140 The Plant World. 



data obtainable for Arizona, but the few figures at hand, 

 when plotted in relaticn to altitude, and supplemented by tlie 

 temperature data accumulating from the vork of MacC(.ugal and 

 of the writercn the Santa Catalina rrountains, show a strong 

 probability that the line lies in the vinicity of the limit of the 

 Carnegica, which is also the limit of Parkinsoiiia microphyUa, 

 Cercidium torreyannvi , Fouquicria splcndens, Encelia jarinosa 

 Acacia greggii,Franscria deltoidea, Cassia covesii and a number of 

 other characteristic desert species. 



The stout form and high water content of the Giant Cactus 

 suggested the necessity of investigating the relation between 

 the daily curve of its internal temperature and the daih' 

 curve for the air, both for the sake of learning whether a given 

 number of hours of freezing air temperature meant as long a 

 subjecting to freezing for the cactus and whether as low a min- 

 nnim A^ould be reached by the cactus. With this object incu- 

 bator thermometers were inserted, to a depth of 3 cm. in two 

 individuals, 8 cm. and 30 cm. high respectively, ani the tem- 

 perature of the air was read simultaneously. Keadings were 

 also made on the north side of an adult cactus at depths of 

 3 and 7 cm. All the observations made showed a rapid response 

 on the part of the cactus to the changes of air temperature, 

 and all showed a greater amplitude in the daily curve for the 

 cactus than in that for the air, excepting in the case of the deep- 

 er thermometer in the adult cactus. The curves shown in fig. 

 I exhibit the data secured from the young cacti during a clear 

 day (Feb. 22, 1911) and one of the coldest nights of the winter 

 of 1910-191 1. Observations on the same cacti on a completely 

 cloudy day showed a rise of temperature for the larger one 

 almost exactly parallel with the march of the air temperature 



Clear days are usually juxtemporaneous with the coldest nights 

 of winter, and under such conditions insolation carries the diur- 

 nal temperatures of the cactus above that of the air, while rad- 

 iation, together with a slight amount of transpiration cooling, 

 carries the nocturnal temperature below that of the air. In 

 other words the surface cells of the cactus are exposed to lower 

 temperature than those indicated by the air thermometer, and 

 the cactus is subjected to a slightly greater number of hours 

 of freezing temperature than is indicated by the air thermo- 



