THE BEHAVIOR OP STOMATA. 



INTRODUCTION. 



This investigation was undertaken to discover what changes 

 take place in the apertures of stomata throughout the day and 

 night, the influence of physical factors upon such changes, and the 

 final effect upon transpiration. Hence the investigation falls 

 naturally into three subdivisions which form the sections of this 

 volume. As previous investigators have usually worked with few 

 plants and in one region, it was felt that considerable light could 

 be thrown upon the subject by using many plants and working in 

 several regions of diverse climate. Experiments which were first 

 carried out at the American Smelting and Refining Company's 

 laboratory near Salt Lake City were therefore continued at the 

 Desert Laboratory at Tucson, Arizona, and at the University of 

 Minnesota, Minneapolis, Minnesota, as well as at Salt Lake City. 

 A wide range of climate was thus available, since the spring climate 

 of Minneapolis is rather humid, the summer climate of Salt Lake 

 City dry and sometimes hot, and the winter in Tucson dry and warm. 

 For a number of reasons, most of the experiments were made at the 

 Salt Lake laboratory, where the water-content could be controlled 

 readily and the range of humidity and temperature from day to day 

 was almost as great as in the desert. 



At the outset, careful consideration was given the methods hitherto 

 employed to determine changes in stomatal apertures. The horn 

 and yucca hygroscopes used by Darwin (1898) were properly 

 criticized by Lloyd (1908) as measures of transpiration rather than 

 of the condition of the stomata. Cobalt-chloride paper is open to 

 the same objection, and therefore only indirectly throws light 

 upon the opening of the stomata. Petroleum ether, when dropped 

 upon a leaf, passes through it if the stomata are open and some- 

 times gives it a water-soaked, translucent appearance. Hence this 

 reagent may be used as a qualitative test for stomatal opening. 

 These methods, however, are no longer in use, and this left but 

 three methods to be considered for the purpose of the investigation. 



The two methods used by Lloyd (1908, 1913) may be regarded as 

 reliable in a high degree. The first method consists in observing 

 the degree of opening in stomata in properly fixed strips of epiderm, 

 the second in observing the stomata in their natural state on the 

 leaves while these are still on the plant. Naturally, the latter 

 method is preferable, since it shows the stomata on leaves which 

 are undisturbed until they are placed under the microscope. But 



