Distribution of the Giant Cactus- 



137 



last one of spring. For all sub-tropical plants the greatest num- 

 ber of consecutive hours of frost and the absolute minimum are 

 the most im]:ortant of these factors, while the others mentioned 

 are more strongly o}:^erative further north. In any consideration 

 of the geographical importance of the operation of these factors 

 it is obviously necessary to consider only the conditions of the 

 coldest winters, which serve as a check on the movements made 

 by species during the milder winters. 



A consideration of the factors which have to do with the 

 distribution and activities of the Giant Cactus {Carnegiea gigan- 



6 A.M. 7 



10 



11 



12 1 P.M. 2 



Fig. I. Cunes showing daily march of internal temperature in Giant Cac'i on a cold day. 

 Dotted line: ;iir temperature. Heavy line, temperature of cactus 30 cm. high. Thin line: 

 temperature of cactus S cm. high. 



tea, Ccreus giganteus) led me to believe that the greatest number 

 of consecutive hours of freezing is the most important climatic 

 datum in determining its northward range, and led me to in- 

 vestigate its ability to withstand freezing temperatures, and 

 the character of the winter cold conditions within its range. 

 The Giant Cactus belongs to a group of about 65 species of trop- 

 ical and sub-tropical arborescent columnar cacti all allied to 

 the genus Cereus. Of this number Carnegia gigantea, Lopho- 

 cereus schoiiii and Leviaireocereus thurberi are the only ones 



