June 1,1925 
Comparative Studies of Pythium debaryanum 
1053 
abundant germination. Each germ 
tube becomes a “main” hypha and 
can be traced as such for a considerable 
distance; short side branches are given 
off profusely, with occasional longer 
lateral branches which soon acquire 
the greater thickness and length 
characteristic of the “main” hyphae. 
Terminal microconidia, scarcely 
larger than the hyphal width, are some¬ 
times found. The size of the normal 
conidia varies with the substratum, 
but is always much larger than that of 
P. debaryanum or D from the same 
medium. Culturing over a period of 
four years has not markedly affected 
the size of the conidia, although the 
oospores have become noticeably re¬ 
duced in size. 
Oospokes or A.—Cultures of the orig¬ 
inal isolations were grown on various 
media for nearly two years before oo¬ 
spores were obtained. The first were 
found on a bit of old mycelium which 
had been used as inoculum for a plate of 
Knopf’s synthetic agar, on which the 
organism makes a scanty, spreading 
growth. Subcultures of this, and 
inocula from two-months-old oatmeal 
agar cultures which had been kept in 
the icebox at least a month, subse¬ 
quently yielded oospores quite regularly 
on Knopf’s agar, later on cornmeal and 
carrot agars. The oospores appeared 
at first on and immediately around the 
inoculum; in later cultures they were 
produced in the general body of the 
medium, synchronously with the co¬ 
nidia. Oospores could usually be found 
in aggregates on distinct hyphal groups 
which could be traced some distance, 
indicating oospore production to be 
a function of special hyphae. 
Heterothallism is, however, not indi¬ 
cated, since the cultures were descen¬ 
dants of single spore cultures which are 
readily made on account of the large 
size of the conidia. The factors re¬ 
sponsible for sexual reproduction as 
contrasted with conidial reproduction 
are not clear-cut, since inoculations of 
a batch of plates occasionally resulted 
in purely conidial formation in some of 
the plates. Inoculations on cornmeal 
agar from old, chilled oatmeal agar 
cultures could generally be counted 
upon to produce oospores, though 
always in moderation. 
The spherical oogonia of A are 
smaller than the conidia and contain 
a more finely granular, nonvacuolate 
hyaline protoplasm. They are soon 
enlaced by adjacent antheridia, which 
may arise below the oogonium or, more 
usually, from neighboring hyphae. 
They are clavate, much larger than 
those of B (compare pis. 6 and 5, 
which were photographed at the same 
enlargement), and from three to eight 
are found regularly with each oogo¬ 
nium. Each is fused at its apex with 
the oogonial wall and sends a fertiliza¬ 
tion tube across to the contracted 
oosphere. At maturity, all antheridia 
around the oospore are found empty. 
All do not, however, empty directly 
into the oosphere; observations on 
living material indicated that a mem¬ 
brane is formed after the emptying of 
one or possibly two antheridia into the 
oosphere, effectively preventing fertili¬ 
zation by the remainder. This is 
shown in Plate 0, A, g, where the con¬ 
tents of the left-hand antheridium have 
been extruded into the space between 
original wall and oosphere. The latter 
has evidently been fertilized by the 
antheridium at the right and formed a 
membrane before the others could 
empty into it, the membrane subse¬ 
quently thickening into the oospore 
wall. Similar cases may be seen in 
Plate 6, A, b and e. 
In one instance observed during fer¬ 
tilization a single antheridium of A had 
already emptied into the oosphere, 
which was rounded up and surrounded 
by a very thin membrane. A second 
antheridium was seen to extrude its 
contents through the fertilization tube 
which touched the oosphere mem¬ 
brane, the contents spreading out, 
however, over the oosphere surface 
and quickly retracting into the anther¬ 
idium, where they degenerated into 
oily globules. Occasionally abortion 
takes place, such as was observed in D. 
The contracted oosphere fails to form 
a definite wall, turns into a mass of 
large, bright oily globules (pi. 6, fig. 
A, 6), which expand and fill the oogo¬ 
nium (pi. 6, fig. A, a), later fusing into a 
single large vacuole nearly filling the 
oogonium except for a surrounding 
layer of granular, hyaline cytoplasm 
(ph 6, A,/). 
More frequently fertilization is fol¬ 
lowed by thickening of the oospore 
walls, accompanied by changes-in the 
oospore contents, which result in the 
final appearance of a large excentric 
vacuole surrounded by a densely gran¬ 
ular but hyaline cytoplasm. Both 
oogonial and oospore walls remain 
smooth and. hyaline; the mature 
oospore lies free in the oogonium but 
occupies a space proportionally less 
than in the case of B. The ratio of 
oospore to oogonium in the former is 
1 :1.2, in the latter 1 :1.1. 
Germination of a few oospores of A 
was obtained by pouring aerated tap 
water over a month-old cornmeal agar 
plate culture containing oospores (pi. 
