142 



The Plant World. 



temperature surrounding the cactus. In the other experiments 

 of each series only the air temperature was read. The freezing 

 mixture of ice and salt was renewed as required. The several 

 experiments differed in the amount of pre-cooling given, and 

 in the length of time the plants were subjected to an air temper- 

 ature below freezing. In each case the cacti were allowed to 

 cool gradually by opening the lid of the freezer and allowing the 

 ice to melt during the next 12 hours. This always resulted in 

 surrounding the cacti with air that vas cool but not below- 

 freezing. After from 12 to 20 hours they were removed from 

 the freezers and placed in a cool room. On the following day 

 they were placed in a green house where the temperature rose to 

 70° to 80° F. in midday. 



Following are given the corrected figures for the tempera- 

 ture of the air (C.) and of the cactus at the outset of the three 

 tests in which thermometers were placed in the tissue of the 

 cactus. 



The temperature of cactus No. 1 fell no lower than -4° 

 and it was kept between that temperature and zero for 42 hours. 

 Cactus No. 5 was precooled, and when placed in the freezer 

 was cooled vigorously and taken as low as -11°. It was not 

 allowed to go above -5°, being kept at temperatures below 

 freezing for 29 hours. Cactus No. 12 was taken only to - 3° 

 but was kept below freezing for 46 hours. In each case the plant 

 was allowed to stand in the greenhouse for several weeks after 



