272 BOTANICAL GAZETTE [APRIL 
America, yet it induces change in other climatological factors which 
are of great importance. ‘“‘Le fait essentiel d’ot dérivent a peu 
prés tous les caractéres du climat de montagne est la raréfaction 
de plus en plus grande de |’air dan les hautes altitudes. De tous 
les phénoménes météorologiques des régions élevées, c’est le plus 
régulier, car c’est le seul qui ne dépende pas des conditions locales 
du relief” (4). Among these changed factors may be mentioned 
increased insolation, increased radiation, change of illumination 
by increase of proportion of violet light, and rapid alteration from 
saturation to extreme dryness (5). 
That any considerable ascent is accompanied by a fall of tem- 
perature is a constant phenomenon all the world over; it is due to 
the diminished heat capacity of the rarefied air. This fall in tem- 
perature may be counteracted within narrow limits by the relief; 
it is frequently noted in the mountains that plants which are indi- 
cators of a colder habitat are growing at a lower elevation than 
other plants commonly found much lower down. This is especially 
true in lately glaciated regions with their usually rugged topog- 
raphy; the cirques will often have a flora distinctly microthermic, 
while the surrounding ridge bears forms suggesting a milder cli- . 
mate. An example of such a contrast is afforded by the cirque 
on the northeast of Mt. Tallac, in which Tsuga Mertensiana is 
growing full 500 feet below Pinus J effreyi on the bluff overlooking 
Fallen Leaf Lake. This inversion of temperature, with the colder 
air sinking to the valleys and the warmer currents sweeping up 
the ridges, accounts in part for the lingering snow drifts that may 
lie in the cirques till late in the summer, or even persist throughout 
the season. It also produces a complexity in zonal maps, the 
limits of the warmer zones advancing up the slopes and the colder 
sinking below their average level. 
In the higher Sierra low temperatures in winter are known 
comparable to those of the east. The lowest temperature reported 
from Tamarack, Alpine County (8000 feet), is—29°F. in January, 
1910, or 51° of frost. On the summit of Mt. Rose the lowest record 
is—10° F. Summer temperatures may become fairly high; Tam- 
arack reports 86°F. in July, and Summit, Placer County (7°17 
feet), 90° in October (table B). This last suggests a new feature 
