1048 
Journal of Agricultural Research 
Vol. XXX, No. 11 
PATHOLOGICAL HISTOLOGY 
The hyphae are both intercellular 
and intracellular, and may be observed 
in all tissues except the fibrovascular 
long before the cells collapse. Con¬ 
striction at passage through the cell 
wall is the rule (pi. 3, C). Long 
strands run between the companion 
cells, branching out at intervals into 
the pith and cortex. Here the de¬ 
velopment of the mycelium is profuse, 
often forming coiled nests within cells 
and branching abundantly and irreg¬ 
ularly. The host protoplasm is rap¬ 
idly disorganized; but nuclei and starch 
grains are still recognizable even in late 
stages of tissue disintegration. 
The large conidia of A are formed 
abundantly within the cells, sometimes 
in the intercellular spaces. In the 
latter case and when formed in the 
elongated companion cells or xylem 
they may be flattened out or com¬ 
pressed in accordance with the shape 
of the surrounding host cell walls. 
Two or three conidia may be found in 
a large pith cell. Oospores of A have 
not been observed within the tissues; 
P. debaryanum (B ), however, forms 
them in abundance, as well as conidia, 
though to a lesser extent. No oos¬ 
pores of D were found within infected 
tissues. This organism forms round 
conidia to a small extent; nests of 
hyphae, irregularly swollen places in 
the mycelium and profuse branching 
were more frequently observed. The 
curious sickle-shaped bodies so abun¬ 
dantly formed in cultures of all three 
were not found in the tissues. 
Discoloration of the host cell walls 
by oxidation products of the disorgan¬ 
ized cell contents is followed by collapse 
and soft rot accelerated by the action 
of secondary organisms on the dead 
cells. The stems of succulent cuttings 
are rapidly reduced to a black flaccid 
mass which remains in the soil if the 
still healthy part is pulled off, and acts 
as a source of infection. 
THE FUNGI 
MORPHOLOGY 
The morphology of Pythium debarya¬ 
num as it occurred in the writer’s 
cultures will be briefly described, for 
comparison with the related fungi here¬ 
in discussed. The hyphae are 3.5 jj, 
to 10.0 /I thick, average 6.4 u, richly 
branching from strongly developed 
main hyphae. The small subsidiary 
branches curve in all directions, re¬ 
sulting in plate colonies which appear 
uniform and finely granular to the 
unaided eye; this type of growth is 
in sharp contrast with the combed- 
silk effect characteristic of P. complec - 
tens, caused by parallel radial growth 
of the branches. Variations from the 
normal hyphae are to be found in 
irregular knotted masses or coils, and 
in clavate, sickle-shaped bodies abun¬ 
dantly developed within two days of 
growth on plates (pi. 4, D). Carpenter 
(8) figures similar bodies in Pythium 
butleri from sugar cane (his pi. 16, 
figs. 1 and 2). In the writer’s cultures 
they were developed at the interface 
of agar and glass, and near solid 
particles in the media, indicating a 
possible appressorial nature. Chains 
of these clavate bodies, which bore a 
striking resemblance to antheridia ex¬ 
cept that they were larger, and not 
near oogonia, were found in plate 
cultures of each of the Pythium species 
under observation. 
Conidia are smooth-walled, sub- 
spherical, terminal, or intercalary, 
16.6 m to 26.2 At in diameter, averaging 
21.7 At. Germination takes place by 
germ tubes, usually one. Zoospore 
formation has not been observed. 
Swollen intercalary bodies are some¬ 
times found in old cultures where the 
media is drying down. These appear 
more irregular in size and shape than 
the conidia formed in the early stage 
of growth, but also germinate by push¬ 
ing out a germ tube. 
Oospores are found in great abund¬ 
ance on corn meal agar and oatmeal 
agar and are far more frequent than 
conidia. The oogonia are 16.8 n to 
25.8 At in diameter, averaging 25.1 Ab 
oospores are 14.7 At to 21.3 At in diameter, 
averaging 18.1 At. The oospores lie free 
in the oogonia. Both are smooth- 
walled. Antheridia are clavate, single, 
less frequently two or three per 
oogonium; a well-marked fertilization 
tube is put forth from the tip of the 
antheridium which is applied to the 
oogonium wall. Fertilization has been 
frequently followed through in living 
material and does not differ essentially 
from the accounts of De Bary and 
Ward, except in the inception of the 
exospore wall, which takes place by 
the tangential extension of a peripheral 
disk as described for P. complectens (4). 
Germination of the oospores has not 
been observed. 
The fourth Pythium (D) resembles 
the above organism in the size of the 
hyphae and in the method of branch¬ 
ing. Round, smooth conidia are pro¬ 
duced in moderation but show greater 
variation in size, ranging from 12.8 ju to 
27.7 u, average 20.1 a*. The most charac¬ 
teristic feature is the intercalary for- 
