Sept. 23,1918 
Stem Lesions Caused by Excessive Heat 
597 
Seedling B. —A heated wire held 1.5 mm. to the right of the stem for five minutes 
resulted in the lesion shown in figure 1, B. The plant remained turgid for 11 days 
and a whorl of new leaves appeared. The stem at the soil surface then decayed, appar¬ 
ently from a root infection, bearing a mat of pink spore masses. 
Seeding C.—A Mazda incandescent lamp placed beside and slightly above the 
seedling for nearly two hours killed the tips of the cotyledons and produced a lesion 
just above the soil surface, without perceptible injury to intermediate tissue. In its 
location this lesion was thoroughly typical of those commonly found in the seed beds. 
Five days later fungus hyphae appeared on the lesion, and the stem broke over at that 
point. The lesion then progressed both up the stem and into the root, and the seed¬ 
ling wilted. Spores of A Iternaria spp. were promptly produced on placing the seedling 
in moist chamber (fig. 1, C). 
Seedling D.—The stem and surrounding soil were subjected for three minutes to 
sunlight through a condensing lens, striking both the stem and soil surface at angles 
of about 45 0 . The resulting lesion extended about 3 mm. below the soil surface and 
4 mm. above it. Mechanical support was required to keep the seedling from falling. 
Hyphae soon appeared on the lesion at the soil surface, and in eight days after treatment 
wilting occurred, a species of Altemaria fruiting on the lesion in moist chamber 
(fig. 1, D). 
Seedling E.—On a hot day*, seedlings planted in loam in a 3-inch pot were placed 
immediately south of a brick wall. At night the seedling shown in figure 1, E, and 
another seedling were leaning slightly, but were apparently uninjured. An examina¬ 
tion of the underground parts showed that these were distinctly shriveled from the 
soil surface to a point 10 mm. below; a third seedling in the pot, still erect and normal 
in appearance, was also constricted just below the soil surface. The plants were re¬ 
potted and kept under observation. The tops of all remained entirely healthy for 
several days. At length the lesion began to extend up the stem, and on the eighth 
day wilting occurred. The entire root and 10 mm. of the stem above the soil line had 
become involved in the original lesion. Spores of species of Dactylosporium, Alter- 
naria, and Fusarium appeared on the lesion in moist chamber. 
Numerous seedlings of the same original lot were kept in the same 
room as the seedlings listed above during the period of the tests. None 
developed lesions which could have been mistaken for whitespot. 
In all of the seedlings whose stems were heated directly the lesions 
were at first a dark grayish green, changing in 24 hours to the light color 
and shriveled appearance characteristic of whitespot lesions on seedlings 
in the nurseries. The immediate darkening is supposed to be due to 
the filling of the intercellular spaces with cell sap, while the ultimate 
light color presumably indicates the loss of liquid from both the inter¬ 
cellular spaces and the lumina of the cells and its replacement by air. 
In all cases the lesions remained definitely limited for several days, and 
were then extended, apparently as a result of infection by fungi not 
commonly capable of attacking uninjured plants. It appeared that in 
most cases neither the heat nor the fungi later entering the lesions stopped 
conduction or evolved toxins in sufficient quantity to cause the death of 
the leaves, as reported for another plant by Overton (jj). Wilting 
finally occurred, it is believed, only when fungi entering at the lesion or 
at some point below it had penetrated the absorbing portion of the root. 
It may be remarked that in many cases of damping-off caused by the 
