INFLUENCE OF LIGHT UPON ACTION OF STOMATA 
At the same time the sunHght attained its maximum brightness. In this 
figure it is indicated by curves that the Hght intensity and the opening and 
closing of the stomata vary similarly, whereas the humidity curve goes in 
the reverse direction. The curves are made of broken lines, as follows: the 
continuous lines connecting the small circles form the curve indicating the 
opening and closing of the stomata; the broken lines of pieces of equal length 
between the small circles form the temperature curve; the broken lines of 
two unequal lengths connecting the small circles form the humidity curves. 
The figures at the bottom of the diagram indicate the hours from 8 o'clock 
in the morning to 6 o'clock in the evening. The figures at the left indicate 
the temperature in degrees Fahrenheit because our Mason hygrometer was 
supplied with Fahrenheit thermometers. In words we have stated the 
temperature in degrees Centigrade. In the graphs the difference is nil. 
Figure 2 indicates the behavior of the stomata of barley seedlings twenty- 
four days old, with 11.3% of moisture in the soil at 9:25 A.M. and 10% at 
4:30 P.M. The loss of water from the soil during this time from the surface 
of the soil and from the plants, through their stomata and otherwise, was 
1.3%. The temperature of the air decreased slowly from 84° F., 28.9° C, 
at 2:40 P.M., and the humidity rose from 62% at 9 A.M. to 72% at 2:40 
P.M., from which time the two curves converged somewhat. Thus the 
atmospheric conditions of this day were moderate, and represent a mean 
average condition as compared with those of the previous day. At 9 A.M. 
there was a heavy fog but nevertheless enough light to cause the stomata 
to open slightly. At ii A.M. there was a short time of fairly bright light, 
but a time of comparative darkness followed, and at 2 40 P.M. the stomata 
were all but closed. -At 4:15 P.M., when it was almost too dark for further 
readings, the stomata were all closed. 
Figure 3 presents a contrast to the case just described. This is the record 
of a similar box of barley seedlings, thirty-seven days old. The soil was 
moist at 9 A.M. but was watered. At 3:40 P.M. it showed 21.7% of mois- 
ture. In the last case a heavy fog prevailed during the early hours of the 
morning, but there was a fair amount of light. In the present instance the 
day was cloudy and, for this country, very dark until 2 P.M., when there 
was a brief interval of light, but by 3 P.M. the dusk of evening had begun. 
The stomata remained closed throughout the day, as the straight line shows. 
The soil was moist and the plant cells were turgid, but with light insufficient 
to stimulate the guard cells the stomata remained closed. The slight 
amount of light at about 2 P.M. was not enough to affect them. The 
temperature reached 28.9° C. at 11.55 A.M., and the humidity fell to 47% 
at 2 P.M., the curves of temperature and humidity showing a rather wide 
divergence in the early afternoon hours and a close convergence, as indi- 
cated before, at 4:55 P.M. 
