Wilson : North American Peronosporales 
55 
causing them to “ damp off,” while on older plants the leaves 
suffer most. These are marked with concentrically ringed brown 
spots. The conidiophores, as is usual in this genus, may emerge 
either through the stomata or by rupturing the epidermis. The 
mycelium is both intercellular and intracellular, and often caus- 
ing a blackening of the vascular bundles. The hyphae are at 
first quite slender, becoming thicker with age, so that they 
measure 3-9 /j. in diameter. The haustoria are not numerous nor 
are they differentiated in appearance from young branches. The 
conidiophores are long and unbranched, usually 100-300 n tall, 
but ranging 35-500 /a. The conidia are pyriform, distinctly 
papillate, 16-60 X 10-45 /“. and producing from 5 to 45 zoospores. 
Intercalary conidia are also found in the cultures which resemble 
to a considerable extent similar bodies in Pythium, but germinat- 
ing as do the typical conidia. “ Varying temperature, alternate 
light and darkness and moisture are essential factors in the 
formation ” of both conidia and zoospores. In nature the fungus 
has been observed to produce conidia sparingly within the tissues 
of the host, a habit not observed in other species of the genus 
except P. infestans. 
Conidial germination, according to the illustrations published, 
may be either of the typical Phytophthora type where each 
zoospore escapes separately, or of the Pythium type where the 
entire mass of zoospores escapes in a vescicle and are later 
liberated by its rupture. The zoospores are not different from 
those of other members of the genus. Chlamydospores are also 
found in cultures. 
The oospores of this species are formed in the same manner as 
that described by Pethybridge for P. erythroseptica. Indeed the 
details of sexual reproduction were worked out on these two 
species simultaneously and independently by Dastur and Pethy- 
bridge, each arriving at the same conclusions, although priority of 
publication made the announcement of Pethybridge precede that 
of Dastur by several weeks. The most interesting point brought 
out by Dastur and not by Pethybridge is that the gametes may 
sometimes arise from the same “ stalk ” instead of from dif- 
ferent ones. When they are on the same hypha the oogonium 
arises as an ingrowing cell at the base of the antheridium. The 
