Feb. 28,1916 
Relation of Stomatal Movement to Infection 
1025 
In another test made on the following day, the humidity ranged from 
9 to 40 units higher inside the humidity box than in the greenhouse 
(fig. 3), and throughout the day the stomata were open wider in the 
former place than in the latter. At 5 a. m. all the stomata were closed 
except those on the lower leaf surface in the humidity box, which were 
slightly open. In general, the initial opening probably occurred soon 
after 5 a. m., for at 7.15 a. m. the stomata were all open, those in the 
humidity box being open wider than those outside. This point opposes 
the theory that the stomata in the humidity box remain well open during 
midday on account of the less intense light due to the additional window- 
Fig. 3.—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 17, 1913 (Table VII). 
glass covering, while during the same period, those outside the humidity 
box close as a reaction to the more intense unobstructed light. If this 
were true, then, the stomata in the humidity box would open later in the 
day than those outside, because the light in the former place would be 
weaker. As a matter of fact, the stomata in the humidity box opened 
earlier and had greater pore width than those outside, even when thus 
exposed to the weaker light. The conclusion that may be drawn from 
this is that the relative humidity is the indicative factor of the causes 
which produce this difference. It should be noted that in the humidity 
box the humidity did not fall below 60 during the day, and the stomata 
were still open at 4.20 p. m., when the last reading for the day was made. 
