Terrestrial plants share a few defining characteristics, structural as well as
functional. Perhaps the most basic shared feature of most plants is their
division into shoots and roots. The separation between these two
portions of the plant came about during the evolutionary move from an aqueous
environment to a terrestrial one, and each part is essential in its own way to
the plants' ability to survive on land.
Figure %: A Generalized Plant
The root, defined as the portion of a plant beneath the soil, brings in
essential water and minerals from the soil. It also anchors the plant to the
substrate, providing stability.
The shoot includes all aerial plant structures such as stems, leaves, flowers,
and fruits. The shoot gathers the carbon dioxide and light energy necessary for
photosynthesis, provides surfaces for gas exchange, and contains the plant's
reproductive organs. Each of these parts, the root and the shoot, is dependent
on the other, for roots cannot perform photosynthesis and shoots cannot take in
water and inorganic nutrients.
Prevention of Water Loss
Plants share other structural qualities as well, most of which stem from their
adaptation to terrestrial conditions. All plants have reproductive structures
that prevent desiccation (drying out) of the gametes. These sex organs,
called antheridia (male) and archegonia (female), are themselves covered
by a layer of jacket cells that help to retain moisture.
In addition to the protection given to the sex organs, the plant surfaces
exposed to air are covered in a waxy layer, called a cuticle, that guards
against water loss. Gas exchange in plants is limited to pores in the leaf
epidermis called stomata, which can open and close to prevent excessive
evaporation of water into the environment.
Autotrophism
Most plants are autotrophs, organisms that synthesize all their own organic
nutrients and do not rely on other organisms for food. The reason that plants
are autotrophic is that they carry out photosynthesis in their leaves. In
the process of photosynthesis, the plant converts water, carbon dioxide, and
light energy into oxygen, sugars, and more water. The oxygen is released into
the surrounding air through the stomata, and the sugars (organic nutrients) are
transported throughout the plant body to areas of growth and storage.
Alternation of Generations
Finally, plants undergo a life cycle that takes them through both haploid
and diploid generations. The multicellular diploid plant structure is
called the sporophyte, which produces spores through meiotic division.
The multicellular haploid plant structure is called the gametophyte, which
is formed from the spore and give rise to the haploid gametes. The fluctuation
between these diploid and haploid stages that occurs in plants is called the
alternation of generations. For further discussion, see Life Cycle,
Alternation of Generations .
