Cold-Resistance; in Smneless Cacti 



129 



latter were placed in a glass cylinder filled with crushed ice and 

 salt. In this way a temperature of 0° C. (32° F.) was reached easily. 

 This experiment was repeated three times, and each time gave 

 results showing that the rapidity of penetration of the cold is 

 dependent upon the thickness of the integument. 



Fig. 5.— Apparatus used for determining the rate of psnetration of temperature through 

 the integuments of various spineless cacti; and, also for making a comparative study of the 

 cooling points, freezing points, and duration of freezing points of different spineless cacti. 



On an average it took G3.3 minutes for the temperature to 

 penetrate Opiintia castillae and 79.3 minutes for it to penetrate 0. 

 Ellisiana, both species of which have thick integuments. The 

 length of time required for the other species was much shorter. 



The following table gives a summary of the experiment, the num- 

 ber of minutes indicating the time required for the temperature 

 to reach 0° C. (32° F.). 



