388 Witson: North AMERICAN PERONOSPORALES 
Like many other plants, the Peronosporaceae do not lend them- 
selves readily to characterization. The conidia vary greatly in size 
and often to a surprising extent in outline, and the conidiophores 
from their very nature are difficult of description. For this reason 
it appears desirable to add the heading “‘icones”’ under which as 
complete a list as practicable of illustrations in American works 
and the chief foreign ones is given for each species. A key to the 
genera is also given in which the three tribes are characterized and 
the genera under the first two included, the third tribe being re- 
served for future treatment. 
A serious handicap in the treatment of the species of this 
family is the lack of information upon many points which are of 
taxonomic importance. While in America the present family has 
received more attention than almost any other group of Phycomy- 
celes, a wide field for investigation is still open. The oospores of 
“many species are unknown, and even when known are of rare 
occurrence in herbaria; the germination of but few species has 
been studied in America, our knowledge on this point being fre- 
quently derived from European sources ; the problems of odgenesis 
are practically untouched ; but few inoculation experiments have 
been conducted to determine the range of hosts which a single 
species will affect. It is therefore apparent that a final, or even 4 
reasonably satisfactory treatment of the group at the present 
time is impracticable. 
In conclusion I wish to express my sincere appreciation of the 
courtesies shown me in my work, and especially to those botanists 
who have so kindly supplied me with material in addition to that 
which was available in preparing the previous paper of this series. 
Conidiophores not clearly differentiated from the mycelium, scorpioid-cymosely 
branched ; conidia germinating normally by zodspores. babeetiee Ra: 
A single genus. Phytophthora. 
enue deaty differentiated from the mycelium. 
onidiophores so nome branched, the branches usually arising at right angles 
mai , successively shorter ; conidia germinating normally by 
zodspores. RHYSOTHECEAE. 
Conidiophores with the main axis indurate above, the — sguaee® ee: 
and basidium-like, Basidiopho 
Conidiophores with arte main axis not indurate above, a lateral cnet de- 
veloped normall 
Conidiophores Seis, stout, sparingly branched; odspores seme 
nently united to the walls of the odgone. 3. Sclerospora. 
