94 Nebraska Agricultural Exp. Station, Research Bui. 6. 
Table 26. — Average relative day and night transpiration and 
evaporation rates during nine consecutive days in 1910 and 30 
days in 191 4. (Taken from Tables 23 and 2£.) 
Mean 
Mean 
Mean 
Day or night 
Transpira- 
Evaporation 
tempera- 
relative 
wind 
tion 
ture 
humidity 
velocity 
Per cent 1 
Per cent 1 
Degrees F. 
Per cent 
Miles 
per hour 
(1) 
(2) 
(3) 
(4) 
(5) 
YEAR 1910 
Day 7 a.m. -7 p. m. 
100 
100 
79.2 
48.9 
6.2 
Night7 p.m.-7a.rn. 
8 
10 
63.8 
78.1 
1.8 
Difference 
15.4 
29.2 
4.4 
YEAR 1914 
Day 7 a. m.-8 p. m. 
100 
100 
85.6 
59.8 
8.1 
Night8p.m.-7 am. 
7 
15 
72.8 
81.5 
6.2 
Difference 
12.8 
21.7 
1.9 
AVERA 
GE FOR TWO 
YEARS 
Day7a.m-7 :30p.m. 
100 
100 
82.4 
54.3 
7.2 
Night 7:30 p. m.- 
7 a. m 
7.5 
12.5 
68.3 
79.8 
4.0 
Difference 
14.1 
25.5 
3.2 
1 The average hourly transpiration and evaporation for the night (12 
hours in 1910 and 11 hours in 1914) is expressed in per cent of the hourly day 
loss. For the exact losses in grams, see Tables 23 and 24. 
Just why the transpiration rate should be reduced relatively 
so much more during the night than the evaporation rate is not 
fully understood. It would appear that there may be two chief 
possible causes, namely: (1) The internal heat developed by 
photosynthesis in the light is absent in the dark of the night. 
This factor may, however, be quite offset by the greater cooling 
effect of a more rapid transpiration in the daytime. (2) A 
comparative study of the condition of the stomatal apertures 
during the daylight and at night has shown that they are closed 
at night, and open in the day. At night the walls of the guard 
cells which form the stomatal aperture lie in such close contact 
that they merely appear as a heavy black line. On the other hand, 
in the daytime a very distinct, colorless aperture is found to exist 
between these cell walls. Occasional stomata show a slight 
aperture even at night. 
As a result of these observations it is evident that the area 
exposed for diffusion is greatly reduced at night in the case of the 
corn leaf, while it is constant with a free water surface. Thus in 
