212 PICTURESQUE EXPERIMENTS 



they are the gate-keepers who regulate both export 

 and import. They are known by actual inspection 

 (with a microscope) to close at night: the result 

 of this is that the evaporation of the leaves is much 

 slower at night, and this is true when allowance has 

 been made for the fact that evaporation is also 

 checked at night by the dampness of the air. 



Fig. 7. 



The microscopical inspection of stomata is not a 

 completely satisfactory method of discovering to 

 what degree they are open. It has, however, been 

 my good fortune to resuscitate and simplify a 

 method of studying the stomatal condition. The 

 method was many years ago tried in a hopelessly 

 cumbersome form by a German, but never came 

 into use. My apparatus is described in the Pro- 

 ceedings of the Royal Society,^ and is known as the 



' A new method of estimating the aperture of stomata. B., 

 Vol. 84, 191 1, 



