NORMAL DAILY PERIODICITY 



37 



is seen that as the opening increases the effect of the wind increases. In 

 using the table, the longitudinal diameter has been assumed to be constantly 

 15 micra. The transverse diameters may then be applied to numbers under 

 2b. If the areas are calculated, the values of r may be deduced and these 

 applied to the numerals under the column r, but I have not proceeded on this 

 method. The ratios desired may then be read in column Q or Q l . 



Table 18. — Ratios of diffusion capacities for stomata of several different dimensions. 



NORMAL DAILY PERIODICITY. 



EXPERIMENTS. 



The following experiments upon Verbena ciliata afford us data upon the 

 problem of the possible regulatory action of stomata upon transpiration. 

 The rate of transpiration was determined by the method described by Cannon 

 (1905). A suitable branch of the plant was selected, and a waterproofed 

 wooden base was built about it for the support of a bell-jar of known capacity. 

 The increase in the relative humidity of the interior of the jar when placed 

 over the branch to be studied was recorded by means of a hygrometer (the 

 Tamprecht polymeter). Each rate amount given is based upon the initial 

 three readings made at 4-minute intervals. Initial temperature and hygrom- 

 eter readings are given in the table, but for the calculation of the rate those 

 at the close of the selected intervals had, of course, to be used. The measure- 

 ments of the stomatal pores were derived by taking the averages of the 

 observed dimensions of both upper and lower epidermis. 



The data afforded show that the daily opening of the stomata commences 

 quite early in the morning, at dawn. The rise in the curve between i h 30 m 

 and 2 h 3o m a. m. signifies in this regard probably nothing, as 0.5 micron is no 

 greater than the error in the measurement. There seems, however, to be a 

 tendency for stomata to open a little during the night, as Darwin observed. 

 From the hour of dawn on till about 8 a. m. the opening goes on steadily, until 

 at this hour the maximum size of the pore has been reached. This has been 



