STOMATAL, RESPONSE TO SUDDEN CHANGE. 



47 



versely, those exposed to normal illumination reduced their rates of transpi- 

 ration when placed in the dark in the ratios 0.22 (No. 176) and 0.21 (No. 177). 

 During the hour of exposure to light there was an increase in the average 

 transverse dimensions of the stomatal opening from 3 to 6 micra. Con- 

 versely, the change from light to dark resulted in a reduction from 3 to 1.5 

 micra. That is, approximately, the diffusive capacity of the stomata in- 

 creased in the light in the ratio 1.8, and decreased in the dark in the ratio 0.6. 



Experiments 46, 47, and 54. Ocotillo, July 16, 17, 1904 (fig. 14). Vertical tube potometer 

 placed on porch at 9 p. m. ; placed in dark at 9 h i5 m a. m. ; replaced in the diffused 

 light at io h i5 m a. m. Hourly rates. 



TABLE 24. Rates of transpiration. 



Epidermis of plant used for experiments 47 and 54 was fixed at 20-minute intervals after 9 h 15 m . All 

 specimens showed the stomata 2 to 6 micra open. 



*40 cm. tube-length contained 8.7 c.c. of water. Leaf-surface 272 cm ! . Maximum transpiration rate 

 per square meter per hour, 221 c.c. 

 t=5.87 c.c. J=L. 52 c.c. 



The transpiration curves took a normal course till 9 h i5 m a. m., when, upon 

 being placed in the dark, the rate fell in i hour to about one-third (46, 47, and 

 54). Readings which were made at 9 h 45 m a - m - show that the change is more 

 rapid during the first part of the period. During the following hour (io h i5 m 

 to n h i5 m a. m.) in the light the rates again rose to within one-third (two 

 cases) and two-thirds (one case) of the rate at 9 h i5 m . 



Measurements of the stomata show an average diminution in size during 

 i hour of darkness from 5 or 6 micra at 9 h i5 m a. m. to not less than 4 micra 

 at io h i5 m a.m., after which, when exposed to light during the hour following, 

 they remained the same. 



Broadly speaking, a negative change of about 60 per cent in transpiration 

 rate was accompanied by a negative change in diffusion capacity of the sto- 

 mata of about 30 per cent. 



