STOMATAL RESPONSE TO SUDDEN CHANGE. 6 1 



leaf is stopped, the rate of transpiration in the other surface becomes great 

 enough to remove all the water-vapor which otherwise would have escaped 

 from both sides; and similar evidence was offered by Comes (1878), who 

 found that the total amount of water lost through both sides separately was 

 greater than through both sides at once. It is, however, in the light of 

 Brown & Escombe's work, not at all necessary to assume a change in the sto- 

 mata to account for Unger's and Comes's results, since the diffusion capacity 

 of the stomata was probably still greater than was necessary to allow the 

 observed rates. The increased vapor tension within the leaf following on 

 the blocking of stomata on one side would account for a higher rate on that 

 side (Comes) than would be found if the stomata on both sides were free. 

 Yet in these instances, as in all the others, while conclusive evidence that 

 considerable changes in the rate of transpiration is given, evidence of equal 

 value that the stomata undergo, or do not undergo, correlated changes is 

 lacking. 



By means of the foregoing data, a summary of which follows, this-lack is to 

 some extent supplied. The light responses are chiefly depended upon, since 

 light affects both transpiration and the activity of stomata. 



SUMMARY. 



The rate of transpiration may increase after the maximum size of the sto- 

 matal pores is reached, and continue to fall after the stomatal closure (short 

 of entire closure) is complete. This occurs normally during the daily course 

 of events. That the stomata commence to close in the early afternoon 

 might be regarded as the cause of the fall in transpiration, were it not for the 

 fact that an increase in the rate of transpiration may occur (as it did, e. g., 

 under the influence of chloroform) while a static stomatal condition prevails. 

 This conclusion accords with the calculation of Brown & Escombe, which 

 showed that the maximum diffusion of water-vapor from the leaf of the sun- 

 flower (Helianthus) was only one-sixth of the possible maximum in view of the 

 number and size of the stomata. ^This clearly indicates that changes in the 

 rate of transpiration within certain wide limits is determined, when the sto- 

 mata are not shut, not by these organs, but by some other means or condition. 



Wide fluctuations in the rate of transpiration may occur without change 

 in the size of the stomatal pores when the period of stimulation is brief. 

 These fluctuations may be, as they usually are, accompanied by incommen- 

 surate changes in the pores, when the period of stimulation is sufficiently 

 prolonged. 



The slow opening of the stomata as compared with the rapid rise in the 

 transpiration rate (as seen, e.g., in experiments 63-66) when the plant is placed 

 in the light might be interpreted as a check on the transpiration, if, at any 

 moment during opening, the greatest possible rate of transpiration occurred. 



