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Session 2 Reading
INTRODUCTION
Plant, any member of the plant kingdom comprising about 260,000 known
species of mosses, liverworts, ferns, herbaceous and woody plants, bushes,
vines, trees, and various other forms that mantle the earth and are also
found in its waters. Plants range in size and complexity from small,
nonvascular mosses, which depend on direct contact with surface water, to
giant redwood trees, the largest living organisms, which can draw water and
minerals through their vascular systems to elevations of more than 100 m
(more than 330 ft).
Impact of Plants
Though the first plants appeared on land only about half a billion years
ago, today they account for by far the largest proportion of the earth’s
biomass. From towering redwoods to almost microscopic species of
duckweed, the plant kingdom is an extraordinarily diverse and long-lived
group that makes the life of animals, fungi, and other organisms
possible. Its members provide oxygen, shelter, and the foundation of the
food web, determining the traits of the organisms that depend on them
in a huge variety of habitats. Their beauty, fragrance, and amazing traits
fascinate and bring intellectual and aesthetic pleasure to many humans.
Only a tiny percentage of plant species are directly used by humans for food,
shelter, fiber, and drugs. At the head of the list are rice, wheat, corn,
legumes, cotton, conifers, and tobacco, on which whole economies and
nations depend. Of even greater importance to humans are the indirect
benefits reaped from the entire plant kingdom and its more than 1 billion
years of carrying out photosynthesis. Plants have laid down the fossil fuels
that provide power for industrial society, and throughout their long history
plants have supplied sufficient oxygen to the atmosphere to support the
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evolution of higher animals. Today the world's biomass is composed
overwhelmingly of plants, which not only underpin virtually all food webs
(see Food Web) but also modify climates and create and hold down soil,
making what would otherwise be stony, sandy masses habitable for life.
Redwood National Park, which covers 446 sq
km (172 sq mi) along California’s northwestern
coast, represents one of the last remaining
ancient redwood forests in the world. Here,
heavy coastal rainfall and lack of exploitation
by humans has encouraged the dramatic
growth of some of the largest trees on earth.
Some of these redwoods are believed to be
more than 2300 years old.
The Great Basin bristlecone pine can
live over 4000 years and is believed to
be one of the oldest living trees on the
planet. All members of the pine family
have needlelike leaves, generally longer
than those of other conifers, which
appear in clusters of two to five,
depending on the species. Pines are
extremely adaptable to a range of
climatic and soil conditions and are
widespread in their distribution.
DIFFERENTIATION FROM OTHER KINGDOMS
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Plants are multicellular green organisms; their cells contain eukaryotic
(nucleated) protoplasm held within more or less rigid cell walls composed
primarily of cellulose. The most important characteristic of plants is their
ability to photosynthesize—that is, to make their own food by converting
light energy into chemical energy—a process carried out in the green,
chlorophyll-containing plastids (cellular organelles) called chloroplasts (see
Chlorophyll; Chloroplast). A few plants have lost their chlorophyll and have
become saprophytic or parasitic in nutrition—that is, they absorb their food
from dead organic matter or living organic matter, respectively—but details
of their structure show that they are evolved plant forms.
Fungi, also eukaryotic and long considered members of the plant kingdom,
have now been placed in a separate kingdom because they lack chlorophyll
and plastids and because their rigid cell walls contain chitin rather than
cellulose. Fungi do not manufacture their own food; instead they absorb it
from either dead or living organic matter.
The various groups of algae were also formerly placed in the plant kingdom
because many are eukaryotic and because most have rigid cell walls and
carry out photosynthesis. Nonetheless, because of the variety of pigment
types, cell wall types, and morphological expression found in the algae, they
are now recognized as part of two separate kingdoms, containing a diversity
of plantlike and other organisms that are not necessarily closely related. One
of the phyla of algae—the green algae—is believed to have given rise to the
plant kingdom, because its chlorophylls, cell walls, and other details of
cellular structure are similar to those of plants.
The animal kingdom is also multicellular and eukaryotic, but its members
differ from the plants in deriving nutrition from other organic matter; by
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ingesting food rather than absorbing it, as in the fungi; by lacking rigid cell
walls; and, usually, by having sensory capabilities and being motile, at least
at some stage.
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