140 
JOHN GRAY AND GEORGE J. PEIRCE 
watered. The soil was slightly caked and the plants showed some wilting. 
Figure 8 represents box A which, after watering, was placed out of doors in 
very bright sunlight. The humidity fell rapidly and remained low for the 
day, varying only 4% from 27% at 1 1 A.M. to 4:10 P.M. The temperature 
reached 32° C. at 11 A.M. and gradually fell to 61 at 4:10 P.M. By i P.M. 
the wilted plants had become erect and the cells had assumed a more nearly 
normal turgescence. By 4:10 P.M. twilight had come on, and the stomata 
remained closed thereafter. 
Figure 9 represents the similar plants in box B, treated in like manner 
except that they were left in the greenhouse. Although the plants in this 
box did not exhibit such extreme wilting as those in box A, nevertheless the 
guard cells proved, on examination, to be completely collapsed. Light was 
abundant throughout the day. By 1 140 P.M. the guard cells had recovered 
the necessary degree of turgidity and began to open steadily until 4:10 P.M. 
After this hour the light was too dim for readings. 
These two cases, therefore, show that both conditions must be present 
together in order that the guard cells may open. The cells must have 
reached their full degree of turgidity before light will affect their action; 
and the more turgid the cells are at the time of exposure, the more responsive 
they will be to the different degrees of light which reach them. Comparing 
the soil moistures of boxes A and B with that of figure 7, it is found that the 
minimum moisture requirements of these oats lay between 9.9%, a favor- 
able condition, and 8%, a wilting condition. 
Rye 
Rye seedlings of different ages exhibited differences in the stomatal 
reactions of the young and the old plants. In the three grain species already 
described there is no difference in the behavior of the stomata of the plants 
of different ages; all reacted in the same way under the same stimuli. As 
already mentioned, rye thrives best, for economic purposes, where the soi 
and air are relatively drier than where the other grains thrive best. Thus 
our rye grew poorly in the humid air of the greenhouse, but finally matured 
into somewhat stunted plants. 
Figure 10 gives the stomatal curve for rye seedlings fourteen days old, 
on a very bright day, with soil moisture at 10% at 9:45 A.M., and with no 
more water added during the day. The temperature reached 36° C. at 
1:40 P.M., and the humidity remained high throughout the day. The 
stomata opened at 8:50 A.M. and remained but slightly open until 1:30 
P.M., when they closed for the day. Sunlight continued until after 4 P.M. 
This and other similar cases showed that the stomata of young rye seedlings 
open to a very slight degree only. 
Rye plants seventy-two days old and beginning to bloom are indicated 
by figure 11. At this time the soil moisture was 20.9% and the temperature 
reached 32° C. at 1:30 P.M. The humidity remained stationary at 31% 
