DlSTRIBUTlOX OF THE GlAXT CaCTUS 145 



Ecliiuocaclus siuif^xojii is reported to range np to 11, ()()() feet 

 altitude, while Opuniia wissoitricnsis lias been collected by 

 Prof. J. Macoun on the Teaee In iver in Alhal)asea at lat. 3() deg. 

 12 min. An indication of the length of greatest exjjosnre to 

 frost for Athabasca can be snrniised from the fact that the 

 weather records for Ila\re, .Montana, 230 miles south of the 

 limit of Opuntia luissojivunsis, show a continuous period of 2>^>i 

 hotn-s (\^ days) of frost in the winter of 1909-1 'MO. 



In summarising it is necessary to bear in mind tliat the 

 pJiNsical conditions of desert regions are such as to gi\e the dail\' 

 temperature curve a nuich greater amplitude than it has in 

 moist regions. At 'J'ucson there are very many da\s on which 

 the mininnun and maxinnim tempera tiu'es are 50° h\ 

 (30° C.) apart, and on exceptional da\s still further than this. 

 ']*lic power of insolation to raise the mid-dav tem]:erature is so 

 great in all deserts that occu];\- low latitudes and altitudes, 

 that however cold the nights ma\' be the daib" maximum is 

 carried al)ove the frost line. There ha\'c, in all ])rol)abilit \', 

 been no da\s at Tucson in many hundreds of years on which 

 the air temperature remained below freezing all day, and the 

 ability of insolation to raise the internal temperature of the 

 Giant Cactus makes it still more certain that none of the in- 

 viduals of this species now li\ ing have endured more than 20 

 hours of frost, and ])criods as long as these are due sold}' to 

 l^artialh' clouch' da\s of the winter rainy season. 



The winter tem])erature conditions of the higher altittides 

 in the desert mountain ranges are very well indi ;ated b\ tlie 

 figures given for I'^Iagstaff, w hicli itself occupies a i ass in a range 

 of mountains surrounded by desert. An\ attemjjt to explain 

 the limitation of the bulk of the desert sj eci s to altitudes of 

 4000 to ,S000 feet in the desert mountains mist lake account of 

 the winter cold factors. l!etween those altitudes the condi- 

 tions of atmospheric and soil aridity are stunewhat less seve e 

 than on the desert lie or and ffotdiills, and 1 ct\\een 5000 and 

 7000 feet these conditions, as indicated by the natural \ege a- 

 tion.are materiallv less severe. Between 5000 and 7000 feet, 

 however, the vegetation is extremely o\ en, giving abundant 

 opportunity for the invasion of desert sjecies into a region in 

 which the atmospheric and soil moisture conditions are more 



