240 The Plant World. 



On the south side it reappears in abundance, and much reduced 

 in size. At an altitude intermediate between the upper Hmits 

 of the two preceding species, it is last seen on the same open 

 southeast mountain side where Q. arizonica dsappears. Plere it 

 is an erect shrub of about the height of the latter species at its 

 upper limit, with smaller leaves than below, and these more com- 

 pactly placed. Both species are strictly absent on a northeast 

 aspect just over the crest of a sharp, sloping ridge. As before, 

 at least where the presence of rocky outcrops favors it, the de- 

 ciduous white oak chaparral {Q. submollis) appears here. 



Similar features of local distribution, only less well marked, 

 are shown by the conifers. The Douglas Fir {Pseudotsuga 

 tayijolia) on the very steep northerly declivities toward the 

 bottom of Stone Cabin Canyon must reach down to within 6,000 

 feet of sea level at most. It reaches its local center of density, 

 as a fine forest, in the cool, hanging gulches draining northward 

 at about 8,000 feet, but at 9,000 feet it appears to some extent 

 on east and west as well as north aspects. Here it is rather scarce 

 once more, probably due less to high elevation than to the un- 

 usually exposed and semi-arid character of these rhyolitic 

 heights. 



Mexican white pine {Pinus strohiformis) was first encount- 

 ered at a spring in the bottom of a gulch at 6,500 feet. A few 

 hundred feet higher it first appeared away from water on a steep 

 slope of north aspect.while strictly absent on other aspects. It 

 increases upwards and on the highest peaks it ranks with P. arizon- 

 ica in numbers,or even exceeds it. While here it still prefers the 

 slopes and pockets at the base of cliffs facing toward the north, 

 it can be found in almost any other situation. The Mayr pine 

 (P. mayriana) was noted at 6,500 feet, mixing with the Arizona 

 pine on similar north-facing habitats. On the sunny, south side 

 of Horse Ridge, at 8,000 feet, a few trees were seen again that 

 probably belonged to this sparsely represented species. 



The principal pine of the forests of southern Arizona (P. 

 arizonica) is encountered in Stone Cabin Canyon, which drains 

 the north side of the high Santa Ritas, on slopes toward the bot- 

 tom of gulches at about 6,500 feet. Like the more shady and 

 precipitous hanging habitats of Douglas fir, its first occurrences 

 at this level are upon northerly slopes, and like the fir and all 



