June, 1919] Variations in Transpiration 493 



many plants show a rhythm in the transpiration rates in dark- 

 ness and attribute it to stomatal movements. Curtis** kept 

 several plants under constant conditions in darkness and deter- 

 mined their hourly transpiration rates by weighing. He refers 

 to the behavior of the transpiration rates as "a pronounced 

 periodicity in the stomata." He made no direct observations 

 on the stomata but concludes that this rhythm was due to their 

 movements. Darwin^ found by the indirect method of deter- 

 mining transpiration with a hygrometer that there was a 

 rhythm in the transpiration rates in darkness and concluded 

 that it was due to stomatal movements. Lloyd^ found in 

 "the plant {Fouquieria splendens) which he used that 

 it behaves qualitatively the same in total darkness as under 

 normal conditions until an hour (usually near 6 A. m.) when, 

 the normal stimulation being absent, the plant relapses to a low 

 condition of activity characteristic of its darkness condition. 

 For the lack of a real explanation we must refer this behavior 

 to the category of induced rhythm." 



Lloyd measured the stomatal openings on different plants 

 but could find no evidence that there was an increase in their 

 dimensions corresponding to this induced rhythm. He con- 

 cludes that some other factor than stomatal activity in the 

 plant must be found to explain the transpiration curve. Liv- 

 ingston and Brown*^ found that in nine out of eleven plants 

 which they investigated there was a decrease in the water con- 

 tent of the leaves beginning after sunrise and reaching a mini- 

 mum a short time after the transpiration attains it maximum. 

 This decrease was as much as 8% in some plants and did not 

 produce wilting of the leaves. A decrease in water content of 

 the mesophyll cells increases the resistance of the cells to water 

 loss. Livingston^ shows that some factor in the plant is operating 

 to reduce water loss even though environmental conditions have 

 not reached their maximum. 



This summary shows that considerable work has been done 

 on the effects of different factors on the transpiration curve; 

 that no single factor has been found to control the rate of trans- 

 piration from the plant, and that transpiration is a resultant of 

 the interaction of several internal and external factors. 



