PHYT0GE0ORAPHY OF THE ARIZONA DESERT 185 



the western foothills of the Baboquivaris nor in the Qui-i-to-woc 

 hills. It is probable that these occurrences mark the northeastern 

 limit of the species. Its altitudinal as well as its geographical 

 limit of distribution thus falls considerably short of that of the 

 sahuaro, while their habitat affinities are clearly indicated to be 

 identical, i.e., the first outlying specimens of the pitahaya are 

 found in the optimum habitat of the sahuaro. 



It may not be amiss to give a brief description of the first organ 

 cactus encountered. It was branched at the base into 21 fluted, 

 cylindrical stems or columns, 4 to 6 inches . in diameter, which at 

 first curved outward, then ascended vertically, the tallest to a 

 height of 10 feet. Five stems examined were similar to those 

 of the sahuaro, and had 18 and 19 ribs each, crowned with fascicles 

 of slender spines numbering from 15 to 19 in each areole. The 

 sahuaro has usually from 20 to 25 ribs and an equal number of 

 spines in each fascicle. Another plant, from which specimens were 

 collected, had 44 columns of all ages and sizes. These arose at 

 or near the base, which was of the diameter of a large sahuaro, 

 or else from primary branches. Sometimes branching takes place 

 higher up, and in these cases it appears to be the result of mechan- 

 ical or other injury to the parent stem. The tallest of these 

 columns measured 12 feet, and was 7 inches in diameter. 



The pitahaya possesses what at first sight appears to be a sickly 

 yellow color, but this yellowish tint proves to be a natural 

 character, distinguishing it from the clearer green of the sahuaro. 

 The hot south side of the highest of the Cababi hills, at whose 

 base a group of pitahayas were found, is covered with jet black 

 rock and brown soil. Here two cacti were found very strongly 

 represented, Opuntia bigelovii and a large Echinocereus that does 

 not occur about Tucson but is common in this region. Its many 

 cylinders, some of them reaching a height of 18 inches or 2 feet, 

 are covered with very long and numerous spines. The plant is 

 marked by a strong yellow color, and is so numerous on this 

 slope as to lend a dominant character to the vegetal covering. 

 Similarly, the Bigelow cholla, were it not for the fact that they 

 rarel}' grow together, could be readily distinguished from its sister 

 species Opuntia julgida by its yellowish tint. A causal relation 



