418 
Journal o f Agricultural Research 
Vol. XXIX, No. 8 
tophthora, the tendency to the well- 
protected method of differentiation 
within the original sporangium finds 
full expression; the vesicle present in 
some species no longer serves to hold 
the undifferentiated spore mass but 
remains as a functionless inheritance 
from Pythium like ancestors, finally 
disappearing in other members of the 
genus. 
RESISTANCE PHENOMENA 
The formation of a protective cork 
cambium tending to inhibit the prog¬ 
ress of infection, such as occurs in this 
disease, has been noted elsewhere by 
various workers. Lutman (12) inter¬ 
preted potato scab as a successive series 
of cork layers laid down in advance of 
the parasite, which, however, was appar¬ 
ently powerful enough to pass through 
each layer and stimulate the produc¬ 
tion of another layer deeper in the 
tissues. He noted the disappearance of 
starch from healthy cells in the vicinity 
of the cork cambium, a condition which 
is also present in this geranium disease. 
Dufrenoy (8) reports protective cork 
formation in a chestnut disease. Tis¬ 
dale, working with flax wilt (21), noted 
suberization of groups of cells adjacent 
to those attacked by Fusarium Uni , and 
pointed out the necessarily chemical 
nature of the initial interacting forces 
involved. Protective cork formation 
accompanied by disappearance of starch 
from adjacent healthy cells is figured 
by Erwin F. Smith in a potato tuber rot 
caused by Bacillus caratovorus (19, 
fig . m). 
The effectiveness of suberized cell 
walls in preventing the progress of 
infection has been emphasized by 
Shapovalov and Edson (17) in their 
work on wound cork formation. In 
the case of Pythium it is in accord with 
the mechanical nature of hyphal pene¬ 
tration as interpreted by Hawkins and 
Harvey (9), although the high resist¬ 
ance of suberized walls to solution by 
a large number of reagents should not 
be overlooked. The inability to obtain 
infection on uninjured geranium stems 
with the organism here reported, and 
the low amount of infection on cut¬ 
tings made two days before inoculation, 
are clearly correlated with difficulty in 
penetrating cork layers, which, as 
related to the former case, are often 10 
cells thick beneath the cuticularized 
epidermis. 
It is of interest to note that the 
organism at present under considera¬ 
tion produces a diffusible substance, 
evidently an acid, capable of changing 
the color of Congo Red agar to a deep 
India purple (vide Cultural studies), 
whereas the other three Pythium spp. 
studied at the same time (to be reported 
on in a later paper) caused no color 
change in this medium; nor did the 
latter ever induce cork formation in 
inoculated plants, which usually suc¬ 
cumbed completely. While the nature 
of the diffusible substance produced by 
the cork-inducing Pythium has not been 
determined, a possible cause and effect 
relation is clearly indicated in the corre¬ 
lation between its production^nd the 
presence or absence of a resistance 
reaction evidenced by the stimulation 
of cell division to form a cork cambium. 
Stimulation of cell division by diffusible 
substances produced by a parasite is 
considered by Kunkel (10) as a possi¬ 
bility in the case of the Plasmodiophora 
disease of cabbage; further evidence is 
available in the numerous experiments 
of Erwin F. Smith with Bad. mori, Bad . 
solanacearum (19) and particularly with 
known by-products of Bad. tumefaciens 
(18, 20). 
SUMMARY 
1. A stemrot of geranium (Pelar¬ 
gonium) cuttings caused by Pythium 
compledens n. sp. is here described. 
2. The disease consists of a progres¬ 
sive basal blackening accompanied by 
pectinization and soft rot of pith and 
cortex. Infection stops at a sharp line 
of demarcation 20 to 40 mm. from the 
base within six to eight days after 
inoculation. 
3. Stoppage of infection is due to a 
host resistance reaction manifested by 
the formation of a cork cambium com¬ 
pletely across and within the stem, 
barring further progress of the hyphse 
after infection has already proceeded 
some distance from the point of in¬ 
oculation. It is accompanied by the 
disappearance of starch from healthy 
cells in the vicinity of the cambium. 
4. This reaction is specific for this 
particular host and organism, and was 
not found in the case of three other 
Pythium spp. studied which caused 
complete destruction, nor in the case 
of Coleus cuttings infected and com¬ 
pletely rotted by this organism. 
5. Characteristic signs of the disease 
can be caused with pure culture in¬ 
oculations of this organism on any 
wounded part of the stem. Reisola¬ 
tion, reinoculation, and constant pres¬ 
ence of characteristic oospores in un¬ 
decayed parts of diseased tissues have 
established pathogenicity. Coleus cut¬ 
tings are susceptible, but not cucum¬ 
ber, radish, or cress seedlings. 
6. The hyphse of this fungus are 
hyaline, coenocytic, and cylindrical 
