INTRODUCTION. 11 



than 0.04 micron in the width of an alfalfa stoma, while the smallest 

 division that can be accurately measured is 0.2 micron. 



The distortion which occurred as a result of the epidermal strip 

 rolling or curling was usually caused by keeping the strip exposed to 

 the air too long. As this shrinking was greater on the one side than 

 on the other, and greater in one direction than the other, it was 

 thought possible that the stomata lying in the direction of the axis 

 of rolling would tend to close, and those lying at right angles to this 

 direction would tend to open slightly. If this occurred it was too 

 slight to be detected. Nevertheless, such strips were not used, 

 except in rare cases, not so much because of doubt in regard to their 

 accuracy as because of difficulty in photographing them. The 

 curled strips apparently checked just as closely as the others with 

 measurements of the stomata in situ. 



The chief reason for confidence in the method lies in the agreement 

 of the stomatal openings observed in the undisturbed leaves with 

 those measured in the strips removed at the same time. Any serious 

 objection to the method must come through showing that there is 

 no such agreement between results from properly performed stripping 

 and those obtained by carefully conducted observations of stomata 

 in situ. Except for the impossibility of observing them during the 

 hours of darkness and poor light without the use of strong artificial 

 light, this latter method might have been used throughout. How- 

 ever, other objections were decisive, in view of the objects sought. 

 These were the impossibility of photographing the stomata in such 

 leaves, and the difficulty of finding enough leaves on a single plant 

 in such position that they could be slipped under the microscope 

 without undue disturbance. 



It must be emphasized that an examination of a leaf in this man- 

 ner does set up a disturbance which sooner or later appears in the 

 stomata, and hence such a leaf can be used but once in the series. 

 This was tested several times by comparing such a leaf with some 

 undisturbed leaf half an hour or an hour later. The usual effect was 

 to produce closure of the stomata. Leaving a leaf under the micro- 

 scope would in extreme cases cause closure in 3 minutes in some sto- 

 mata, but as a rule 10 minutes or even more elapsed before a change 

 occurred. As the experiments demonstrated that the stripping 

 method was as reliable as direct observations, it was used because of 

 its greater convenience, and direct examination merely for checking 

 and comparison. 



The accuracy of the method depends greatly upon the process of 

 stripping. To gain speed, the scalpel used in making the cut to 

 start the strip, and the forceps used in seizing the epiderm and in 

 stripping, were bound together with rubber bands, the end of the 



