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EDITH BELLAMY SHREVE 



and in the open. The two plants which were grown in the green-house moved 

 upward under green-house conditions but downward when placed under the 

 conditions of higher evaporation in the open. Plant No. 8, which had grown in 

 the open, behaved in the same manner as the green-house grown plants. 



Experiments 1-7 show that young turgid joints which have not 

 recently suffered from drought, when placed under conditions 

 of average evaporative intensity and subjected to the normal 

 day to night changes in light intensity and air temperature, 

 show short periodic movements, consisting of a rise during the 



Fig. 4. Movements of Opuntia. March 2-4, low temperature with an alter- 

 nation of diffuse light and darkness. March 5-7, high temperature with an alter- 

 nation of diffuse light and darkness. March 8-10, normal clay to night changes 

 in temperature and light. Two lowest graphs indicate temperature and light 

 conditions. See experiment 8. 



daylight and a drop during darkness hours. The rise is greatest 

 in the forenoon, frequently ceasing, or even being replaced by a 

 drop, after four o'clock. In other words, during the time of 

 greatest evaporative intensity of the day the plant is erecting 

 itself and during the time of lowest evaporation it is drooping. 

 This characteristic movement is influenced by the amount of 

 water in the soil and may be stopped or reversed b}^ allowing 

 the plant to become desiccated and subsequently irrigating. A 

 very high evaporative intensity may also give a downward move- 

 ment during the day to a joint which has grown in the green- 

 house and sometimes to a very young joint which has made its 



