314 MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 



rate days and was very active at 32° C. A period of continuous 

 elongation of the spine was comprised between April 10 to 17, 

 1916, during which time the air temperature ranged between 14° C. 

 and 28° C. The temperature of the body coincided with the lower 

 night temperature of the air and did not rise above 32° C. (Fig. 5). 



The maximum enlargement of the spine was at rate of .075 mm. 

 per hour, while that of the neighboring apical tract was not more 

 than a third of this rate. After the spine had reached nearly mature 

 length the apical tissue accelerated showing a rate as .088 mm. per 

 hour. Growth began on rising temperatures of 15° C. and above 

 and was observed at 40° C. of the body. The main part of the 

 growth took place in the daytime and no action directly attributable 

 to light effects could be detected. 



EcJiinocactus and Carncgiea are active during the period in 

 which the temperature is within the tonic range, as taken from 

 thermometers inserted in the tissues. This implies that such plants 

 grow during the daylight period in the open and as far into the 

 night as the temperature permits, the maximum rate being attained 

 during midday. Numerous tests show but little variation in the 

 acidity of Echinocactns and Carncgiea, and it is to be inferred that 

 the respiration of the sugars is of a kind in which the disintegration 

 is carried through to its iinal limits. 



A number of records of growth of the succulent leaves of 

 Mescmhryanthemum incquilaterale were obtained for comparison 

 with Opnntia, Carncgiea and Echinocactns. Determinations of the 

 acidity of the sap show that while the total range is not as great as 

 that found in Opnntia versicolor by Richards,^ yet the daily course 

 of variation is marked, as may be seen from the following measure- 

 ments of Mcsonhryauthcuium. 



Acidity in Cubic Centimeters of N/ioo NaOH. 



December 7. December 8. 



Pure Tuice Total Acidity Total Acidity Pure luice Total Acidity Total Acidity 

 C cm P^"" Gf"- Dry per Gm. Fresh „gj. r^ni P^r Gm. Dry per Gm. Fresh 

 ^ ■ ■ Material. M..-,i-.i P ' ' i\t„.„.:„i tvi„.„„:„i 



8:00 A.M 0280 1.584 



12:00 M 0279 1.509 



4:30 P.M 0232 1. 191 



s " Acidity and Gas Interchange in Cacti," Publ. No. 209, Carnegie Inst, 

 of Washington, 1915. 



