84 THE PHYSIOLOGY OF STOMATA. 



By using paired leaves of Verbena, as above described, the observations 

 shown in table 45 were made. In each case the control measurement was 

 obtained from one leaf of the pair; the other, at the close of the indicated 

 period of wilting, from the other leaf. 



In the last case some stomata were open as widely as 6 micra, though they 

 were few. Extreme flaccidity was usually reached in 15 minutes at the 

 latest, under the severe but normal conditions to which they were subjected. 



Ocotillo. August 6, 1906. Primary leaves. Closure of stomata during wilting. 



Different leaves taken in a series, the first to be regarded as control. 



jh 53 m p. rn., 3 to 9 micra. 



2 p. m., 3 to 9. Another piece of epidermis from the same leaf gave at 



2 h O4 m p. m., 3 to 9, wilting evident. 



2 h o7 m p. m., 3 to 7.5, but more stomata measuring 3 micra than before. 



2 h I2 m p. m ., o to 8, wilting marked, leaf curling. 



2 h i8 m p. m., o to 4.5. 



2 h 23 m p. m., o to 2. 



It is to be noted that the opening of the narrowest stomata is not increased, 

 but is steadily decreased concomitantly with the widest open. 



Watermelon. Unirrigated plant. 8 a. m ., October 6, 1906. 



The stomata, which are small (measuring 18 by 6 and 4.5 by 1.5 micra), 

 measured at this hour from o to 1.5 micra. A leaf was wilted for 50 minutes. 

 Measurements were made at i -minute intervals for 6 minutes, followed by 

 2-minute intervals, then by 3-minute intervals, and a final measurement 

 at 50 minutes. The greatest measurements were steadily decreased by 0.5 

 micron per minute for 3 minutes, after which it remained at less than 0.5 

 micron for the 16 minutes. All stomata were closed at the end of 50 minutes. 

 There was, moreover, no increase in the number of open stomata, as would 

 be expected should closed ones open, which might occur though the open ones 

 failed to increase their size. At 13 minutes a single stoma measuring 2 micra 

 was found. This was one in hundreds examined. 



This experiment has the advantage of having been made in the morning, 

 when presumably the stomata were in condition for opening both physically 

 and physiologically. No temporary opening, however, occurred, as the 

 closure was constant and gradual. 



The conclusion to be drawn is that no matter whether stomata are open 

 or closed, neither in the morning nor in the afternoon does a temporary opening 

 occur during wilting. 



In any particular plant closure does not commence in all stomata at the 

 same time. Local conditions may account for this. Thus, in Verbena no 

 change was noted in one case for i minute, in another for 3 minutes. 



