STOMATAL MOVEMENT. 



39 



In most instances the majority of the stomata were about the average size. 

 A test of the method of selection was made by taking a leaf at random and 

 measuring all the stomata included in it. The average was found to be 

 practically the same as the one obtained by the first method. In the opinion 

 of the observer the widths given represent very little error, but where the 

 openings were small the lengths could not always be determined accurately. 

 Every measurement was made to include equal numbers of stomata on the 

 two sides of the leaves. The necessity for the inclusion of the stomata of 

 both sides of the leaves in measurements of this kind has been pointed out 

 by Livingston and Estabrook.* 



Table 15 contains the results reduced to microns, including the length (I) 

 of the openings between the guard cells, the width (w) of the opening, and 

 the square root of their products (v7te?).t 



A comparison of the curves shows that the method under discussion gives 

 approximately the same results as Lloyd's absolute alcohol method. In 

 one respect this method seems to have an advantage over the absolute 

 alcohol method, namely, that the guard cells are not subjected to the danger 

 of loss of water from evaporation during the interval of transfer from the 

 leaf to the alcohol, a factor which might be serious in the dry atmosphere of 

 Arizona. 



TABLE 15. A comparison of stomatal measurements taken by the absolute alcohol method 

 and the picric acid method, Cissus and Parkinsonia, November 17, 1912. 



EXPERIMENTATION . 



EXPERIMENT XIV. 



During the progress of experiment V (see page 22) leaf samples for sto- 

 matal measurements were taken as near the middle of each transpiration 

 period as the manipulation of the experiment would permit and treated with 

 picric acid in cedar oil, as described above. The values of T/E are repeated 

 from page 23 and are plotted in fig. 20 with the values for the square roots 

 of the products of the stomatal diameters. 



""Livingston, B. E., and Estabrook, A. H., Observations on the degree of stomatal 

 movement in certain plants, Bull. Torr. Club, xxxix, 15-22, 1912. 



fBrowne and Escomb, Static diffusion of gases, Phil. Trans. Rov. Soc. Lond., B, 

 vol. 192, pp. 270-292, 1900. 



