1028 
Journal of Agricultural Research 
Vol. V, No. 22 
the humidity remained comparatively high—above 60. A comparable 
difference is noted in the humidities and stomatal pore widths taken on 
this date and on May 20. After 11 a. m. the humidity on June 3 was 
generally above 60 and the stomata had pore widths of more than 5/* 
until after 1 p. m.,when the opening gradually decreased until closure 
occurred about 3 p. m. On May 20, after 11 a. m., the humidity was 
generally slightly above 50 and the stomatal pore opening was reduced 
from 5 ju at 10 a. m. to about 2/x at 11 a. m., after which time it seldom 
exceeded this amount. 
A few readings were made in the field at various times during the season 
to get an indication of the stomatal movement under such conditions. 
On June 21 the stomata were found to be well open at 3 p. m. and later at 
a humidity of 60 or above (Table VIII). On June 23 the stomata were 
widely open from 8.30 to 10.40 a. m., even though the humidity dropped 
to as low as 40 at 10.10 a. m. The readings were not continued long 
enough to determine whether this low humidity would produce stomatal 
closure during midday. However, the readings taken on July 18 indicate 
that at 2 p. m. the stomata had a smaller pore width than at any other 
reading during the day and at that time the lowest humidity (57.5) of the 
day occurred. Two readings were made at the same time in this field. 
The one made near the center of the field, where the plants were large and 
close together, showed the stomata to be open (8.7 upper, 1.8 lower) at a 
humidity of 57.5, while the other made at the edge of the field, where the 
plants were small and far apart, showed the stomata to be closed at a 
humidity of 43.5. The maturity was determined to be the same for both 
sets of leaves used. In this case the soil-moisture content was noted to 
be much lower at the edge than in the center of the field, as the low 
humidity would indicate. 
Table VIII. —Stomatal pore openings on leaves of sugar-beet plants growing in the field 
at Rocky Ford , Colo., in IQI3, together with the temperature and relative-humidity records 
taken among the leaves at that time 
Date and time of readings. 
Temperature. 
Humidity. 
June 21: 
3.00 p. m. 
°F. 
85 
60 
3.41; p. m. 
4-3° P- m. 
77 
65 
June 23: 
8.30 a. m. 
74 
60 
9.20 a. m. 
79 
52 
10.10 a. m. 
33 
39- 5 
10.40 a. m. 
35 
46.5 
July 18: 
9.00 a. m. 
72 
9 i 
10.30 a. m. 
74 
87 
11.15 a. m. 
82 
67 
2.00 p. m. 1 2 . 
83 
57- 5 
2.00 p. m. 2 . 
89 
43- 5 
Average stomatal pore widths. 1 
Upper leaf sur¬ 
face. 
M 
IO. 4 U) 
4.96 M 
1.72 (9) 
i°- 8 (3) 
*3- S (4) 
10.6 (7) 
12.9 ( 5 ) 
6 - 3 ( 6 ) 
4- 6 (3) 
o (10) 
Lower leaf sur¬ 
face. 
10.08 (5) 
6.6 ( 6 ) 
5-1 (7) 
9- 9 (4) 
IO * 3 ( 4 ) 
7. 1 8} 
10.8 (3) 
6. 4 ( 3 ) 
4* 1 ( 3 ) 
14.4 (4) 
1. 8 (10) 
o (10) 
1 The number of readings made is given in parentheses following each average. 
2 These readings were taken at two different places in the same field. 
