INDUCED RHYTHM IN STOMATA. 



77 



Another point of interest is suggested by the fact that the stomata which 

 were exposed to diffused light in the afternoon opened more than those on 

 the sunny exposure. The plant is not a desert plant, and this result may be 

 due to a masking effect of a heat reaction upon the light reaction. The tem- 

 perature in direct sunlight at the time of the year when the data were obtained 

 is great, and greater on the western exposure than upon the eastern, so that 

 plants exposed to the western sun often show signs of wilting. Care was, 

 however, taken to avoid using wilted material, the difficulty of which has 

 been referred to elsewhere. 



When, however, the stomata examined are taken from the plants under the 

 normally increasing illumination and temperature, the reverse is found to 



l.40p.m. 2.00 2.20 2.40 3.00 3.20 



150 



125 



100 



75 



50 



25 



1 I 



I I I 



] I 



1 I 



IShade 

 a.m. 



IV Sun 

 a.m. 



in Shade 



nSun 



p.m. 



9.30a.m. 9.50 10.10 10.30 10.50 10.10 



FIG. 28. Curves constructed on data of table 43. Ampelopsis quinqucfolia. 



obtain, as shown by the measurements given. Stomata were collected at the 

 same hours from the eastern and western exposures of the building. The 

 plants on the eastern side were exposed to direct sunlight till about 1 1 a. m. ; 

 that on the west side began to be illuminated directly at that hour. The 

 stomata exposed to the early morning sun opened more rapidly than those in 

 diffused illumination, and the maximum was attained an hour earlier. Ampe- 

 lopsis appears to accord with Fouquieria and Verbena in having a sustained 

 maximum opening, lasting for about 3 or 4 hours (fig. 29). The data further 

 indicate that in the plant exposed to the western sun the stomata began to 

 close earlier, again a result, possibly, of wilting. The reverse condition, 

 namely, submitting the plant to a given period of darkness, has been experi- 

 mented with, with more uniform results. 



