|From the BULLETIN OF THE TORREY BoranIcat,Cius, 34: 387-416, 1907.] 
Studies in North American Peronosporales—Il. Phytophthoreae 
and Rhysotheceae 
Guy WeEsT WILSON 
The family Peronosporaceae, which includes all the genera of 
the order except A/dugo, may be briefly characterized as follows: 
Seite Tate Hit idiov! ial, variously branched; 
Wirral: ] 
7 
conidia borne singly at the apex of the ultimate branchlets of the 
coniophores, germinating by zoospores or rarely by a germ-tube ; 
Ospores globular, variously sculptured, germinating by a germ-. 
tube 
Of the three well-defined tribes which constitute this family, 
the first two are discussed in this paper. It is usual to follow the 
older authors in considering the species embraced in the genera at 
Present under discussion, as clearly distinguished from the remain- 
ing members of the family by the method of germination of the 
conidia, which in the Phytophthoreae and Rhysotheceae is normally 
by zodspores, while those of the Peronosporeae germinate by means 
of a germ-tube. It is, however, well known that under certain 
conditions the conidia of these species do not throw out zoospores, 
but produce one or more germ-tubes. This is probably due to 
conditions which cause the potential zodspores, which are already 
formed within the conidium, to germinate before escaping from the 
‘conidial membrane.* With this real or apparent intergradation 
of characters, it is desirable to have a more stable basis for group- 
ing the genera within the family. Characters which are much 
more easily observed and subject to less important variation are 
afforded by the conidiophores, the habit of branching of which 
conforms to the method of conidial germination. 
*See Hartig, Unters. Forstbot, Inst, Miinchen 1: f/. 3. 
387 
