1 8 THE PHYSIOLOGY OF STOMATA. 



METHODS.* 



In order to answer the questions which I set before myself, it was necessary 

 to study separately the phenomena of transpiration and of stomatal action. 

 The methods employed will now be described. 



TRANSPIRATION. 



Inasmuch as it was impossible to use potted plants or to dig up and start 

 complete individuals, the method of weighing, so insistently maintained by 

 some as the only strictly reliable method of measuring the transpiration, was 

 not feasible. The use of such indicators as cobalt chlorid, as used by Stahl 

 and others, while useful for certain kinds of comparative study, was not 

 adapted to the conditions surrounding the experiments, the small leaf area 

 of the ocotillo, the extremely low relative humidity, and the high tempera- 

 tures all contributing to make it very difficult or impossible to arrange a small 

 closed chamber suitable to the method, which is, moreover, not at all adapted, 

 at least as at present carried out, to the demands of quantitative work. For 

 similar reasons, and others which will be discussed later, the type of hygro- 

 scope used by Francis Darwin was not available. Two other methods were 

 tried. One is that inaugurated by MacDougal and elaborated by Cannon 

 (1905 6), in which a Lamprecht polymeter is used to indicate the rise of relative 

 humidity within a closed chamber in which is confined the whole or an attached 

 portion of the plant to be studied. Knowing the capacity of the chamber 

 and the temperature and having taken readings of the hair hygrometer at 

 constant intervals, it is then possible, theoretically at least, to calculate the 

 amount of water- vapor which has escaped into the chamber from the surface 

 of the plant or part inclosed. While the method offers the advantage of 

 allowing transpiration to be studied while the plant remains undamaged and 

 growing in the normal surroundings, the objection may still at this time be 

 raised that the hair hygrometer is an inadequate instrument, as at present 

 constructed. It may, however, be quite possible to construct an instrument 

 of great delicacy and accuracy, and this will probably be accomplished in the 

 future. For the purpose of my work this method proved to be of only limited 

 value, and, while a few of the results have been used, they are considered to 

 be of less value than those obtained in the way now to be described. 



The second means at hand was the potometer, an instrument which has 

 been repeatedly criticized on well-known grounds. f For some time the Mac- 

 Dougal potometer, consisting of a horizontal tube of small caliber graduated 

 to 100 mg. was used. A considerable number of experiments were made 

 with this type of potometer, but the form of the whole instrument was at 

 length found to be too cumbersome and unwieldy to lend itself to my purposes. 

 I could not move the instrument about without difficulty, and it was very easy 



*The main points were presented before the Botanical Society of America at its meeting 

 in December, 1904. 



fBurgerstein (1904) and Clements (1905). 



