1024 
Journal of Agricultural Research 
Vol. V, No. 22 
by 1.30 p. m. During this time the relative humidity fell from 43 to 16 
while inside the humidity box it ranged from 70 to 46 and the average 
width of the pores of the stomata increased from 9 to 12.6/1. The stomata 
on the upper leaf surface were also open wider and remained open longer 
than those on the lower, while all were closed by 7 p, m. These points 
Fig. 2.—Stomatal pore widths on mature leaves kept under different relative humidities in a humidity 
box (H. B.) and free in the greenhouse (G. H.) at Rocky Ford, Colo., on May i6, 1913 (Table VII). 
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seem to indicate that the relative humidity as supported by soil moisture, 
transpiration, etc., must remain, in general, above a certain percentage 
in order that the maximum influence of light may be realized. Other¬ 
wise, if the humidity is too low, the light factor becomes in some way 
less operative, and the stomata open to a less extent and close earlier. 
