84 CARNEGIE INSTITUTION OF WASHINGTON. 



cactus and Carnegiea, and with the smaller Mesembryanthemum. 

 Determinations of the acidity of the sap of this plant show that while 

 the total range is not as great as that found by Richards in Opuntia 

 versicolor (Acidity and Gas Interchange in Cacti, Carnegie Inst. Wash. 

 Pub. No. 209, 1915), yet the daily course of variation is marked. 



The leaves are triangular in cross-section, and as the pairs emerge 

 from the sheathing bases of the antecedent pair the inner or upper 

 faces are appressed. The upright position implied is held until a half 

 or a third of their length is attained. The tips of a pair were harnessed 

 together and, being turgid and firm, were arranged to press upward on 

 the bearing lever of the auxograph. 



The general features of the daily behavior of this plant were quite 

 similar to those of Opuntia in that elongation accelerated in mid- 

 forenoon, about 9 to 11 a. m., and continued until 1 to 3 p. m., when it 

 was checked and a shrinkage ensued which generally ended at 5 or 

 6 p. m. or sunset. After this time, temperature being favorable, a 

 low rate of growth continued through the night and until the daily 

 acceleration occurred a few hours after sunrise. 



The daily course of transpiration has not been determined, but it is 

 allowable to assume that the imbibition capacity of the growing regions 

 is lessened by acidity, as it is in Opuntia. 



The results of measurements of growth of the apical part of the globu- 

 lar Echinocactus and of the cylindrical Carnegiea afford some interesting 

 comparisons, since both are massive succulents, but present a type of 

 respiration somewhat different from that of Opuntia and Mesembry- 

 anthemum. 



The spines of Echinocactus arise from special meristem tracts lateral 

 to the growing-point, and as the growth is wholly basal the rigid tips 

 afford an excellent bearing for an auxograph arm. A preparation was 

 kept under observation at a point some distance from the walls of a 

 greenhouse late in April 1916. Temperatures of the body near the 

 surface were taken by a thermometer with a thin bulb left in place 

 during the course of the observation. Growth began at 22° C. to 

 24° C. about 8 a. m., continuing during the warm dayUght period and 

 until nearly 8 p. m. Nothing higher than 37° C. was shown by the 

 body. The daily rate varied from zero to 0.05 mm. per hour and no 

 retractions were discernible. The length remained fairly constant 

 when growth ceased. The temperature of the body of this plant did 

 not fall below about 14° C. during any part of the period. 



The same plant was available for experimental purposes in March 

 1916. The cluster of spines, the tips of which had emerged for a length 

 of 4 to 6 mm. in 1916, began to show freshly colored sections at their 

 bases indicative of elongation, and one of these was brought into bear- 

 ing in the cup-shaped end of the vertical arm of an auxograph. The 

 preparation was placed near the south end of an unheated glass-house, 



