8 BEHAVIOR OF STOMATA. 



the impossibility of doing this at night or in poor light reduces the 

 applicability of this method very considerably. The porometer 

 method, devised by Darwin and Pertz (1911) and modified and 

 improved by Laidlaw and Knight (1916), was not considered 

 favorably for several reasons, chief among which were the unnatural 

 conditions introduced by pulling a current of air through the inter- 

 cellular spaces of the leaf, and a doubt as to what porometer readings 

 actually represented. To the best of the writer's knowledge, no 

 comparison has been made between a series of direct observations 

 upon the condition of the stomata and porometer readings made 

 at the same time. Until the method is checked in this manner, 

 it is felt that its reliability is questionable. 



For the reasons given, it was felt that the most desirable method 

 to employ was the first one used by Lloyd (1908). This consists 

 in stripping the epiderm from the leaf and quickly plunging it into 

 absolute alcohol, which immediately fixes the epidermal cells, 

 keeping them in the shape they were in when immersed. The 

 guard-cells can not lose an appreciable amount of water in the 

 fraction of a second between stripping and immersion in alcohol. 

 The effect of the alcohol is to dehydrate the cell-wall before pen- 

 etrating the cell proper to any extent. The dehydration of the 

 cellulose wall causes it to become very stiff and hard. As a con- 

 sequence the cells retain their original form in spite of the fact that 

 the alcohol next removes the water from within the cell. This 

 form is maintained for some time, provided the walls are kept 

 dehydrated. Theoretically, therefore, the method seemed sound, but 

 it was evident that it could not be considered reliable until checked 

 by direct observation of the stomata of living leaves, not only for 

 each species and variety of plant studied, but for a wide range of 

 stomatal apertures for each as well. Not only was this done, but 

 in addition several parallel series were made with the more important 

 plants, one series being the usual set of epidermal strips collected 

 each hour, the other a set of measurements of stomatal apertures 

 on the living leaves of the same plant, also made each hour. 



The check upon the method was ordinarily made in the following 

 fashion: A stand for the microscope was erected in such a manner 

 that a leaf could be brought into the field with the least possible 

 change of position and consequent disturbance. This leaf was then 

 lightly clamped between a long cover-glass and slide, and about 10 

 stomata quickly measured. Not more than 2 minutes was allowed 

 for this operation, in order to prevent any serious change while the 

 leaf was exposed to these unnatural conditions. Then the leaf was 

 released and stripped in the usual manner, great care being exercised 

 to secure the area just examined. After the customary two days of 



