Distribution op the Giant CactxJS. 



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Curves drawn to show the changes of intensity of climatic 

 factors, as followed from one locality to another distant one, 

 commonly show a smooth rise or fall. However, on passing 

 from lower to higher latitudes o' altitudes the number of con- 

 secutive hours of freezing becomes gradually greater until the 

 point is reached at which davs without a mid-dav thaw are first 

 encountered; there is then a sudden rise from about 22 hours 

 of frost to from 36 to 42 hours, according as the fall and suhse- 



FiG. 2. Young Ciant Cacti fro>en 42 and 29 hrs., respectively, showing destruction of 

 tissues at base. 



quent rise of temperature are abrupt or gradual. In other 

 words this factor is unique in that the curve expressing its 

 changes of intensity possesses a sudden vertical rise, or indeed 

 a number of such rises. The line along which this takes place 

 in the severest of winters is bound to be an important limit of 

 plant distribution, at least it is so along the line of sudden rise 

 which lies nearest the absolute frost line. It has not been pos- 

 sible to locate this isohoral line exactly from the climatological 



