640 
Journal of Agricultural Research 
Vol. V, No. 14 
In order to decide between these two assumptions, other evidence is 
necessary; and this may be found in a consideration of the transpiration 
during the night—i. e., when the radiation received by the plants is nil. 
In Table XXXVIII are summarized the transpiration and wet-bulb 
depression (in percentage of the maximum) and the air temperature 
(in percentage of the maximum range) for the hours 3 to 4 a. m. and 
8 to 9 p. m. It is evident from the table that a simultaneous diminu¬ 
tion in the wet-bulb depression of one-fourth of its maximum and in 
temperature of one-third of its maximum range results in a drop of 
only 3 per cent in the transpiration rate. This would seem to indicate 
that the high correlation obtained between transpiration and air 
temperature (or wet-bulb depression) is largely due to the direct corre¬ 
lation between radiation and temperature (or wet-bulb depression). 1 
Table XXXVIII .—Comparison of transpiration , temperature , and wet-bulb depression 
at 3 to 4 a. m. and 8 to g p. m. 
Crop or period. 
Per cent of maximum 
transpiration. 
Per cent of maximum 
temperature. 
Per cent of maximum 
wet-bulb depression. 
3 to 4 
a. m. 
8 to 9 
p. m. 
Differ¬ 
ence in 
a. m. 
and 
p. m. 
read¬ 
ing. 
3 to 4 
a. m. 
8 to 9 
p. m. 
Differ¬ 
ence in 
a. m. 
and 
p. m. 
read¬ 
ing. 
3 to 4 
a. m. 
8 to 9 
p. m. 
Differ¬ 
ence in 
a. m. 
and 
p. m. 
read¬ 
ing. 
Wheat. 
2 
e 
2 
■2 
40 
37 
O 
D 
O 
Oats. 
4 
6 
2 
2 
24 
22 
21 
37 
16 
Rye. 
3 
7 
4 
3 
42 
39 
*7 
39 
22 
Sorghum. 
2 
5 
3 
2 
34 
32 
25 
48 
23 
Amaranthus. 
1 
3 
2 
4 
42 
38 
12 
45 
33 
Alfalfa. 
2 
3 
1 
4 
35 
3 i 
15 
42 
27 
June period. 
2 
3 
1 
3 
49 
46 
19 
47 
28 
October period. 
1 
7 
6 
5 
32 
27 
2 
38 
36 
Mean. 
7 
34 
26 
If we ascribe to radiation a causative effect equal to that indicated by 
the correlation coefficient with transpiration, it becomes possible also to 
investigate the influence of wind velocity on transpiration by a process 
of elimination similar to that employed above. 
Transpiration in still air is somewhat less than in moving air, since 
the latter tends to reduce the distance that the transpired moisture must 
move in order to find free-air conditions. In other words, the wind 
tends to increase the diffusion gradient, and so increases the transpira¬ 
tion (or evaporation) rate. But a slight movement appears to satisfy 
this condition, and the correlation coefficients between wind and trans¬ 
piration (Table XXXVII) show that the variation in wind at Akron, 
where some wind nearly always occurs, has little influence on the trans- 
1 In opposition to this view it may be argued that the plants from 3 to 4 a. m. are more turgid than from 
8 to 9 P- m. This is undoubtedly true, but it is also true that during the last named period the plants 
are more turgid than at a or 3 p. m., the period during which the maximum transpiration rate was observed. 
