98 CARNEGIE INSTITUTION OF WASHINGTON. 



perature, under the same conditions, in leaves of the mesophytic type; 

 (5) leaf water-content varies as much as 100 per cent from winter to 

 summer in a single species and varies considerably with the soil water- 

 content at any one season. 



The calorimetric method for the determination of leaf temperature 

 has been improved by the use of a more accurate means of finding 

 the specific heat of the leaves. A known amount of heat is introduced 

 into the calorimeter by means of a mercury calorifer, and arrangements 

 for the calibration of this instrument have been made with the Bureau 

 of Standards. A check on the constants of the calorimeter will thus 

 be provided. 



Autonomic Movements of Stems of Opuntia versicolor, by Edith B. Shreve. 



Work has been continued on the movements of Opuntia stems, which 

 has been reported upon in previous years. Dr. MacDougal suggested 

 that the large variations in amount of acid in this plant, which have 

 been found by Dr. H. M. Richards, might affect the water-holding 

 capacity of the colloids and thus set up the motion. Plants 

 were placed under controlled conditions, where an increase or decrease 

 of acidity could be predicted,^ and their movements were measured. 

 The tests were conducted under (1) constant high temperature and 

 varj-ing light conditions; (2) constant low temperature and varying 

 light conditions; and (3) constant darkness and varying temperatures. 

 With certain exceptions, later satisfactorily accounted for, a downward 

 motion or curvature was always accompanied by an increase in acidity 

 and an upward motion by a decrease in acidity. 



As noted in the Year Book for 1913 (p. 79), the seasonal position has 

 been shown to be a function of the turgidity, depending in turn upon 

 the soil water-content. This has been confirmed by application to 

 potted plants, where the branches can be made to straighten or curve 

 by changing the amount of water in the soil. The statement made 

 last year, that the upward motion takes place only in direct sunlight, 

 has been found to be wrong, as a more careful measurement and at 

 shorter intervals showed that the rate is very much retarded but not 

 stopped in diffuse light or in darkness, and under some conditions the 

 decrease in rate is not very great. 



Transpiration studies were continued with the same results as before, 

 namely, a higher relative rate occurs at night than in the day time 

 if a moderate amount of water is present in the soil, and a higher 

 actual transpiration rate occurs at night if the soil water-content is 

 high. Plants were cut and allowed to develop roots in water and then 

 used for the measurement of water intake and outgo. It was found, 

 under conditions of average transpiration, such as occur in the green- 

 house in summer, that the water intake at night is less than the outgo, 



^Dr. H. A. Spoehr kindly aided in the prediction of the acidity conditions. 



