STOMATA AND RELATIVE HUMIDITY. 91 



due to a reduced relative humidity would be, and it is quite conceivable that 

 the stomata would be more sensitive to a reduced than to an increased 

 humidity. 



On this part of the subject Francis Darwin has offered results (1898, p. 557) : 



The fact that many plants close their stomata when brought from a greenhouse to a 

 laboratory can not be doubted by anyone who investigates the subject; nevertheless it 

 does not seem to be widely recognized that this is the case, and that it occurs in cases where 

 there is not the slightest visible flaccidity. 



This observation is supported by suitable evidence, and inasmuch as the 

 drier air would tend to increase the transpiration, the behavior of the hygro- 

 scope would seem here to indicate stomatal behavior, approximately at least. 



With such evidence as I have available I am able only to say that in the 

 plants which are here under consideration such is not the case. This may be 

 wholly due to the fact that these are desert plants living often under a very 

 low relative humidity for long periods, and are therefore "attuned" to the 

 rigorous conditions to which they are subjected. But even at times of high 

 relative humidity no obvious effect of this condition has been observed. 

 Two other experiments may be added, with, however, negative results. 



Verbena ciliata. December, 1906: After 1.25 hours in a nearly saturated 

 atmosphere, showed no decrease in stomatal size when exposed to a relative 

 humidity of about 50 per cent. 



December 29, 1906: After 48 hours' darkness in a saturated atmosphere, 

 on exposure to light, and to a relative humidity of 50 per cent, the stomata 

 were as in the normal. 



Mentha piperita. December 5, 1906: Two plants in pots were removed 

 from outdoors, where the relative humidity was very high, after drenching 

 rains on the 2d and 3d (3.25 inches), into a sunny window in a warm, dry 

 room at noon. One was covered with a glass bell- jar arranged to keep the 

 humidity high, while the other was left uncovered to the dry air of the room. 

 At the end of 2 hours the stomata of the latter were, if anything, more open 

 than those exposed to high humidity. 



The conclusion is, in these plants, that, as long as wilting does not take 

 place, a low relative humidity does not reduce the stomatal opening. 



