval, papillate ; zoospores oval, biciliate, escaping 
by the rupture of the papilla ; oospores intramycelial, the epispore 
more or less ridged. 
Type species, Peronospora infestans Casp. 
Herbarium material of the species of this genus is very unsatis- 
factory for study, as the conidiophores form a very dense covering 
to the host, and being quite flaccid and often very long they form 
at maturity a dense felt in which the individual conidiophores are 
effectively obscured. This is especially true of P. zxfestans, while 
Some of the foreign species are not difficult to study. 
Key to the species 
Conidia pons ns one, rarely two, borne at the apex of an aborted cyme; conidio- 
phore simple or branched below 
Host ast i 
Host Araceae. 
Conidia numerous in a simple or compound cym 
Conidia sessile or ieee. -stalked ina ac cyme, 
Conidia small, about 35 /. 
Conidia large, 50, or more. 
Conidia sessile in a compound cyme. 
ost Solanaceae. 
Host Sicsieionlateab, 
1. P. Phaseolt, 
2. FP. Colocastae. 
3. P. Nicotianae. 
4. P. Cactorum. 
z P. infestans. 
. P. Thalictri. 
1. PayropHTHora PuHaseo. Thaxter, Bot. Gaz. 
14: 274. 1889 
The present species differs rather markedly from the other 
American species of the genus in the method of branching of the 
Conidiophores, but in other respects they are quite similar. The 
conidiophores are very long, simple, or more commonly branched 
