270 TRANSFER OF WATER THROUGH THE PLANT. 



increases, they curve more strongly, and the cleft widens ; but 

 when their turgescence diminishes, the cleft becomes straighter 

 and narrower, it being clear that with increasing turgescence 

 the guardian cells must become more convex on the side of 

 least resistance, and more concave upon the side of greatest 

 resistance. 



727. Relations of stoinata to external influences. In a classical 

 series of experiments upon the relations of stomata to their sur- 

 roundings, Mohl 1 has shown that when the uninjured leaves of 

 certain orchids, lilies, etc., are wet with water, the clefts of the 

 stomata open ; but these plants form exceptions to the general 

 rule, for it was found that in the greater number of cases studied 

 the cleft closes when the stoma is brought in contact with water. 

 In Amaryllis and the grasses, this closing takes place with great 

 rapidity. 



728. When a thin film of epidermis with its stomata is de- 

 tached, and examined under the microscope, the behavior is the 

 reverse of that above. In a detached film the guardian cells of 

 the stoma are partially freed from the action of the contiguous 

 proper epidermal cells, and as a result the cleft widens when 

 w r ater is applied, the turgescence being increased ; but if a solu- 

 tion of sugar in water is employed, the cleft grows narrower, 

 since the turgescence of the cells is at once diminished by 

 osmosis. 



According to Mohl, in a wilted leaf the clefts of the stomata 

 are partially or wholly closed, but the application of water causes 

 them to open. If kept wet, they soon close again. 



729. The cleft of a stoma opens more widely in the light 

 than in darkness ; thus leaves of Lilium which have been kept 

 in the dark in a saturated atmosphere for some days have the 

 stomata closed, and when wet the cleft opens only slightly. 

 Upon exposure to sunlight, the cleft gradually opens. 



730. According to Van Tieghem, 2 stomata are always open 

 in sunlight and closed in darkness. In order to cause open 

 stomata to close, it is merely necessary to suddenly change the 

 amount of light. This closing of the stomata takes place in half 

 an hour when a bright light is replaced by diffused light. 



It has been found that heat has no marked effect upon the 

 opening and closing of stomata ; thus when a plant is kept in 

 darkness at a temperature of from 15 to 17 C., they are closed, 



1 Botanische Zeitung, 1856. 



2 Traite de Botanique, 1884, p. 636. 



