312 MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 



globular Echinocactus and of the cylindrical Carnegica afford some 

 interesting comparisons, since both are massive succulents, but pre- 

 sent a type of respiration something different from that of Opimtia.'' 



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 

 daylight period and until nearly 8 P.M. Nothing higher than 37° C. 

 was shown by the body. The daily rate varied from zero to .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 



Fig. 4. Auxographic record of variations in length of spine of Echino- 

 cactus, March 13 to March 17, 1917. Shortening from 8 P.M. to 8 A.M. due 

 to low temperature. X 10. 



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 bearing in the cup-shaped end of the vertical arm of 



■'^ AlacDougal, "The End-results of Desiccation and Starvation of Succu- 

 lent Plants," Physiological Researches, Vol. i, No. 7, 1915. 



