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138 john gray and george j. peirce 
Wheat 
Similar measurements were made of wheat stomata, and the data follow. 
Thus figure 4 shows stomatal and other measurements taken in a box of 
wheat seedlings twenty-three days old. The conditions were nearly the 
same as those described in connection with figure i, and there is much 
similarity in the diagrams. The light was bright from the beginning of the 
day, and was intense between 12:30 and 2:30 P.M. The maximum tem- 
perature was 38° C. at 1:15 P.M., the maximum humidity 57%. The 
humidity fell to 46% at 2:15 P.M., but rose slightly from then on with the 
decrease in warmth. The soil moisture was 20.4% at 8:25 A.M. More 
water was added and the soil kept very wet all day. With this abundance 
of moisture and light the stomata opened, but they did not respond as 
rapidly as the barley, as shown by the slower start. The period of greatest 
expansion of the stomata was shorter than in barley, and the period of 
closing was also shorter. The temperature and the humidity both remained 
high as late as 4:15 P.M., when the stomata almost closed. 
The degree of turgidity of the cells of a plant depends, other things being 
equal, upon the percentage of moisture in the soil in which they grow. 
Without considerable turgidity the guard cells shrink or collapse. In this 
condition light does not so stimulate them that they open. This fact is 
indicated by wheat seedlings, the minimum water requirements of which, 
under the conditions of our experiments, were found to lie between 16.7% 
and 17.8% of soil moisture. 
Another set of wheat seedlings thirty-seven days old, grown in 17.8% 
of soil moisture, illustrate the intimate relation between light and the open- 
ing of the stomata. The morning of observation was dark and cloudy until 
10:30 A.M.; then there followed an interval of weak light to about 1:30 
P.M., a cloudy period at 2 P.M., a clearing of the clouds at 3:00 P.M., and 
the dusk of evening coming on at about 4:45 P.M. The stomata opened 
slightly during each light period and closed with each recurring darkness, 
and thus opened twice during the single day. The temperature and the 
humidity were high throughout the period of the readings. 
In figure 5 no such sensitive response to light is recorded. Here are 
shown the records of wheat seedhngs fifteen days old, with soil moisture at 
16.7%. The plants were erect and showed no outward signs of wilting, 
but the guard cells appeared to be somewhat collapsed. The temperature 
rose to 34° C. at 2:05 P.M., and the humidity rose slightly in the forenoon 
and fell in the afternoon to 42% at 3:15 P.M. The sun shone brightly at 
8:30 A.M. and the sky was cloudless throughout the day. External stimuli 
were such in this instance as to indicate a wide opening of the stomata; but, 
because of the shortage of soil water, they remained tightly closed through- 
out the day, protecting the leaf from evaporation and thus conserving such 
moisture as it contained. In the preceding case, as in this one, there is a 
striking similarity between the humidity curve and the stomatal curve. 
