io6 



THE PHYSIOLOGY OF STOMATA. 



course of events. In these figures the extreme measurements are plotted 

 in thin lines and their means in heavy lines. These latter are again plotted, 

 together with a grand mean, d, in fig. 37. A portion of the curve of grand 

 means is theoretical, this being represented by a broken line. 



The large proportion of stomata which failed to open is to be explained 

 by their immature condition. The leaves were young, and were forced in 

 the glass-house. Clearly, the maximum measurements are the more nearly 

 normal for fully active stomata. 



The stomata in ocotillo commence to open before dawn and continue 

 to open steadily till the maximum is reached at io h 30 m a. m. The stomata 

 of the lower surface of the secondary leaves appear to be somewhat less uni- 

 form in their behavior. Part of this appearance of the curve may very well 



6 9 12m. 3 p.m. 6 9 



Fig. 38. — Daily periodicity of stomatal movement in Fouquieria splendens, December 17-18, 

 1906, from data derived from rather young leaves. Extreme measurements in thin lines; 

 means in a heavy line. 



be due to error in measurement. The position of the leaves, however, tends 

 to render the illumination of their lower surfaces much more irregular than 

 the upper, and this may be the explanation. The amplitude of movement 

 of the stomata of the primary leaves is one-fourth greater than that of the sto- 

 mata of the upper surface of the secondary leaves. Francis Darwin (1898, 

 p. 589) was unable to arrive at any more definite conclusion as regards the 

 stomata of upper and lower surfaces of the leaf of Narcissus. 



After the maximum opening is attained, the stomata begin to close, at 

 first rather slowly and later (after 3 p. m.) more rapidly. The rise in the 

 curves from 9 p. m. on is due, I believe, to the lack of observations between 

 that hour and 3 a. m. The differences in the size of the openings on clear and 

 cloudy days are small. 



