INFLUENCE OF LIGHT UPON ACTION OF STOMATA 
As the humidity began to fall the stomata began to open: but as the curves 
show, the stomata did not begin to fluctuate with the changes in humidity, 
but rather were guided in their movements by the amount of light. Fall 
of humidity, therefore, is probably not a factor influencing the behavior of 
the guard cells. 
Figure 6 is representative of many sets of wheat seedlings examined on 
moderately bright days with varying light intensity. In this case the light 
was brighter than in the two preceding. The plants were ten days old, 
and the soil moisture in the box varied from 20.9% at 9:25 A.M. to 22.2% 
at 4:30 P.M. The temperature and the humidity both remained within 
the limits of 76° and 64%, converging very closely at 4:15 P.M. Con- 
ditions were such as to keep the stomata open all day, although a dark 
period about 11:30 A.M. caused a partial closing, as indicated by the drop 
in the line in the figure. By 2 o'clock the light was strong enough to cause 
the maximum opening for the day, as shown by the stomatal curve. 
Oats 
The reactions of the stomata of oat seedlings very closely resembled 
those of barley and wheat. That there are special difl'erences, however, is 
revealed by the accompanying graphs. On examining an extensive series 
of stomatal reactions, figure 7 was chosen as indicating the optimum con- 
dition for oats. The soil was comparatively dry at 8:30 A.M., there being 
only 9.9% of soil moisture, and no water was added during the day. The 
temperature rose to 35.5° C. at 1:30 P.M., and the minimum humidity, 
44-46%, prevailed between 12:45 and 2:55 P.M. The seedlings were 
fifteen days old and well developed. The sun was unclouded from 8 A.M. 
on the day of these readings, and the light reached its greatest brightness 
between 12 o'clock noon and 2 P.M. The first readings showed the Stoma ta 
open. They continued to open still wider until the maximum was reached 
at 12:45 P.M. The maximum width was maintained for a few minutes 
only, after which they began to close. The closing was very gradual, how- 
ever, as they were still slightly open at the last reading, which was at 
4:30 P.M. 
When one compares this curve with that in figure i for barley and that 
in figure 4 for wheat, all representing optimum conditions for their respec- 
tives species, it is obvious that the stomata of oats, when they have reached 
their maximum expansion, take longer to close, as well as to open, than do 
those of the other two plants. Their guard cells react less promptly to 
light stimuli than the guard cells of the stomata of barley and wheat. Of 
all the grains examined, wheat stomata are the ones most sensitive to light, 
if one may judge by the speed of reaction of the guard cells. 
To determine the amount of soil moisture necessary for oats, two boxes 
of plants, each sixty-three days old and very similar in character, are used. 
At 9 A.M. both contained 8% soil moisture, and each was abundantly 
