4 
73 [300] 
there are many plants, the a top near: are strictly circumscribed by 
elevation or equivalent temperatu for example, the chestnut does not 
tise higher in the Swiss Alps, in se aies parallel of 45°, than 2,400 feet: on 
Etna, in latitude 38°, it reaches no higher than 4,000 feet. Many of the 
plants found * plains in the north of Europe occupy t the mountains of the 
The olive, in 44° of latitude, its most northern range, will not wow 
_at a greater elevation than 1,200 feet. In general, it is — that, as w 
ewan the equator, vegetation becomes more and mor affected by i 
vation; and that, as we recede from it, the effects of slehetion gradually 
_ ©The cause of the influence of elevation upon plants is ascribed, in the 
first place; to reduced temperature ; secondly, to a eae intensity of solar 
light ; and, thirdly, to a decrease in humidity. The rate at which tempe- 
rature decreases as we ascend from the surface’ of the peed noes basowe 
ing to latitude: Humboldt has shown that, in the temperate orrid 
zones, the decrement of heat is essentially different. In the picts zone, 
the temperature of the region lying at the height of between 3,000 an 0 
feet, on which the clouds repose that are visible to the natives of the plains, 
decreases much. more slowly than either above or below that elevation ; 
but, in the temperate zone, the decrease is more gradual. In proof of this, 
the following table has been formed by Humboldt: 
ieee eae 
pogo zone. Ss age zone. 
— t. 02210, ’ 50—47°. 
Elevation above | 
e sea in feet : “Y 
Mean temperature of | Difference. | Mean temperature of | Difference. 
the year. the year. 
0 80° 53° 
12° - 
3,000 68° 4}° 
ee “40 — 9° 
6,000 64° 320° ‘ ‘ 
9 — Ren * 
9,000 55° 230 ap: 
—- 11° 7a 
~ 12,000 440 
en 10° 
15,000 34° 
por 
The eee of the density of the air, as we ascend, produces a cor- 
responding increase in the intensity of the light ; ; a circumstance in ehashk 
i ues been said to correspond with high latitudes; but this is 
ibtfu 
u ay diet me ee ascend, a basi cause, as powerful a Lig 
of temperature, would be found for the effects of elevation upon vege 
But it is certain that the humidity of the air does not change eral 
we te A with the character of vegetation; on the contrary, it 
