138 THE PHYSIOLOGY OF STOMATA. 



INDUCED RHYTHM IN STOMATAL MOVEMENT. 



The question of induced rhythm in stomatal movement has been attacked, 

 but with conflicting and, for the greater part, with negative results. 



WILTING. 



During the progress of wilting there ensues a gradual closure of the stomata 

 without any preliminary opening such as was believed to occur by Francis 

 Darwin. The beginning of closure occurs somewhat later than initial wilting 

 and seems to be a result of water loss by the leaf as a whole. There is no 

 adaptive closure, meaning by this a closure in anticipation of wilting. Closure 

 "before visible wilting" has no meaning, inasmuch as the progress of wilting 

 and concomitant stomatal behavior must be investigated in each particular 

 plant by quantitative methods. For this purpose a suitable method, as 

 applied to Verbena ciliata, has been devised. 



There can be little doubt that a high relative humidity favors the opening of 

 stomata indirectly, by the suppression of water loss, when the supply of water 

 is reduced toward the point of danger to the plant. Aside from this condi- 

 tion, however, there seems to be no relation between the degree of relative 

 humidity and the condition of the stomata. The plants of the desert about 

 Tucson are subjected to extremely low relative humidities, without any 

 apparent effect upon the stomata. Changing a plant from one extreme to 

 another is not followed by closure or opening, although the rate of transpi- 

 ration may be greatly affected, since in a saturated space the transpiration 

 rate may be reduced to a very low value and the stomata remain open ; while 

 at a low relative humidity, as, e. g.,oi 5 per cent, the stomata do not close, 

 provided there is a sufficient supply of water for the plant. There appears 

 to be no regulation of transpiration by the stomata correlated with changes of 

 relative humidity. 



These conclusions, taken together with those of Brown & Escombe with 

 reference to the relation between the diffusion capacity of stomata for carbon 

 dioxid, indicate strongly that these organs in the plants studied are not 

 adaptive in the active sense. They are means of communication between 

 the interior of the plant and the gaseous environment, but the amount of 

 water- vapor which escapes is to a very marked degree independent of the size 

 of the pores. If a regulation of water loss exists, it is more probably brought 

 about by some other means. Their ability to conserve the water content 

 of the plant is a function of their size and numbers, and in many plants of 

 such adjunctive anatomical characters as have been copiously described by 

 many observers (especially Porsch, 1905). None of these features, however, 

 are regulatory in their function, but are effective only through the dampening 

 effect of constant value upon transpiration. 



