310 



EDITH BELLAMY SHREVE 



ness and diffuse light; (4) constant low temperature and con- 

 stant darkness. 



In the case of the continued darkness with an alternation of 

 high and low temperatures, the first twenty-four hours shows 

 the characteristic movements but the second period of low tem- 

 perature fails to show the downward movement and after this, 

 one plant continues a general downward movement, while the 

 other moves upward during exposure to high temperature and 

 downward under the influence of low temperature. 



PllT-t SO. 7 



Kar. 1! 



Fig. 5. Movements of Opuntia. March 8, normal day to night changes in 

 temperature and light. March 9-11, high temperature with an alternation of 

 diffuse light and darkness. March 12-13, high temperature with an alternation 

 of normal day to night changes in light. Two lowest graphs indicate tempera- 

 ture and light conditions. See experiment 8. 



All of the plants resumed their characteristic movements 

 when they were again placed under normal conditions, but not 

 until one period of low temperature and darkness had been 

 experienced. 



Thus it appears that an interruption of the normal day to 

 night changes in temperature and light intensity breaks the 

 sequence of day to night movements. It is evident that both 

 light intensity and temperature influence the movements, but 

 a given light intensity or temperature or any combination of 

 the two will not produce a given movement unless they have 



