ii8 



Elementary Plant Physiology. 



insolation in the middle of the day, and from rapid cooling 

 and loss of water at night. The plants used have been accus- 



Fig. 88.— Positions of leaflets of bean in daylight and in darkness. After Detmer. 



tomed to regular alternations of periods of darkness and light 



until they have acquired a rhythm or habit, in accordance 



with which the movements 

 will be repeated a number of 

 times, even when kept under 

 continuous darkness or illumi- 

 nation. After a time, however, 

 the rhythm will be lost in 

 many species. After becoming 

 fully acquainted with the be- 

 havior of the plant used in the 

 above test, place it in sunlight, 

 and raise the temperature of the 



air around it, or of the soil, and note effect on movements 



of leaves. 



129. Geotropism : reaction to gravity. — Bend over 



Fig. 89. — Position of pinnules of 

 Mimosa Jiudica at niglit. 



