PLANT GEOGRAPHY. 123

 

as the pine, spruce, hemlock, cedar, and fir. In
the northern portions of the zone, beeches and
alders are found, and willows, when the soil is
moist. The meadows are covered with grasses
and flowers, and afford abundant pasturage.

The northern limit of trees is marked on the map of
the plant regions.

323. The Polar Zone, or the Zone of Alpine
Shrubs, Mosses, Lichens, and Saxifrages, extends
from the limits of the sub-arctic zone to the pole.
In this zone, no trees occur except those of a stunted
growth. Alpine shrubs, or those of tortuous, com-
pact growth, such as the Alpine rhododendra, the
dwarf birch, willow, and alder, occur. Sedges and
grasses are found. The pastures of the preceding
zones are absent; in their place we find extended
areas covered with lichens.

The northern plains of Siberia are covered with exten-
sive marshes, called Tundras, where the ground, during
most of the year, is frozen to great depths. The short

summers only suffice to thaw the surface, when a few
mosses and lichens appear. :

25,000 feet.

20,000

 

Near the extreme northern limits of the North
Polar zone, from the limit of the isotherm of 41°
Fahr. for the month of July, such plants only are
found as can thrive during the brief Arctic sum-
mer of from four to six weeks. Shrubs are en-
tirely absent; lichens and mosses occur, together
with stunted Alpine herbs. In Spitzbergen, lich-
ens and mosses are found, the former being espe-

‘cially numerous.

324, The Vertical Distribution of Vegetation.—It
is difficult to make a good systematic arrangement of vege-
tation into vertical zones, since the temperature and moisture,
on which such an arrangement must be based, are subject
to very considerable variations. Thus, the position of the
mountain-ranges as regards the prevalent wind, the direc-
tion of the mountain slopes, and the extent of the elevated
plateaus, all exert such a powerful influence on the mean
annual temperature and the rainfall, that even in the
same range, opposite slopes, or even different parts of the
same slope, afford very marked climatic contrasts. In
ranges that are widely separated, the differences are still
greater. The following chart exhibits the characteristic
flora in tropical America, Africa, Europe, and Asia, at
similar elevations.

 

15,000

10,000

5,000

(1.) Between the level of the sea and 5000 feet,
the vegetation is, in general, the same as in the tropical
and sub-tropical zones. Palms, bananas, and tree-ferns oc-
cur in the lower parts, and barley, potatoes, sugar-cane,
rice, cotton, etc., as marked on the chart.

(2.) Between 5000 and 10,000 feet, the vegetation
is, in general, the same as in warm temperate zones. In

America, the birch and cedar occur in the lower portions ©

of the region, and Peruvian bark and the cinchona trees,
so useful in medicine, in the upper portions. In Africa
and Europe, the pine, birch, and oak occur; and in Asia,
the oak ; here also the vine is cultivated.

(3.) Between 10,000 and 15,000 feet, the vegeta-
tion, in general, is that of the cold temperate zones. De-
ciduous trees occur; rye, wheat, barley, and oats are cul-
tivated.

(4.) Between 15,000 and 20,000 feet, the flora cor-
responds, in general, to that of the polar and .arctic zones.
A few rhododendrons and birches occur on the warmer
Asiatic slopes, and occasionally crops of barley are culti-

Fig. 107, Vertical Distribution of Vegetation. (After BLack.)

vated. The greater part of this zone is covered by eternal
snow, as is the case with all greater elevations,

In the descriptions here given, it will be noticed that
the correspondence of the vertical and horizontal zones is
but of a very general character. The names of the plants
on the chart mark the limits at which they will grow.

325. Plant Regions——In some localities, a few
plants occur over extended areas, in such vast
numbers as to give a characteristic appearance to
the country they cover. A brief mention will be
made of such regions, especially as they illustrate
the influence of the presence or absence of moist-
ure on the vegetation. :

. 826. Forests occur wherever the moisture is
abundantly and regularly distributed throughout
the year. Asa rule, forests are limited to those
portions of the world where the rain falls at all