342 EDITH BELLAMY SHREVE 



AVhat factors cause the day to night turgor changes, i.e., why 

 the difference between absorption and transpiration is positive 

 in the day and negative at night is of course another question. 

 The author is now at work upon these more fundamental causes 

 and will report upon the progress of the work at an early date. 



That the phenomena which have been studied in Opuntia 

 versicolor are practically universal for Opuntia seems probable 

 from measurements which have been made on other species. 

 The seasonal movement has been observed in the following: 

 Opuntia fusicaulis, 0. fulgida, 0. mamillaia, 0. tetracantha, 0. 

 arbuscula, 0. leptocaulis, 0. bigelovii, all of the cylindrical form, 

 and in 0. leavis, 0. blakeana, 0. linguiformis, 0. fusicaulis, and 

 0. monacantha, of the flat-jointed type. The day to night move- 

 ment has been measured on 0. lepticaulis, 0. fusicaulis, and 

 0. monacantha and found to be the same as for 0. versicolor. 



SUMMARY 



(1) Twelve species of Opuntia and also Carnegiea gigantea, 

 have been found to show seasonal movements of stems and 

 branches, which consist of a drop during desiccation and a rise 

 during recovery. These movements have been correlated with 

 turgidity changes. 



(2) The form of the adult cactus plant and the position of its 

 branches is determined by the water relations existing during 

 the period of growth and secondary thickening of its various 

 parts and not by any peculiarities residing in its growing point 

 or in its mode of initiating branches. 



(3) Opuntia versicolor, 0. fusicaulis, and 0. leptocaulis were 

 measured for the investigation of a short period movement which 

 is shown to consist of an upward movement during the day 

 and a downward one at night, when normal conditions of tem- 

 perature, light intensity, soil-water content, and evaporative 

 power of the air, exist. 



(4) A detailed study of Opuntia versicolor showed that this 

 short period movement is influenced by temperature, light in- 

 tensity, evaporative power of the air, and the water-content of 



