136 PLANT WORLD. 



register the rate at which water is leaving the leaves as 

 vapor. The proof of the matter can be had by simply weigh- 

 ing the whole system at different times, when the difference 

 in weight should equal the difference in the amount of water 

 which has disappeared from the tube. In using the two 

 methods in conjunction, one takes a piece of the plant to 

 be studied with a sufficient amount of foliage so that the 

 removal of a few leaves will introduce a negligible error, 

 arranges it in a "vertical tube potometer," and, by changing 

 arrange is in a "vertical tube potometer," and, by changing 

 the surrounding conditions, as from darkness to full sunlight, 

 one causes changes in the rates of transpiration, and, by 

 removing leaves and preserving the stomata, one is able to 

 obtain the data sought. We may now pass on to examine 

 the nature of the evidence thus discovered. 



By arranging a series of experiments to be allowed to 

 run for a period of something over twenty-four hours, the 

 course of transpiration for such a period may be obtained; 

 and by taking specimens of the stomata at given hours we 

 may trace the course of normal stomatal movement for the 

 day also. The degree of correlation between the two will at 

 once be displayed upon the examination of the data, or better 

 still by plotting these in the form of curves. Data thus 

 obtained show that, after the maximum degree of stomatal 

 opening has been obtained, the rate of transpiration con- 

 tinues to become more rapid. During the period in which 

 the maximum opening of the stomata is retained, the rate may 

 again fall, and during the latter part of the day, when the 

 stomata are closing, the movement may continue steadily, 

 while positive fluctuations in the transpiration rate occur. The 

 conclusion must therefore be reached that the movements of 

 the stomata between the extremes appear to bear little 

 relation to the fluctuations which may occur in the rate of 

 transpiration. Such changes are, however, not xery marked 

 during a short period of time under normal circumstances. 

 By placing the experiment under conditions of marked and 



