78 CARNEGIE INSTITUTION OF WASHINGTON. 



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

 are determined by the water-relations existing during the period of 

 growth and secondary thickening of its various parts and not by an}' 

 peculiarities residing in its growing-point or its mode of initiating 

 lateral branches. 



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

 for a short-period movement, which consists of an upward movement 

 during the daytime and a downward movement at night, under normal 

 conditions of temperature, light, soil, water-content, and evaporative 

 power of the air. 



(4) A detailed studj^ of 0. versicolor showed that this short-period 

 movement is influenced by temperature, light, evaporative power of 

 the air, and the water-content of both soil and tissues, separately as 

 well as in various combinations. But the influence of these factors 

 is an indirect one, acting through other intermediate processes. 



(5) The day-to-night movements have been shown to be caused 

 directly by turgidity changes in the stems. 



(6) 0. versicolor is less turgid at night than in the daytime, as is 

 shown by the fact that the plant absorbs more water through its roots 

 in the day than it loses in the same time by transpiration, and at 

 night it loses more than it absorbs. This is the opposite of the behavior 

 shown by non-succulents which have been studied. 



In this paper no mention is made of a possible correlation between 

 acidity and the movements, because further experimentation has thrown 

 doubt upon the existence of such a correlation, and surely shows that 

 if a relation exists it is b}^ no means a simple one. In the Year Book 

 for 1914 (page 98) it was said: 



"Plants were placed under controlled conditions where an increase or 

 decrease of acidity could be predicted and their movements measured . . . 

 a downward movement was always accompanied by an increase in acidity 

 and an upward movement by a decrease in aciditj'." 



The predictions referred to were made on the basis of the results of 

 other workers; but later, when tests were made on the material used 

 for the experiments, it was found that the acidity changes under con- 

 ditions of high temperature and darkness did not agree with the pre- 

 dictions. Consequently the statement quoted above from last year's 

 report does not hold for all conditions of temperature and light. 



The acidity results which were obtained from tliis work show that, 

 at least for the time of year when the tests were made (April to June) , 

 0. versicolor shows a marked increase in acidity when kept at 90° F. 

 in darkness for 8 to 16 hours. Plants were placed under the controlled 

 conditions at the close of hot, clear days. Many determinations were 

 made, so that there is no doubt in the mind of the experimenter 

 about the correctness of the results for the conditions and material 



