42 THE PHYSIOLOGY OF STOMATA. 



very unlikely that the course of transpiration in the growing leaf is less liable 

 to the usual fluctuations observed in day and night. 



From data obtained from other experiments and observations hereafter 

 presented, the daily maximum rate of transpiration is shown to occur at or 

 between the hours of n a. m. and 3 p. m., while the maximum stomatal 

 opening is attained at 8 to io h 30 m a. m. The minimum rate of transpiration 

 falls in the night later than 8 p. m., while stomatal closure proceeds to its 

 minimum at about sunset. It thus appears that, as in Verbena, the maxi- 

 mum stomatal opening is reached usually about 2 hours before the maximum 

 transpiration rate occurs, and the evidence makes it probable also that the 

 transpiration rate often begins to fall before the stomatal closure commences. 

 It is not clear that the course of the stomatal movement comports wholly with 

 that of Verbena, since the maximum opening may be maintained by some 

 stomata as late as 3 p. m., as has been occasionally observed. 



According to the summary given by Burgerstein (1887, p. 53) the trans- 

 piration maxima of various plants occur between 1 1 a. m. and 3 p. m., though 

 Sorauer (1880) found that in seedlings of apple, pear, and cherry it occurs 

 as early as 9 a. m., the rate falling more slowly thereafter than it rose in 

 the morning. 



Curtis (1902), in accurate weighing experiments, found that the maximum 

 transpiration was attained (by potted plants) at or between the hours of 

 12 day and 2 p. m., and in plants kept under constant illumination (electric 

 light) this maximum was attained as early as 11 a. m. The minimum was 

 nearly reached at sundown. 



Cannon (1905) has recorded observations on the ocotillo, finding that the 

 maximum occurred, in April, at about noon. The minimum was reached 

 at about sundown. The low rate then reached was maintained till sunrise, 

 though the details of the course during the night are not given. 



Livingston (1906) found that in a desert species of Euphorbia the maximum 

 occurred from 10 a. m. to 2 p. m. The minimum rate was reached somewhat 

 after 2 a. m., but this rate was nearly reached at or about sundown. 



With the permission of my colleague, Dr. B. E. Livingston, to use cer- 

 tain of his data prior to their publication, I am able to add that in two 

 plants of Verbena ciliata studied by him during the present year (1906) the 

 maximum rate of transpiration occurred at 3 p. m. The data were obtained 

 at 3 -hour intervals (by weighing), and it may very well be that the maximum 

 occurred before 3 p. m., but certainly not before noon. The character of the 

 curves derived from Livingston's data consists well with the curves which 

 I have obtained. All of them have been plotted together for the sake of 

 ease of comparison. 



By calculating the ratios between transpiration rate and evaporation rate, 

 as determined by a new type of evaporimeter elaborated by himself, Living- 



