62 Witson: NortH AMERICAN PERONOSPORALES 
oospore tuberculate or ridged. According to the investigations of 
Zalewski * and Stevens + this is the more specialized group. The 
former author points out the more complex character and apparently 
more complete development of the epispore, while the latter deals 
with cytological phenomena only. The second group of species is 
characterized by a reticulate epispore and contains the remaining 
species of which the odspores are known, and in all probability 
the two species in which they are at present unknown. There are 
three or four well-defined types of reticulation represented, all of 
which are found among the American species. The first of these 
is represented by A. Bliti and A. platensis and may be considered 
typical of this group. The reticulations are very evident, the 
meshes large and the areolae deep and unoccupied by any eleva- 
tions. The pattern is often somewhat imperfectly developed. 
From this type the other species vary in a striking manner, yet the 
primary characters remain the same. In A. Tragopogonis and A. 
Swertiae the areolae are not so deep and the reticulations are 
crested at their angles with more or less prominent tubercles. In 
A, Portulacae the variation takes the form of tubercles within the 
areolae while the reticulations themselves are similar to those of 
A, Bit. Unique within the genus is A. occidentalis, which has the 
epispore finely reticulate and the areolae so shallow as to give the 
impression, at first sight, of pits rather than reticulations. The 
conidia are quite similar to those of A. tropica, but the odspore- 
characters indicate a closer relationship to A. platensis or A. Swer- 
ttae. According to Zalewski the reticulate spores have a less pet- 
fectly developed epispore which reaches its highest development in 
A. Tragopogonis. This arrangement of species is confirmed by the 
work of Stevens. 
The material upon which the present paper is based is con- 
tained in the herbaria of the New York Botanical Garden and of 
Columbia University and in the private collections of Dr. L. M. 
Underwood, Dr. J. C. Arthur and the author. The literature of 
the genus has been carefully looked over by means of Dr. Farlow’s 
Bibliographical Index and other aids. No localities are cited 
from which material has not been examined, but all published 
* Bot. Cent. 15: 215-224. 1883. ; 
t Bot. Gaz. 32: 77-98, 157-169, 238-261. p/. r-g-+ fext figs. 1901. 
