The various species of cacti illustrate this necessary correlation between plant structures and environment probably better than any other large group of plants. Opuntia, the most important genus., is abundantly 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.
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.
Where it grows best the summer temperature often reaches a maximum of 115 degrees F. and the daily temperature for weeks at a time ex-
