Oct. 15, 1924 
Geranium Stemrot 
415 
squares, each square then being planted 
in the center of the fresh plate. Before 
inoculation all plates were kept over¬ 
night in their respective compartments 
to avoid lag effects. 
No growth takes place at 11° or at 
37.5° within 48 hours or for any subse¬ 
quent period at the latter temperature. 
Very slow growth is evident at 5° after 
144 hours (3 mm., not shown in fig. 3). 
The optimum temperature centers 
around 30°. A marked difference in 
temperature ranges above and below 
the optimum may be observed, the 
growth rate falling away much more 
rapidly above the optimum tempera¬ 
ture. 
GENERAL DISCUSSION 
TAXONOMY 
The general characters of this fungus 
and the fact that the zoospores are not 
formed within the original sporangial 
wall, but within an evanescent vesicle 
containing the undifferentiated extruded 
contents of the sporangium, place this 
organism in the genus Pythium (Pring- 
sheim). Preliminary complete differen¬ 
tiation before entrance into the vesicle 
has not been observed, although scores 
of sporangia have been germinated and 
studied; the only other method of ger¬ 
mination noted was by germ tube, as 
described above for older sporangia. 
The characters of the sporangium 
and oospore place this organism in But¬ 
ler’s subgenus Sphserosporangium (6), 
among the species with smooth-walled 
oospores lying free in the smooth oogo¬ 
nium (P. de baryanum, P. uitimum , and 
P. vexans). It differs from the first, 
which was studied at the same time, in: 
(a) Smaller oospores and oogonia; ( b) 
finer mycelium; (c). very marked cul¬ 
tural differences; (dj inability to attack 
water cress, cucumbers, and radishes; 
(e) preponderance of zoosporangia 
rather than conidia; (/) the very char¬ 
acteristic antheridium. It differs from 
P. uitimum in being parasitic, in pro¬ 
ducing zoosporangia rather than co¬ 
nidia (which is a distinguishing char¬ 
acter of P. uitimum ), and in the shape 
of the antheridium. 
In this last character it comes near¬ 
est to De Bary’s Pythium vexans, which 
he characterized (. 1 , 2) by: (a) The pe¬ 
culiar insertion of the oogonium, which 
is sessile on the outside of the mycel¬ 
ium or inserted with a broad base into 
the mycelial tube; (6) the broad ap- 
pressed antheridium fused to the 
oogonium, although his figure {2, PI. 
5, jig. 3) shows one clavate antherid¬ 
ium; (c) saprophytic habit; ( d ) no spo¬ 
rangia or conidia were observed. But¬ 
ler (6) adds that the tapering of the 
hyphae into very fine filaments distin¬ 
guishes it from any other species he 
studied. He was more successful than 
De Bary in finding sporangia, which 
were “rare, scarcely ever spherical or 
oval, but irregularly pear-shaped, ovoid 
or subangular.” 
The organism described in this paper 
resembles P. vexans in the shape of 
the broad appressed antheridium, but 
differs in the following respects: (a) 
The oogonia are not inserted with a 
broad base, but are borne on a slender 
stalk (PI. 5, B, C, D); (6) it is para¬ 
sitic on Pelargonium and Coleus; (c) 
sporangia are formed abundantly on 
various media and are oval to spherical 
(PI. 4, F), except when intercalary; 
(d) the hyphae are cylindrical with 
rounded tips (PI. 4, E) and do not 
taper to fine points. It is considered 
a distinct species, for which the name 
Pythium complectens n. sp. (referring 
to the clasping antheridium) is pro¬ 
posed. The technical description fol¬ 
lows. 
Pythium complectens, n. sp, 
Hyphae coenocytic, hyaline, 1.70 y to 
4.85 y\ cylindrical with rounded tips, 
forming a strongly parallel silky growth 
on solid media; acid produced chang¬ 
ing color of Congo Red agar; sporangia 
abundant, vacuolate, produced singly, 
spherical when terminal, oval to sub- 
spherical when intercalary, without 
papilla, average 21.8 y in diameter, 
range 16.4 y to 27.3 y, germinating by 
extrusion of undifferentiated contents 
into vesicle in which zoospores are 
formed, not proliferating; zoospores 
broadly lenticular, with two cilia at 
hilum, with single vacuole, 10 to 26 
formed per sporangium, 5.9 y to 8.5 y 
wide by 10.6 y to 11.5 y long, rounding 
up and germinating by a tube; oos¬ 
pores single, smooth walled, spherical, 
free in the oogonium, wall light yellow 
to sepia brown, abundant in host tis¬ 
sues, average 16.2 y in diameter, range 
11.3 y to 20.8 y; oogonia smooth, sub- 
spherical, borne on a slender stalk, 
average 18.6 y in diameter, range 13.2 y 
to 23.3 y; antheridium single, one- 
celled, arising from adjacent hypha 
or below oogonial stalk, persistent, 
varying from a trumpet shape flaring 
out at region of attachment, to a broad 
irregularly lobed mass clasping a large 
part of the oogonium and fused with 
it. Parasitic on Coleus and Pelar¬ 
gonium cuttings, causing a black stem- 
rot; inducing a resistance reaction in 
the latter host characterized by the 
formation of a cork cambium, barring 
further progress of the hyphae after in¬ 
fection has proceeded some distance 
from the point of inoculation. 
