INTRODUCTION. 17 



"rather dark," "too dark to see clearly," are merely descriptive and 

 can not serve as measures of light. 



In these studies, field plants grown under natural conditions were 

 used exclusively. All possible factors were accurately measured 

 and the correlations were drawn with due regard to the limits placed 

 by lack of data on other reactions by the plant. Too much empha- 

 sis can not be placed on the importance of having the plant under as 

 normal conditions as possible. Of course, a plant on which one leaf 

 after another is removed is not entirely under normal conditions, but 

 an alfalfa plant with approximately 1,000 leaflets is not very much 

 affected by the removal of 50 during a 24-hour series. This is shown 

 by a series started at 8 p. m. on June 26, 1916. A large plant of al- 

 falfa was stripped during the night as well as the next day at 6 a. m., 

 June 27 ; another set was added to the series by stripping a plant close 

 by, and hence under as nearly the same conditions as possible to the 

 first plant. At noon on June 27 a third plant was added to the series. 

 If the loss of the leaves made any appreciable difference in the sto- 

 matal movement there should be no agreement in the curves of the 

 three sets; if these curves coincide, the effect of continued stripping is 

 not appreciable. As the graphs show, no difference can be detected. 

 When a large proportion of the leaves of a plant are removed, there 

 can be no doubt that stomatal movement in the remaining leaves will 

 not be normal, but this was avoided by removing comparatively few 

 leaves from each plant. 



In the following descriptions and graphs, stomatal apertures are 

 expressed in percentages of the maximum. This checks out the effect 

 of variations in size of stomata and readily allows comparison of 

 stomatal movement in the various plants investigated. It also 

 allows the relation of light and humidity to be more clearly shown 

 than is possible in any other manner. As it was not possible to meas- 

 ure fractions of 0.1 micron, the degree of opening is given to the 

 nearest twentieth of the maximum. In the earlier experiments the 

 diffusion capacity n Va6 was also calculated, but the curve produced 

 was essentially similar and had no advantage not found in the method 

 used. 



Acknowledgment is due Dr. P. J. O'Gara, of the American Smelt- 

 ing and Refining Company, for aid and assistance throughout this 

 investigation. As the 1916 work was carried on for the American 

 Smelting and Refining Company, the use of all material and photo- 

 graphs from that year is by their permission. At various periods 

 since that time, the company, through Dr. O'Gara, has extended the 

 use of its laboratory and given material assistance in the continuance 

 of the work. Acknowledgment is also due Dr. Clements for making 

 possible the further prosecution of the work, especially in connection 

 with the effect of stomatal movement upon transpiration. 



