July i, 1921 
Pathogenicity of Corticium vagum on the Potato 
475 
in which the growing point is exposed to the fungus in the soil. This 
relation was determined with a fair degree of accuracy by the rate at 
which the plants appeared through the soil at the various temperatures. 
Uniformly in all the experiments the plants at 24 0 C. came up first. 
These appeared after a period of from 18 to 25 days, depending upon the 
vigor and state of dormancy of the tubers used for seed. In general, the 
plants in soil held at 18 0 , 21 0 , and 27 0 appeared from 2 to 5 days later 
than those at 24 0 , while those grown at 9 0 and 12 0 were delayed as much 
as 10 to 15 days. The plants at 30° were very irregular in their growth 
and in a few cases were as seriously retarded as plants at 12 0 . 
A quantitative expression of the time estimate of the rapidity of 
growth is found in the data presented in Table VIII. The results are 
in general representative of the growth obtained in all the different 
experiments where adequate time was allowed before removing the plants. 
A decidedly different growth relation is shown at the various tempera¬ 
tures with the change of the seat of action from the tubers and growing 
points underground to the leaves and growing points above ground. 
The measurements obtained on December 2, 33 days after planting, 
together with the appearance of the plants December 15, as shown in 
Plate 88, B, indicates an optimum soil temperature somewhere near 18 0 
C. for the latter periods of potato growth. This, in general, agrees with 
the conclusions of Orton (9) and Smith (15). 
Table; VIII .—Effect of various soil temperatures on the rate of growth of the normal 
potato plant at different stages in its early development & 
Date. 
Age of 
plants. 
Average height of plants at various temperatures. 
95 °C. 
12° C. 
15.2*0. 
18° C. 
2I. 4 °C. 
24° c. 
27. 2° C. 
30. 5 °C. 
Nov. 25.. . 
Dec. 2 .... 
Dec. 15 b . . 
Days. 
25 
33 
46 
Inches. 
Not tip 
Just Up 
Inches. 
Not Up 
2. 6 
Inches. 
1.4 
4-3 
Inches. 
2. I 
5-8 
Inches. 
2.7 
5 * 0 
Inches. 
3 - 2 
4 - 8 
Inches. 
2 - 5 
3 - 8 
Inches. 
2. 0 
3-2 
a The figures represent the height of the plant above the soil. 
b Plate 88, B, shows the relative heights of plants at this date. 
The most interesting results were obtained with respect to the effect 
of the high temperatures upon the general morphology of the potato 
plant. Uniformly, at 30° C. the plants showed a decided swellng of the 
underground stems (PI. 93, B). This feature was accompanied by a 
repression of the stolons and by what appeared to be a compensating 
development of leafy structures or deformed branches at the under¬ 
ground nodes. Excessive branching also occurred at or near the 
apical region of the stem just as the latter emerged through the soil. 
Upon further growth these branches gave a decided rosetted appearance 
to the plants. The aerial branches were in general very slender, 
with shortened intemodes, and bore very small, narrow leaves which 
