Transpiration as a Factor in Crop Production. 
93 
2. During that part of the day when the transpiration de- 
creased hourly (2 p. m. to 8 p. m.) there was an actual average 
decrease from hour to hour in: 
Transpiration (per cent) 22.50 
Evaporation (per cent) 20.00 
Temperature (degrees F.) 1.50 
Relative humidity (per cent increase) 2.00 
Wind velocity (miles per hour) 45 
3. Averaging together the hourly differences thruout the 
13 hours of the day regardless of whether plus or minus, we have 
as an hourly average for the day the following changes: 
Transpiration (per cent) 25.00 
Evaporation (per cent) 26.00 
Temperature (degrees F.) 2.30 
Relative humidity (per cent) 3.70 
Wind velocity (miles per hour) 50 
4. As an average for the 13 daylight hours (7 a. m. to 8 p. m.) 
a 10 per cent change in the transpiration rate is accompanied by 
the following changes: 
Transpiration (per cent) 10.00 
Evaporation (per cent) 10.40 
Temperature (degrees F.) 90 
Relative humidity (per cent) 1.50 
Wind velocity (miles per hour) 20 
RELATIVE DAY AND NIGHT TRANSPIRATION AND EVAPORATION IN RELATION 
TO CLIMATIC FACTORS. 
Table 26 is summarized from Tables 23 and 24, which show 
the hourly transpiration and evaporation rates for periods in 
1910 and 1911 during which the plants were full grown and the 
leaf -area remained rather constant. The 24 hours were arbitrarily 
divided into 12 hours each for day and night in 1910, and in 1914 
the night included 11 hours and the day 13 hours. 
It is very interesting to note that as an average for the two 
years the average hourly night transpiration amounted to only 
7.5 per cent of the hourly day loss. The corresponding difference 
in climatic factors is summed up as follows: The night tempera- 
ture was 14.1° F. cooler, the relative humidity 25.5 per cent 
higher, and the wind velocity 3.2 miles an hour lower. There 
was also the normal light and darkness of day and night. 
These differences in climatic factors were sufficient to reduce the 
average hourly night transpiration to one-thirteenth of the day 
transpiration. At the same time, the evaporation from a free water 
surface was reduced only to one-eighth of the day evaporation. 
