Sept. 33,1918 
Stem Lesions Caused by Excessive Heat 
599 
On the other wall, exposed to the south, the percentage of affected 
shoots was much greater than on the level surface of the rest of the 
field. Wind action was not excluded as a possible cause of the cowpea 
lesions, but the protected location of the nursery, so far as south winds 
are concerned, made the evidence rather conclusive that insolation rather 
than wind was responsible for the lesions on the rye. Munch {8) reports 
whitespot on maple, vetch, and peas, and believes that in some cases 
germinating seeds, as well as seedlings which have already broken soil, 
are killed by overheated soil. 
Whitespot is not always fatal, even when the lesion girdles the stem. 
Two seedlings of Pinus ponderosa which had been girdled by definite 
whitespot lesions, slightly shrunken but not severe enough to cause 
breaking over, were marked for later observation. At the end of the, 
season the lesions had disappeared and the plants seemed in every way 
normal. In leaf lesions due to heat, Sorauer (x5, p. 638) has after several 
weeks observed a regeneration of chloroplasts in slightly affected tissues. 
All things considered, whitespot lesions are believed to be caused 
mainly by excessive heat. While light as such may possibly take part 
in some cases, it evidently does not enter into all cases of injury. The 
relative unimportance of light as distinguished from heat is indicated by 
the numerous lesions under slat frames, the extension of all serious 
lesions to the north sides of stems, and the experimental production of 
lesions below the soil surface (seedlings D, E, and others). The pre¬ 
liminary experiments here reported were mostly at excessive temper¬ 
atures, and absolute proof that heat alone is the cause of the common 
lesions in the seed beds must await further experiments at temperatures 
which more commonly occur in nature. 
BASAL LESIONS ON SEEDLINGS SEVERAL MONTHS OLD 
A type of trouble which is probably related to the whitespot described 
in the foregoing was observed in 1915 in the seed beds of a nursery of 
the United States Forest Service, located at an elevation of 7,300 feet 
in the Wasatch Mountains, Utah. The plants affected were spruce and 
Douglas fir which had been raised from seed the preceding year. They 
had made a normal height growth during their first season and remained 
green throughout the winter under a heavy coating of snow which covered 
them for more than five months. Two or three weeks after the snow 
melted many of the seedlings began to turn yellow and ultimately died. 
Examination showed dead bark, beginning at the soil surface and ex¬ 
tending up the stem from 3 to 9 mm. In many cases the lesion extended 
farther up the stem on the south than on the north side, and on some 
seedlings lesions were found which were entirely limited to the south side 
of the stem, and had started to heal over from the edges. In no case 
was there found any such swelling above the lesion as occurs above stem- 
girdle lesions on older stock. In many of the advanced cases the cortex 
