298 
Journal of Agricultural Research 
Vol. XXX, No. 4 
antly. Fragments of agar containing 
ends of these strands are cut from the 
plate and transferred to new plates 
until no bacterial growth accompanies 
the fungus, when it is transferred to 
corn-meal agar for a stock culture. It 
often happens that strands of Pythium 
grow in a manner resembling Aphano- 
myces so closely that they can not be 
distinguished until they are trans¬ 
ferred to new plates of prune agar, 
where they soon develop a white 
matted growth very different in charac¬ 
ter from the sparse arachnoid growth 
of Aphanomyces. Oftentimes many 
fragments of roots and stems must be 
selected and plated before a culture of 
the desired fungus is obtained. 
MORPHOLOGY OF PARASITE 
MYCELIUM 
In longitudinal sections of diseased 
pea stems the vegetative stage of the 
parasite is revealed as a hyaline non- 
septate mycelium, composed of hyphae, 
varying considerably in diameter among 
themselves, but individually not sub¬ 
ject to abrupt fluctuations in respect to 
this dimension. (PI. 3, A.) Branch¬ 
ing is exhibited in moderate, usually 
not in great abundance, the branches 
generally being produced at angles 
approaching a right angle. Not infre¬ 
quently branches show little linear 
growth, then remaining as short diver- 
ticulate spurs on the axial filaments. 
The fungus is largely intracellular, the 
hyphae being oriented longitudinally 
within the cells, their development 
between the cells being relatively 
meager and apparently more or less 
accidental. Appearances often suggest 
that the fungus passes through the cell 
walls of the host perhaps with less ease 
than, for example, some parasitic spe¬ 
cies of Pythium. Whereas in cortical 
tissue invaded by the common damp- 
ing-off fungus, the hyphae pass pro¬ 
miscuously from cell to cell, without 
much evidence of the membranes pro¬ 
viding any obstacle, in tissue invaded 
by the root rot parasite cells crowded 
with mycelium may lie adjacent tq 
others entirely free of the fungus. It 
should be mentioned, however, that the 
cortical tissue invaded by the root rot 
parasite is of a distinctly less succulent 
character than the cortical tissue of 
seedlings of various hosts subject to 
damping-off, and that strains of Pyth¬ 
ium effective in producing the latter 
type of injury exhibit little or no 
aggressiveness toward pea plants readi¬ 
ly attacked by the species of Aphano¬ 
myces under consideration. A some¬ 
what similar mycelial distribution was 
reported by Weatherwax (17) in 
filaments of Spirogyra dubia Kg. 
invaded by Aphanomyces phycophilus 
DeBary. 
SEXUAL REPRODUCTION 
The purely vegetative condition 
represents a rather brief phase in the 
development of the fungus, mycelial 
growth coming to a pause as the 
cortical tissue begins to collapse. 
Oogonia and antheridia now make their 
appearance. Owing to the crowding 
of the hyphae within the host cells, the 
relation between the branches bearing 
these organs can not usually be dis¬ 
tinguished. There is no reason to 
believe, however, that the sexual 
apparatus in diseased plants shows any 
significant departure from these struc¬ 
tures as they develop on suitable 
artificial substrata where they can 
be accurately studied. The oogonia 
before fertilization are thin-walled, 
subglobose bodies with densely granu¬ 
lar, vacuolate contents. After ferti¬ 
lization the oogonial wall becomes 
conspicuously thickened, the thicken¬ 
ing being subject to peculiar irregulari¬ 
ties, with the result that the inner con¬ 
tour is represented by a more or less 
sinuous line, giving the whole structure 
a peculiar internally scalloped appear¬ 
ance. (PI. 4, A to H.) As in other 
species of Aphanomyces, the oogonial 
cavity is very largely but not com¬ 
pletely occupied by the single oospore, a 
subspherical structure with a thick 
colorless wall, the thickness of the latter 
not given to great variations either 
with respect to different individuals or 
with respect to different portions of the 
same individual. The contents of the 
normal mature oospore consists of a 
EXPLANATORY LEGEND FOR PLATE 3 
A—Portion of longitudinal section of basal part of pea stem in an early stage of infection, showing 
the development of the fungus in certain cortical cells and extension into adjacent cells. The 
ellipsoidal body near top of figure represents a developing oogonium. From material of a plant 
grown in soil inoculated with a pure culture of the parasite, killed in Flemming’s weaker solution, 
embedded in paraffin, cut on the microtome, and stained with Flemming’s triple combi¬ 
nation. X470 
B.—Epidermis of hypocotyl of pea seedling derived from a surface-sterilized seed planted on sterile 
water agar in a test tube, the resulting culture inoculated 5 days after planting with a pure 
culture of Aphanomyces euteiches. Showing penetration by 3 extramatrical hyphae into tissue 
of plant—one, Ba , entering close to or at the juncture of adjacent epidermal cells; another, Bb, 
entering between two epidermal cells; and a third, Be, entering between a guard cell and an 
epidermal cell. X470 
