Oct. 15, 1924 
Geranium Stemrot 
409 
gions of protoplasm. Gilia may be 
seen lashing about at the margin, which 
is now well indented. The general mass 
increases in volume with the appear¬ 
ance of single vacuoles within the zoo¬ 
spores, which are now well marked off 
and begin to separate at their tips. 
They move slightly apart and over 
each other for a short period, then 
break apart singly or in entangled 
pairs and swim away through the dis¬ 
rupted membrane. In one case the 
first few zoospores to escape left through 
the same part of the vesicle; but as a 
rule there is no localized point of exit, 
the vesicle wall apparently splitting at 
several places through the impacts of 
the moving zoospores. In some cases 
the entire mass of entangled zoospores 
may break out of the membrane and 
move off some distance before separa¬ 
tion takes place 
No trace of the vesicle can be seen 
after the escape of the zoospores. The 
open tube, which has acquired enough 
wall material to insure rigidity, re¬ 
mains on the old sporangial wall. It 
is now rarely more than one-third the 
diameter of the spore, and the addi¬ 
tional length observed in the early 
stage of germination evidently had 
gone to make up the vesicle. 
The process of zoospores formation, 
from the protrusion of the tube to the 
escape of the zoospores, takes about 
15 minutes. It is more or less-syn¬ 
chronous in most of the sporangia in 
a, drop, which may be seen swarm¬ 
ing with zoospores half an hour after 
sowing. 
Zoospores. —From 10 to 26 are 
formed from each sporangium. They 
are broadly lenticular, 5.9 n to 8.5 m 
wide by 10.6 n to 11.5 m long, contain 
a single small round vacuole sur¬ 
rounded by finely granular proto¬ 
plasm, and bear two cilia at the 
Tiilum (PL 5, A). After swimming 
around for half an hour to an hour, 
they come to rest, and the ciliary 
motion gradually ceases as the zoospore 
rounds up. A very slender germ tube 
is put forth, which broadens back to 
the base as it grows in length and 
branches out into a mycelium. 
Asexual fruiting bodies from older 
cultures show an increased tendency 
to direct germination, particularly in 
the case of those with large vacuoles. 
The tubes do not differ at first from 
those extruded in the indirect process, 
except that more than one may be 
formed from a single spore. The tips 
continue to grow, branch, and form a 
mycelium. 
Oospores. —These are formed in 
great abundance within the cells of dis¬ 
eased tissues, on oatmeal agar, and 
geranium agar. They are globose, 
smooth walled, lie free within the 
oogonial wall, and average 16.18 ju in 
diameter (300 measurements), ranging 
from 11.32 m to 20.85 m- The con- s 
tents are hyaline, with fine granules 
and oil drops surrounding a large 
rounded excentric vacuole half the 
diameter of the oospore. The walls 
are hyaline when young, later becom¬ 
ing yellow to brown. The formation 
of discolored oxidation products on 
substrata is evidently a factor in wall 
coloration, since oospores from oat¬ 
meal and corn-meal agar, which are 
not discolored, remain light yellow, 
whereas oospores from host tissue and 
geranium agar, both of which are 
browned, are dark walled. 
The oogonial wall is smooth or 
slightly collapsed, and often bears 
traces of the oogonial stalk. 
The most characteristic feature of 
the mature sexual fruiting body is the 
antheridium. It is one-celled, per¬ 
sistent, and varies in shape from a 
trumpet form flaring out at the 
region of attachment, to a broad 
irregularly lobed mass clasping or 
wrapped around a large part of the 
oogonium and fused with it. The ex¬ 
istence of a lateral pressure is indi¬ 
cated by the collapse of that part of 
the empty oogonial wall immediately 
underneath. The cylindrical or cla- 
vate type so characteristic of most 
Pythium spp. has not been observed in 
host tissue. 
EXPLANATORY LEGEND FOR PLATE 4 
A—Oospore formation, early stages (a to h). Note clasping antheridium; stalked oogonium 
(at ax) 
B.—Three stages in maturation of same oospore (a to c) 
C— Oospores in contracted oosphere stage (a) and exospore stage ( b) 
D. —Mature oospores 
E. — HyphEe, showing cylindrical nature and rounded tips 
F. —Sporangia-terminal, and intercalary (at x) 
G. —Sporangial germination: (a), Tube protruded; ( b ), contents partly in vesicle; (c), contents 
extruded, undifferentiated 
H. —Three zoospores entrapped in sporangial wall, after most of the undifferentiated contents 
had emerged 
