1020 
Journal of Agricultural Research 
Vol. V, No. 22 
This, then, would indicate that the stomata on old leaves exhibit very 
little movement; that those on heart leaves open, but not so widely as 
on mature leaves, and close earlier; that on cotyledons and mature 
leaves they open widely, indicating their great activity. Therefore, in 
the study of the environmental factors influencing stomatal movement 
only mature leaves have been considered, since they were always avail¬ 
able and responded readily to changes in environment. They also rep¬ 
resent that portion of leaf growth which is most susceptible to infection 
by Cercospora beticola. If it is true, as claimed by IIjin (4) and others, 
that variation in the osmotic pressure of the guard cells regulates stomatal 
movement, then it might be concluded that the leaves which exhibit the 
greatest stomatal movement are also the most active metabolically and 
are consequently the most important to plant development. 
ENVIRONMENTAL FACTORS 
It is generally agreed by various investigators that the chief external 
factors influencing stomatal movement are light and temperature, while 
a difference of opinion exists as to the influence of relative humidity. 
Some believe that humidity greatly affects the degree of stomatal open¬ 
ing, while others consider it of only minor importance. Wilson and 
Greenman (12) found that the stomata on plants of Melilotus alba which 
were left covered with a glass case, thus being in a nearly saturated 
atmosphere, were well open, while on those which were left standing in 
the drier open air the stomata were nearly all closed. Darwin (2) gave 
evidence to prove that stomata were very sensitive to changes in the 
humidity, closing on being taken from a high to a low humidity and 
•opening under the reverse conditions when all the plants were exposed 
to* approximately the same light. According to Lloyd (6) “ there is a 
small amount of evidence that a high relative humidity favors, as a con¬ 
dition, the wider opening of the stomata in the ocotillo” and in regard 
to Mentha piperita , also a desert plant, he concludes “. . . in these 
plants, that as long as wilting does not take place a low relative humidity 
does not reduce the stomatal opening.” 
As shown by the present study, the writers believe that, while light 
may be considered a fundamental factor in stomatal movement, yet 
stomatal closure is effected by low relative humidity, even though light 
is active. The relative humidity present at any time, together with an 
optimum temperature, has been found to be a good criterion of the 
amount of stomatal movement that may be possible under the existing 
conditions. 
LIGHT 
In this study no attempt has been made to determine the exact rela¬ 
tion of light to stomatal movement. Only a few scattered readings have 
been made to determine what effect direct sunlight has on stomatal 
