Witson: NortTH AMERICAN PERONOSPORALES 399 
_ genus cannot stand under the present tribe. That the method of 
germination is more nearly analogous to that of the Peronosporeae 
is the view held by the present author. The genus Rhysotheca is 
therefore the most highly developed and the typical genus of the 
group of genera the conidia of which germinates by zoéspores, while 
_Plasmopara stands in the same relation to Peronospora as does 
Pseudoperonospora to Rhysotheca. 
Key to the species 
_ ncageaie low, sie ee 300 « or less, 2-3 times 
bra , the branches short. 1. R. Geranit.\ 
Coiiopore tall, averaging over 300 4, 2- Sis usually 4 
or 5, times branched, the branches long 
Ultimate branchlets elongate, cylindric. 
Primary branches short, ee branched. 2. R. Umbelliferarum. 
Primary branches elongate, lax 
ae i 3- : ‘fois drenched: 
small, about 15 & 17 4“. 3. R. Epilodii. 
Con UD about 27 & 35 1 . R. Heliocarpi. 
Coin 4 —5 times sile gy very lax. 5. &. obducens.\ - 
Ultimate branchlets more or less conic. 
Ultimate sienciaes broadly truncate. 
Secondary branchlets very short. 6. R. Halstedit. © 
Secondary branchlets lax. 
Secondary branchlets sparingly branched. 
Conidia about 12 & 15 m. 7. R. australis. 
Conidia about 1 18 uw. 8. &. illinoensis. 
Secondary branchlets much _ branched ; 
conidia about 14 & 22 4. 9. &. viticola, Vv 
Ultimate branchlets narrowly truncate. 
nidia elliptic, about 15-20 4 long. 
Conidiophores 2-4 times Fae 10, R, Viburni 
Conidiophores 3-5 times branched 11. R&R, ribicola, ° 
Conidia globose-elliptic, about is-5ba long. 12. 2. Gonolobi. 
1. Rhysotheca Geranii (Peck) 
Peronospora Geranii Peck, Rep. N. Y. State Mus. Nat. Hist. 28: 63. 
1876. 
Peronospora nivea Geranii Farlow; Ellis, N. Am. Fungi 278. 1879. 
Plasmopara Geranii Berl. & De-Toni, in Sacc, Syll. Fung. 7: 248. 
1888. 
Hypo ophyllous ; infected areas conspicuous, definite in outline, 
or often covering the entire leaf, white-downy ; conidiophores 
fasciculate, monopodially 2-3 times branched, the branches short 
