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POPULAR SCIENCE MONTHLY. 



The various species of cacti illustrate this necessary correlation be- 

 tween plant structures and environment probably better than any other 

 large group of plants. Opuntia, the most important genus, is abun- 

 dantly represented in the flora of our arid southwest (Figs. 1 and 2) 

 and reaches its maximum development on the Tucson plains in 

 southern Arizona. No less than ninety-two species of Opuntia are 

 growing wild in southwestern United States and northern Mexico, 

 selecting for the most part situations that are so dry that few other 

 plants persist where they thrive. 



In this article I desire in particular to call attention to the cholla 

 {Opuntia fulgida Engelm.) a cactus which grows to the size of a small 

 tree and which reaches its maximum development on the Tucson plains. 



Fig. 2. The Cholla and Tuna, growing together in southern Arizona. 



The cholla has probably not yet reached the limits of its variation and 

 distribution, and is one of the most interesting and characteristic plants 

 of the arid regions of the western continent. The organs of this plant 

 are most wonderfully adapted for performing their various functions, 

 to the best advantage of the plant, under what would be with most 

 plants an extremely adverse environment. 



The cholla is one of the largest of the cacti having numerous 

 branches. It grows best where fully exposed to the intense glare and 

 heat of the desert sun and where the annual rainfall averages from four 

 to twelve inches. It grows on the dryest upland, on open, porous, limy 

 soil that for months at a time is as dry as powder. 



Wliei-c it grows best the sTimmer temjierature often reaches a maxi- 

 mum of 115 degrees F. and the daily temperature for fl-ecks at a time ex- 



