AUTONOMIC MOVEMENTS IN SUCCULENTS 



311 



been preceded by the proper combination of light and tempera- 

 ture. The duration of these conditions is also of importance. 

 For example, an upward movement takes place under conditions 

 of strong light intensity and high temperature, provided the 

 plant has been subjected to a low temperature and darkness for 

 a period immediately preceding. Further, a downward move- 

 ment at low temperature and with weak light intensity takes 



Plan". So. 4 



Plait No. 4 : 



6 p.m. 6 

 Mar. £2. 



6 p.m. 6 a. I 



•' p.m. 6 a.m. 6 p.m. 



Apr. 1. 



Fig. 6. Movements of Opuntia. March 20-24, constant darkness with an 

 alternation of low and high temperatures at twelve hour intervals, followed by 

 one day of normal conditions. April 1-4, constant darkness and constant low 

 temperature. Two lowest graphs indicate temperature and light conditions. 

 See experiment 8. 



place only when these conditions follow high temperature and 

 strong light. These periods need not be as long as twelve hours, 

 as may be seen from the graph on March 11 and 12, when the 

 heating unit failed to work and is so recorded by the thermograph 

 trace. 



The facts brought out in the last paragraph show that the 

 influence of temperature and light intensity on the movements 

 cannot be a direct one. The temperature and light must exert 

 an influence on some other processes which in turn influence 



