1050 
Journal of Agricultural Research 
Vol. XXX, No. 11 
mation in plate cultures, within seven 
days after inoculation, of irregular 
swollen bodies filled with a dense pro¬ 
toplasm which has withdrawn from 
adjacent regions of the hyphae, laying 
down septa as it contracts (pi. 4, E). 
Such a condition is a not uncommon 
phenomenon in a variety of old fungus 
cultures; in this case, however, it is 
produced quite normally after a few 
days of growth. The general contrac¬ 
tion of the protoplasm into these 
bodies with abundant septation of the 
empty hyphae occurs in 7 to 10 days 
after inoculation of the plates. Such 
colonies are granular but otherwise 
almost transparent and without the 
filamentous structure usually visible to 
the eye in fungus cultures. Hand-lens 
inspection reveals a multitude of short 
straight crystal-like bodies, which are 
the septa formed in the clear hyphae by 
the retracting protoplasm. Germina¬ 
tion of these resting bodies takes place 
by a tube, as in the case of the round 
conidia. 
A second characteristic of D is the 
scarcity of oospores, contrasting sharply 
with the profusion of oospores pro¬ 
duced by B on plates of nearly all 
media. None were obtained for 14 
months after the original isolation, 
though 16 media were tried. They 
finally appeared in hanging drops of 
sterile oatmeal decoction four days 
after inoculation, and have since 
appeared sporadically in cornmeal 
agar plates. The conditions neces¬ 
sary for oospore formation are not 
known, aside from the facts that long- 
continued growth and transfers on 
artificial media preceded their appear¬ 
ance, and that they are most likely 
to appear immediately around and 
on the mycelium used as the inoculum, 
rather than on the fresh growth. 
In measurements the oospores of 
D vary little from those of B, ranging 
from 15.9 p to 19.9 p, averaging 17.8 p. 
One to four antheridia are present, 
all emptying their contents. They 
are usually broader at the apex than 
are antheridia of B, and differ also in 
that they often lie appressed to the 
oogonium along their entire length. 
Contraction of the oosphere and 
fertilization through a tube occurs 
as described in this group. Complete 
maturation is however the exception 
rather than the rule; the contracted 
fertilized oosphere often degenerates 
into a mass of oily globules which later 
merge into a large vacuole filling the 
oogonium except for a peripheral 
layer of hyaline granules, no oospore 
wall being formed. Oospores that 
have come to maturity lie free in the 
oogonium and are smooth walled. 
The walls are distinctly thinner than 
those of B (pi. 5, C), and are hyaline 
or yellowish. The contents are gran¬ 
ular except for a single eccentric 
vacuole. 
Attempts at germinating the oo¬ 
spores of D, including the use of freez¬ 
ing, alternate temperatures, etheriza¬ 
tion, and soaking in water, have not 
succeeded. The round conidia and the 
swollen irregular bodies are clearly the 
stages on which the organism is de¬ 
pendent for dissemination and the 
tiding over of unfavorable conditions, 
since oospores have not been found in 
nature and are so scarce in culture. 
The sickle-shaped bodies character¬ 
istic of these Pythium spp. were de¬ 
veloped in plate cultures two days after 
inoculation. 
Type A, the organism first isolated, 
is characterized morphologically (1) by 
the abundant formation of spherical 
terminal conidia on all media and in 
the host plant, much larger than those 
of B (21.7 p to 48.9 p, average 36.2 p); 
(2) by a comparatively small produc¬ 
tion of large oogonia (25.5 p to 34.7 p , 
average 31.7 p) and oospores (21.3 p to 
29.8 p, average 26.6 p) accompanied reg¬ 
ularly by three to eight antheridia. 
Comparative measurements from this 
organism and B are shown in Figures 
1 and 2. Hyphae are 3.5 p to 9.2 p wide, 
average 6.4 p, and consist of thick main 
branches with curving lateral branches 
arising at acute and right angles, con¬ 
stricted at the point of origin. The 
sickle-shaped bodies are also present, 
often growing in chains (pi. 4, D). 
The conidia appear within two days 
on culture media, and are formed by 
the swelling up of a hvphal tip. Inter¬ 
calary conidia have not been observed, 
in marked contrast with the abundance 
of this type in cultures of the other 
organisms studied. The conidia are 
smooth and thin walled, and are filled 
with a densely granular protoplasm 
which becomes vacuolate with age. 
Conidia from potato cylinders, beef agar, 
and potato agar without dextrose are 
abnormally hyaline and collapse sooner 
than those from media rich in sugars 
and readily available food, as sugar- 
beet agar and oatmeal agar. Drying 
out of the media in all cases is followed 
or accompanied by collapse and death 
of the conidia. Those in the depth of 
the media, therefore less exposed to 
drying, are more likely to remain 
plump and viable, cultures sometimes 
remaining viable after 10 months. 
The conidia of A are formed in such 
abundance on all media that the 
colonies rapidly assume a finely granu- 
