transparent protoplasm, and containing a darker
opaque spot called the nucleus. Examined by a
sufficiently powerful glass, all living protoplasm
is seen to be in constant motion, currents passing
through the different parts in somewhat definite
directions.

As the germ cell develops, in all the higher
forms of life, it multiplies, and various organs
appear, peculiar to the form of life from which
the germ cell was derived. All living bodies
contain organs, and living matter is therefore
sometimes called organic matter, to distinguish it
from non-living or inorganic matter.

Science has not yet disclosed the nature of the change
whereby non-living matter is converted into living pro-
toplasm. To produce living matter the intervention of

already living matter is, so far as is known, absolutely
necessary. 3

312. Intermediate Position of Plants—Proto-
plasm forms an essential part of both plants and
animals. Plants alone, however, possess the power
of manufacturing protoplasm directly from inor-
ganic or non-living matter. Plants prepare food
for animals, who are, consequently, dependent on
plants for their existence. Both plants and ani-
mals are consumers of the proteid compounds.
Plants alone are producers. In the scale of ex-
istence plants, therefore, occupy a position inter-
mediate between minerals and animals.

313. Plant Geography treats of the distribu-
tion of plant-life over the earth.

Plant geography differs essentially from botany. Bot-
any arranges plants into regular classes, according to pe-

culiarities in their organs of growth and reproduction. '

Plant geography considers them only in reference either to
the more prominent appearances, by which they give a
distinct character to the vegetation of a country, or in
regard to their general usefulness to man.

In this limited view, all the minuter differences in
structure or organization are passed over, the general form
being the main geographical element of a plant, and the
element with which physical geography is principally in-
terested.

The plants of any section of country, taken
collectively, are called its flora.

314, Conditions Requisite for Plant Growth —
Plants require for their growth certain conditions
of light, heat, and moisture; and since the requi-
site amount of each of these varies with different
species of plants, we find in every climatic zone a

characteristic flora. The soil must contain those
mineral ingredients which form a)part of the
structure of the plant, and, moreover, must con-
tain them in a condition in which they can be
readily assimilated by the plant.

PLANT. GEOGRAPHY. 119

 

‘The substance of plants consists mainly of
water derived from the air and the soil. Analy-
sis shows that vegetable matter is composed almost
entirely of water, and various compounds of' car-
bon, hydrogen, oxygen, and sulphur. The water
is derived from the moisture of the soil and of the
air; the carbon, from the carbonic acid of the air.
The exceedingly small proportion of mineral mat-
ter comes directly from the soil. i

The nature of the soil, then, is far from being
the most important element in the distribution of
mere vegetation ; for, even when a soil is absent,
if the other requisites of light, heat, and moist-
ure are present, the simpler vegetable forms soon
appear, and slowly prepare, even on a bare,
rocky surface, a soil which is able to sustain
higher and still higher species. This is effected
by the breaking up of the hard mineral matter,
and the accumulation, year after year, of the de-
caying plants. In this way a vegetable mould is
produced. The bare surfaces which the conti-
nents possessed, when they first emerged from the
oceans, gained their covering of soil principally
in this way.

Moisture, Heat, and Light are the prime essen-
tials of vegetation, and it is on their distribution
that the distribution of vegetation is principally
dependent.

‘\ 815. Distribution of Vegetation—The influ-
ence of heat and moisture is noticed as we pass
from the equator to the poles, or from the base
of a tropical mountain to the summit. Thus
arises a horizontal and a vertical distribution of
vegetation.

The greatest luxuriance of vegetation is found
in the equatorial regions, where both heat and
moisture are most abundant. Here a greater va-
riety of species occurs, and the individual plants
are larger, and more brilliantly colored, both in
their leaves and flowers. As we pass toward the
poles, the number of the species diminishes; trees
disappear, being replaced by shrubs and herbs,
and these, in turn, by lichens and mosses, until

‘finally, amid the snows of the polar latitude,
even the simplest forms of vegetable life are
often wanting.

316.\Horizontal Distribution of Vegetation.

(1.) According to Meyen, we may divide the earth’s
surface into zones according to the latitude, and the moun-
tainous elevations into zones according to the altitude.
Since. the distribution of heat is not only dependent on
the latitude or altitude, we may advantageously modify

this plan as has been suggested by Dove, and divide the |
zones by the isotherms.