Witson: NortH AMERICAN PERONOSPORALES 63 
reports have been taken into account in determining the distribu- 
tion of species; and mention is made of all reported hosts upon 
which no specimens have been seen. The determinations of all 
hosts have been verified, with the resulting omission of a few of 
those previously published. These discrepancies are noted in the 
proper places by the insertion after the specimen of the previously 
published host name. All specimens containing odspores are 
marked by an asterisk (*). Inasmuch as the hosts, or species 
closely related to the hosts, of all the extralimital species of the 
genus occur in North America, these species have been included 
in the key, and brief mention made of them in their place in the 
Sequence of species. In conclusion I wish to express my appre- 
ciation of the courtesies which have been shown me in this work 
by the loan of specimens, by critical suggestions and by the deter- 
mination of hosts. 
ALBUGO (Pers.) S. F. Gray, Nat. Arr. Brit. Pl. 
2: 540. 1621 
Uredo § Albugo Pers. Syn. Meth. Fung. 223. 1801. 
Cystopus Lévy. Ann. Sci. Nat. III. 8: 371. 1847. 
Conidiophores simple, cylindric or clavate, crowded into sub- 
epidermal sori without peridium or paraphyses ; conidia cylindric 
or globular, borne in chains, smooth, hyaline or with light-yellow 
Contents ; odspores globular, produced in various parts of the 
host, often separate from the conidia and forming more or less 
Conspicuous masses; spores liberated by the rupture of the epi- 
dermis of the host ; germination always by zoospores. 
Type species, A. Cruciferarum S. F. Gray = Uredo candida 
Pers, 
Key to the species 
Odspore tuberculate ; conidia globose or more or less cylindric, not as long as broad, if 
discoid the membrane of equal thickness throughout. 
Oéspore with prominent tubercles ; conidia similar, or the terminal smaller. 
Oéspore with a few very large tubercles; conidial membrane of equal thickness 
throughout. 
Conidia globular, hyaline. 
Conidia and odspores large ; 
ts 
1. A. candida. 
Conidia and odspores small; hosts ak 
; 2. A. sibirica. 
oraginaceae. 
Conidia discoid, yellow. 3. A. tropica. 
