1205 °° PHYSICAL GEOGRAPHY.

 

(2.) According to Schouw, we may divide the earth’s
surface into regions characterized by assemblages of pecu-
liar floras, and separated by natural barriers.

The great number of the regions required to give thor-
-oughness to Schouw’s system, renders its use inadvisable
in an elementary book.

(3.) According to Humboldt and others, we may
divide the earth’s surface into zones, according to the
physiognomy of the plants inhabiting them. Here plants
of entirely different species are grouped by their mere out-
ward resemblances into what are called forms.

The first method is the one most suitable for our pur-
poses. We shall follow, in the main, Dove’s modification,
as adopted by A. R. Johnston, and divide the surface of
the earth into zones, according to the isotherms, or lines
of mean annual temperature. The values of the isotherms
are given in round numbers. This system is based on the

fact, that the character of the vegetation is dependent

mainly on the temperature, which, in-its turn, regulates
the quantity of moisture.

317. Horizontal Zones of Vegetation.

(1.) The Tropical Zone, extends between the
isotherms of 73° Fahr. on each side of the equa-
tor.

(2.) The Sub-Tropical Zones, extend in each

hemisphere from the isotherm of 73° Fahr. to 68° _

Fahr.

(3.) The Warm Temperate Zones, extend in
each hemisphere from the isotherm of 68° Fahr.
to 56° Fahr.

(4.) The Cold Temperate Zones, end in each
hemisphere from the isotherm of 55° aoe to 41°
Fahr.

(5.) The Sub-Arctic Zone, extends in the north-
ern hemisphere from the isotherm of 41° Fahr.
to the September isotherm of 36.5° Fahr.

(6.) The Polar Zone, extends in the northern
hemisphere from the September isotherm of 36.5°
Fahr. to the poles.

318. The Tropical Zone, or the zone of palms,
bananas, spices, and aromatic plants, lies on each
side of the equator, between the isotherms of 73°
Fahr. It includes most of the land within the
tropics of both hemispheres.

The excessive heat and moisture of this zone
produce an especial luxuriance in the vegetation.
Trees attain enormous size, the foliage is bright,
the flowers brilliant, and the number of species
great. The forests are characterized by the great
variety of trees, and when allowed to attain their
densest growth, are almost impenetrable, from the
numerous parasitic plants with which they are
covered, or the gigantic, rope-like climbers that
twine among them.

Palms, bananas, tree-like grasses, and orchids
are among the most characteristic plants.

Orchids are curious plants, inhabiting damp forests.
They attach themselves to trees and rocks, drawing nearly

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ne
Fig, 104.’ Palm-Trees,

all their nourishment from the air. Asa class, they are
noted for the fragrance, vivid coloring, and curious forms
of their flowers. The well-known vanilla bean is obtained
from an orchid. The humble grasses of our latitude, in
this zone, are represented by the bamboo, which often at-
tains the height of 60 feet.

The banyan-tree,. a species of fig-tree, is found in the

 

 

 

 

 

 

 

 

Fig, 105. Banyan-Tree,

East Indies. From a colossal trunk numerous air-branches
are sent out, which, descending to the ground, take root,