144 
JOHN GRAY AND GEORGE J. PEIRCE 
throughout the period of observation, the maximum being 27° C. at i :io 
P.M.; the humidity was lowest at the same hour, namely 49%. In the 
figure line (a) represents the behavior of stomata of cultivated oats, mature 
plants seventy days old, in soil containing 24.5% of moisture. Line (b) 
indicates the behavior of the stomata of a large plant of Avena fatua, then 
in bloom, which had been in the greenhouse for thirty-five days, in soil 
containing 15-20% of moisture at the time of observation. No water was 
added to either box during the day. The sky was clear and the light bright 
from 8:00 A.M., reaching its maximum brightness between 11:00 A.M. 
and 1:45 P.M. At the latter hour the sky clouded, but by three o'clock 
it was Hghter again. The sun set behind our mountains at four o'clock, 
and the plants were in shadow from that time on. The two curves show 
that the stomata remained open as long as there was bright light; that, on 
the dimming of the light soon after one o'clock, the stomata of the cultivated 
species closed and did not open again durmg the rest of the day, although 
the light later on became somewhat brighter again. On the other hand, 
the stomata of the wild oat closed very gradually, and they were completely 
closed only at sun-down. The stomata of the wild oat reacted to the dim- 
ming light, but only much more slowly than those of the cultivated species. 
Wild oats growing in hard, dry soil were examined on various days. 
The plants were usually erect and exhibited no outward microscopic signs 
of wilting, but the stomata were shrunken and showed no movement. 
These wild plants survive in such dry surface soil, and on hot days on which 
the cultivated species wilt or burn, even though abundantly supplied with 
water in the soil. In the behavior of the stomata of these two species of 
oats we see one reason for the differences in their occurrence and in their 
requirements. Thus, the wild species can and does maintain itself in 
changing conditions in which the temperatures, humidity, soil moisture, 
and illumination may cover a very wide range; whereas the cultivated 
species requires a fairly high proportion of soil moisture, and it can with- 
stand only moderate dryness of the air and moderate heat. 
The Significance of Stomatal Movements 
The foregoing experiments show that, in the species studied, there are 
definite times during which the stomata are open and other times during 
which they are closed. While the stomata are open carbon dioxide enters 
the leaves of these plants more rapidly than while they are closed, other 
things being equal. Furthermore, carbon dioxide will enter a green leaf 
not only in accordance with the openness of its stomata, but also in accord- 
ance with the rate at which it is being used in the leaf. This use constitutes 
the photosynthetic process resulting in the production of sugar, starch, etc., 
a process which goes on at rates proportioned, among other factors, to the 
intensity of the light. We see, therefore, that, so far as these five species of 
annual plants are concerned, a factor which regulates the rate of food 
