San Bernardino District. 633 
the moisture drains rapidly away. Cercocarpus ledifolius, Spiraea (Holodiscus) 
dıscolor var. dumosa, Plaladelphus serpyliifolius, Aplopappus cuneatus, Ceano- 
thus cordulatus and Arctostaphylos patula grow in such situations besides 
Eriogonum saxatıle, Silene Parish, Gilia pungens and Hieracium horridum. 
Alpine Formation. The most southern latitude at which the true boreal 
alpine plants exist in North America is on San Jacinto Mountain, where 
are found a few characteristic species growing along the banks of perpetual 
snow in the cool, shaded canyons, which descend the north side of the main 
peak at altitudes of between 10,000 and 10,800 feet. 
The plants of the summit of San Jacinto Peak (10,700— 10,805 (10,987) feet altitude = 3290 m) 
are: Aquilegia truncata (alpine form), Carex Preslii, Castanopsis (Castanea) chrysophylla, Draba 
corrugata, Heuchera rubescens, Holodiscus discolor var. dumosa, Monardella odoratissima, 
Oxyria digyna, Pedicularis semibarbata, Pinus flexilis, P. Murrayana, Ranunculus Eschscholtzii, 
Ribes cereum, R. lacustre var. molle, Silene Parishii, Spraguea umbellata, and Trisetum subspi- 
catum ?!), 
We know that on Grayback Peak Pinus albicaulis makes its appearance 
at 11,500 feet and continues alone nearly to the summit 11,725 feet (= 3574 m) 
above sea-level. The summit of Grayback is flat and consists of porous de- 
composed granite unfavorable to the growth of plants. San Jacinto is more 
fortunate having on its precipitous northern face some steep, shaded canyons 
preserving perpetual snows. Nevertheless, the flora of these high peaks is 
scantier than their altitude would lead one to expect. A single species Ranun- 
culus Eschscholtzii has been found on both peaks, Arenaria (Alsine) verna var. 
hirta and Antennaria alpina have been collected on Grayback, while from San 
Jacinto Carexr Preslii and Oxyria digyna have been added to the short list?). 
Chapter V. Mexican Subtropie Zone and Mountain Regions, 
Within this zone are several regions which have been included in the tem- 
perate zone. The distinction is somewhat difficult to draw between the flora 
of temperate and tropic climates, because of the fact that many plants range 
north or south of their usual limits by taking advantage of mountain summits, 
or river valleys. 
1. Sonoran Desert Region. 
This comprises the desert region lying contiguous to the valleys of the 
lower Rio Grande and Gila rivers and immediately surrounding the shores 
of the Gulf of California. The geology, physiography (ante P- 2 
climatology (ante p. 148) have been described previously in detail. ree 
districts are recognizable and are described below. 
ı) Harz, H. M.: Loc. cit. page 33. 
2) Parısh, S. B.: See Bibl. p. 81. 
