CHANGES IN CONTENTS OF STOMATA. 



95 



series of photomicrographs herewith presented shows an average degree of 

 uniformity. 



The opening of the stomata begins at dawn and proceeds slowly till 10 a. m. 

 in the early spring and till about 8 h 30 m a. m. in the summer (plate 7, fig. 3). 

 The openings measure, in average-sized stomata at this time, 6 to 12 micra, 

 although in many cases the opening is still larger, reaching the extreme size 

 of 16 micra. Generally, in an average piece of the epidermis, the transverse 

 measurements will vary between 6 and 10 micra, but frequently the variation 



3a.m. 



I2p.m 



Fig. 32. — Daily periodicity of stomatal movements in Verbena ciliata. 

 Extreme measurements are plotted by thin lines, and means in a 

 heavy line. July 2, 1906. Broken lines are curves for parts of 

 other days (July 1 and 3). 



will be much less. To the eye often a remarkable degree of uniformity of 

 size obtains, as indeed the illustrations show (see plate 7, fig. 2). It may 

 be remarked that the illustrations are from pieces taken at random, the only 

 consideration having been, on account of the curvatures in the mounted 

 epidermis, to get as many stomata in one optical plane as possible. 



The maximum opening is maintained for about 3 hours, after which closure 

 sets in (plate 7, figs. 4 and 5). It seems that the initial closure is somewhat 

 rapid, though the total movement occupies about the same length of time 



