138 The Plant World. 



which extend northward above the limit of frost. The distri- 

 bution of Carnegiea stretches from the mouth of the Yaqui 

 river, in Sonora, northward along the Gulf of Lower California 

 and the Colorado river to the mouth of the Bill Williams river 

 in Arizona, and eastward to the Santa Rita mountains in south- 

 ern Arizona, and up the various tributaries of the Gila river 

 to approximately 3900 feet (1190 m.) elevation. The vertical 

 limit of occurence of Giant Cactus in the desert mountain 

 ranges in the center of its area is 4500 feet (1375 m.), while its 

 abundant occurrence ends rather abruptly at 4200 feet (1280 m.). 

 The distribution of the giant Cactus to the westward of the Col- 

 orado river is undoubtedly limited by the low summer rainfall 

 of interior southern California. 



In order to ascertain the character of the winter cold con- 

 ditions wnthin and just outside the range of the Giant Cactus 

 an examination of climatological records was made for the four 

 localities in Arizona at which thermograph records are regularly 

 secured. Yuma, Phoenix and Tucson are within the range 

 of Carnegiea, Flagstaff being considerably above it. The figures 

 in the following table are for the winter of 1909-1910 — the 

 most severe for several decades. 



STATION YUMA PHOENIX TUCSON FLAGSTAFF 



Elevation (feet) 141 UOS 2663 6907 



Elevation (meters) 4.3 337 812 2106 



Number of days with freezing tem- 

 perature 13 14 15 193 



Number of hours of freezing tem- 

 perature 32 89 126 2200 



Greatest number of consecutive 



hours of freezing temperature 8 13 19 132 



Minimum temperature during great- 

 est number of hours (Fahr.) . . 25 23 17 —22 



Same (Centigr.) -4 -5 -8 -30 



The most important figures of this table are those which 

 show that the Giant Cactus is capable of withstanding 19 hours 

 of continued freezing temperature and as low a minimum as 

 17°F.(-8.3°C.). It may be noted that Flagstaff possesses a 

 climate much more rigorous than that of the other stations. 

 The fact that Tucson is only 1300 feet below the limit of the 

 Giant Cactus, while Flagstaff is 3000 feet above it indicates that 

 the cold conditions at Tucson are nearer those which limit the 

 Giant Cactus than are those of Flagstaff. 



