54 LIVING PLANTS 



gardless of the surroundings of the plant. 

 The "sleep" of the plant on any given night 

 is not the direct result of the absence of light 

 at that time but of the darkness of previous 

 nights. If a healthy individual is placed in 

 continuous light or darkness, it continues to 

 "go to sleep" at regular intervals for several 

 days, but finally sickens or adjusts itself to 

 the new conditions imposed upon it. The 

 habit is regained on restoration to normal 

 conditions. 



The sleep position may be produced in the 

 middle of the day by suddenly excluding the 

 sun's rays, but the normal position will soon 

 be regained unless the temperature is greatly 

 reduced. Mimosa is one of the plants in 

 which the night position is wholly a response 

 to low temperatures. 



If a screened plant is suddenly exposed to 

 the sun's glare in the middle of the day, the 

 leaves sink and the pinnules close as if struck 

 or jarred. Many interesting deviations from 

 these typical reactions have been observed. 

 Thus, if a healthy plant is placed in a dry 

 room at 15° Centigrade the blades will be 

 found extended in the morning, while the pe- 

 tioles have assumed and retained a depressed 

 position. The reactions described above are 

 not especially characteristic of mimosa, since 

 they are exhibited by many hundreds of spe- 



