Distribution of the Giant Cactus- 



141 



j;rapli trace. The actual conditions of temperature for a given 

 individual are a function of the amount of heating or precooling 

 it has received during the preceding day. 



In order to observe the actual eflectsofexpc.sures to freezing 

 temperature varying v.ithin the Hniits of duration common to 

 central Arizona, a number of }oung Giant Cacti were potted, 

 of such size that they could be placed in large freezers. The 

 fact that the Giant Cactus is able to survive for months without 

 renewed supplies of water I'd me to pi eservecnly the mi in ] arts 



T'IG. 3. Young Giant Cacti frozen 6 and 1 5 hrs., respectively, showing uninjured condition 



of the root system, and to leave the potted plants without watering 

 previous to their use. 'Ihree sets of experiments were made, 

 in which a total of 11 plants of 18 to 28 cm., height were used 

 and pot-grown seedlings less than 1 cm. in height. In each of 

 the three sets of experiments there was one of the larger cacti 

 in which an incubator thermometer was inserted to a depth of 

 ^ cm., with its scale projecting above the lid of the freezer, 

 while there was another thermometer placed to show the air 



