9 o 



THE PHYSIOLOGY OF STOMATA. 



by choosing mature but not moribund leaves. The plant itself was in vigor- 

 ous condition, and by its strong growth indicated a plentiful supply of soil- 

 water, and this makes any explanation involving an increase of soil-water 

 improbable. On the other hand, that such a difference in the relative humid- 

 ity as above described should affect the stomata of a plant well supplied with 

 water seems, in the light of other experiments, even more improbable. The 

 following data will be seen to bear out this view : 



On July 13, 1904, there was a hard shower lasting an hour, during which 

 1.1 inches of rain fell. In spite of this, however, the stomata showed no 

 change in dimensions, measuring o to 3 micra before and after the shower. 

 The plant had previously been irrigated. 



A few days before this (July 8) an attached branch was allowed to remain 

 covered by a bell-jar for one hour and a quarter, during which the relative 

 humidity rose from 42 per cent to 87 per cent. At the close of the period, 

 at 4 h 24 m p. m., the stomata measured exactly as those of a control branch. 



On July 11, 1904, at 9 h 3o m p. m., the stomata of the same branch meas- 

 ured o to 2 micra. The branch was left covered by the bell-jar for 12 hours, 

 when at 9 h 3o m a. m. the stomata measured 2 to 6 micra, as also did, however, 

 the stomata of the control. It is to be concluded, therefore, that the opening 

 was normal and unaffected by the continuous high relative humidity. 



The same behavior has been shown by Verbena ciliata. An exposure of 2 

 hours to a high humidity under a bell-jar on July 3, 1905, at mid-day (exper- 

 iment 140) failed to produce any changes in the stomata, though at the close 

 of the period the rate of transpiration was apparently greater. This result, 

 it is true, may be incorrect, since there may have been moisture deposited on 

 the leaves or absorbed by the glandular hairs, which, upon exposure to the 

 day air beneath a fresh bell-jar, was sufficient to affect the hygrometer. 



The same behavior of the stomata was shown also by all the Verbena 

 experiments in the 1906 series (200 to 300), in which bell- jars were used. In 

 no case was I able to detect obvious differences between the experimental 

 part, detached from the plant, and the control part. 



Similarly the method used in my experiments with Ampelopsis to deter- 

 mine the presence or absence of an induced rhythm in stomatal movement 

 made it necessary to confine leaves in a small chamber, which, while light- 

 tight, produced a high relative humidity. After 12 to 14 hours under these 

 conditions, in the total absence of light, no tendency on the part of the stomata 

 to open was observed, while exposure to darkening was followed by closure 

 in spite of increasing humidity. One instance only (July 19, 1905) shows 

 a behavior which, if not attributable to personal error, might have been due 

 to the increased humidity. 



It is evident, though, that the further opening of stomata under increased 

 humidity is of less importance, for the teleological point of view, than closure 



