Seemseetieetincennen anal 
1903] FLORA OF SOUTHERN CALIFORNIA 213 
The rainclouds hug the mountains, and in winter one often may 
pass from the bright sunshine of the desert valleys through mist 
and drizzle to heavy showers by ascending for a few miles some 
canon leading up the mountains. Hence it is in the cafions that 
the botanist learns to expect the most abundant and varied vege- 
tation. 
The desert mountains are also the scenes in midsummer of 
‘cloud-bursts,”’ or violent thunderstorms. These discharge in 
a short period and over a limited area a great quantity of water, 
and for a few hours the parched cafions are filled with rushing 
torrents. I have found a measurement of but one of these 
“cloud-bursts,” and that only partial. At Campo on a day in 
August, 1891, there fell in such a storm 16” (406™") of rain, 
before the gauge was washed away, preventing a complete 
measurement. 
On the seacoast the aridity of the atmosphere is modified by 
frequent fogs and damp air-currents from the sea; and these 
ameliorating influences extend their benefits, in a less degree, 
inland, but do not pass beyond the mountains to modify the 
dryness of the desert air, while the strong winds almost con- 
stantly blowing there, produce a further desiccating effect. 
PHYTOGEOGRAPHICAL AREAS. 
It will readily be understood, from the preceding account, 
that the region under consideration consists of three divisions, 
differing from one another in both topography and climate. 
Consequently each of these will be found to possess a distinct 
and characteristic flora, and to constitute a separate life area. 
In its relation to the biological divisions proposed by Dr. 
Merriam* the entire territory may be regarded as a part of the 
Sonoran province, in the midst of which arise two isolated peaks 
belonging to the Boreal province. A scanty Arctic flora occupies 
the summits of these peaks, and beneath it a diligent study can 
disentangle the Hudsonian and Canadian zones. This has been 
worked out very carefully and thoroughly by Mr. H. M. Hall,?7 
**MERRIAM, J. Hart, N. Am. Fauna 3:20, map 5; National Geo. Mag. 6:229; 
Yearbook U. S. Dept. Agr. 1897 : 203. 
7 HALL, HARVEY M., A “botanical survey of San Jacinto Mountain. Univ. of 
Cal. Pub. Bot. zr: I-144. 
