INFLUENCE OF LIGHT UPON ACTION OF STOMATA I4I 
from 11:50 A.M. to 2:30 P.M. The light was bright from 8:30 A.M. till 
I .-30 P.M. From that hour it waned until twilight, which came on shortly 
after 4 P.M. The stomata of these older plants more noticeably responded 
to the light than did those of the younger plants described immediately 
above. They opened at 9:30 A.M. and remained open until 1:30 P.M.; 
but though their width was uniform throughout this time, they were not 
as widely open as the stomata of the other grain plants. Nevertheless, in 
the case of rye also, the width of the stomatal opening corresponded to the 
intensity of the sunlight. The fact that the stomata never opened to their 
full width may be an important and significant adaptation, or reaction, to 
their environment. If, for example, the stomata opened wide where the 
air is dry, transpiration might be so excessive as to dry out the plant and 
destroy it. One of us has shown that when the guard cells are killed or 
paralyzed by sulphur dioxide fumes, the rate of transpiration goes up, other 
things being equal, and the plant, the organ, or the part may dry out and 
die, because the plant has lost control, locally or generally, of its water. 
The minimum water requirements of rye were not especially studied; 
but our observations indicate that they are below 10%. Rye plants ap- 
peared to develop quite favorably on soil containing 10% or less of water, 
whereas if the amount reached 20% or more the plants did poorly. 
To provide a check upon the previous work we conducted the following 
experiments upon these four species of grain grown in pots. One pot of 
each species was placed in bright sunlight and one pot of each was put into 
the dark. The temperature and the humidity were kept the same for both 
sets by means of an electric fan. In all cases the stomata were slightly 
open when the experiment began. The results were so similar in all four 
species that only those obtained from the wheat plants will be reported here. 
Wheat is selected for this because we were able to reverse the positions of 
the pots, in the case of the wheat, on the following day, thus giving us a 
check on the check furnished by the first set of experiments. Figure 12 
gives the two very dissimilar curves constructed from the records of the 
behavior of the stomata of wheat in light and in darkness. Temperature 
and humidity were both high, and there was an abundance of moisture in 
the soil. At 8:30 A.M. both sets of plants were examined. The stomata 
were open. Each set was watered and one, A, placed in the light, the 
other, B, in darkness. The rise and fall of the line A in figure 12 corresponds 
with and indicates the increase and decrease of sunlight during the day. 
B shows that the stomata closed at 10 A.M. and so remained as long as they 
were in darkness. Figure 13 records the behavior of the same two pots of 
plants,, B put into the light, and A into the dark. Conditions of light and 
of soil moisture were very similar to those of the day preceding, but the 
plants were subjected to the opposite effects of light and darkness. Hence 
the stomata of the plant placed in the dark closed promptly and remained 
closed, whereas those of the plant which was brightly lighted continued to 
