Oct. 15, 1924 
Geranium Stemrot 
405 
the more turgid upper phellogen cells, 
as well as the healthy parenchyma 
above, are deeply stained with the 
Orange G, indicating their cellulose 
nature. Starch has completely dis¬ 
appeared for a considerable distance 
above and below the cork layer. Im¬ 
mediately underneath the collapsed 
stratum of suberized walls are one to four 
layers of infected cells, whose walls 
are still rigid, slightly swollen, often 
split and pulled apart, and character¬ 
ized by failure to take up completely 
any of the stains in the Flemming 
triple or the iodine green-acid fuchsin 
combination after fixation with Merkel’s 
or Flemming’s Strong. They remain 
a clear amber color, as in sections 
of fresh material. Negative reac¬ 
tions were obtained for lignin, suberin, 
pectin, and cellulose, except that solu¬ 
tion took place in concentrated sul¬ 
phuric acid. The contents are dis¬ 
organized, and hyphae (often oospores) 
are present, reaching up to the sub¬ 
erized layer but not beyond it. 
Below this “ nonstaining” layer is 
the remainder of the infected tissue, 
bordering on the hollowed part of the 
stem. The walls are swollen and stain 
deeply with the safranin of the Flem¬ 
ming triple, indicating pectinization. 
This was confirmed by positive reac¬ 
tions with Bismark Brown, Methylene 
Blue, and Ruthenium Red. Examina¬ 
tion of walls in the ‘‘ 1 nonstaining” re¬ 
gion bordering on the pectinized cells 
showed in many cases an extension 
of the safranin along the outer lamina. 
Thus, a single cell often showed com¬ 
plete reddening of the basal wall im¬ 
mediately adjacent to the pectinized 
tissues, the remaining walls being 
amber colored except for a short con¬ 
tinuation of safranin on the outer parts 
of the two walls neighboring on the 
wholly stained wall. The fact that 
safranin is taken up only on the surface 
of the “nonstaining” wall implies action 
of pectinase from without, and pre¬ 
cludes the possibility that it is due to 
incomplete washing out during the 
staining process, which would have 
left the safranin in the innermost 
lamella. That complete pectinization 
and solution up to the suberized layer 
ultimately takes place was shown by 
examination of cuttings which had been 
infected for several weeks and had 
formed the usual demarcation line. 
Here complete hollowing out had taken 
place and the suberized layers, now 
several cells thick, bordered directly 
on the cavity (PI. 3). 
The formation of a cork layer across 
the stem acts as an effective barrier to 
further progress of the hyphae, which 
have not been observed above it except 
where fissures have occurred through 
the action of nematodes or mechanical 
injury. It is clearly a specific reaction 
on the part of this host to this organ¬ 
ism, since it was not present in the case 
of the other three Pythium species stud¬ 
ied nor in the case of Coleus cuttings 
jnfeeted with this fungus. 
MORPHOLOGY AND DEVELOPMENT 
OF THE FUNGUS 
The hyphae. —Hyaline, coenocytic 
when young, cylindrical, with no 
tapering in main or lateral branches 
(PI. 4, E). They are very slender, 
measuring 1.70 ju to 4.85 ju (75 meas¬ 
urements) . Branching is abundant 
and irregular; lateral hyphae often ex¬ 
tend beyond the parent hypha and be¬ 
come main branches in turn. The an¬ 
gle of branching varies from 45° to 90°, 
followed by a sharp swing forward, 
which results in a characteristic closely 
parallel growth on solid media, like 
combed silk. Smaller subsidiary 
branches are given off profusely and 
curve irregularly. A transparent 
Liesegang-ring effect in the otherwise 
dense growth has been found to be 
caused by a comparative scarcity of 
these smaller branches in these rings, 
thus affecting the otherwise uniform 
density of the growth. When growing 
plate cultures are inverted under the 
microscope, the hyphae at the edge of 
the colony sometimes cease elongation 
at their original diameter, and put 
forth a slender prolongation from the 
tip, which broadens out farther on into 
the normal diameter and resumes the 
usual course of growth. 
Sporangia. —The asexual fruiting 
bodies are always borne singly. They 
are usually terminal, less frequently 
intercalary (pi. 4, F), and sometimes 
sessile, owing to continued lateral 
growth of the hyphal tip from the base 
of the sporangium. Terminal and ses¬ 
sile sporangia are uniformly oval to 
spherical. Intercalary sporangia are 
irregularly oval to subspherical, often 
asymetrical, with flattenings at the 
places of attachment. Measurements 
EXPLANATORY LEGEND FOR PLATE 2 
A. —Section of stem at demarcation. Healthy cells at top; cork cambium center; below, single 
layer of crushed cells, suberized; split and swollen walls of infected cells below, deeply 
stained; hyphse (at x ); pectinization of lower cell walls, causing cavity 
B. —Later stage, cambium layers increased; decay of diseased tissues has progressed up to the 
protective cork cambium 
