is one of the first parts of a plant that starts
to grow. A primary root develops from a
plant's seed and quickly produces branches called
secondary roots. At the tip of each root
is a rootcap that protects the delicate
tip as it pushes through the soil. Threadlike root
hairs grow farther back on the root. These
hairs greatly increase the plant's ability to
absorb water and minerals from the soil.
Kinds of Roots
There are two main kinds of
root systems -- fibrous and taproot.
Grass is an example of a plant with a fibrous
root system. It has many slender roots of about
the same size that spread out in all directions.
A plant with a taproot system has one root that
is larger than the rest. Carrots and radishes
have taproots. Taproots grow straight down, some
as deep as 15 feet.
plants have modified roots that perform special
functions. Roots that grow from the stem above
the ground are called adventitious roots.
They include the prop roots of corn and
certain other plants. Prop roots grow down into
the soil from the lower part of the stem and help brace the
plant against the wind. Some species of orchids
and other plants that live on tree branches send
out aerial roots, which cling to the
branches. Aerial roots absorb water and minerals
from the surface of the tree and from the air.
Mistletoe is one of the few plants with roots
that penetrate the limbs of a tree. These roots,
called sinkers, absorb food, water, and
minerals directly from the tree.
Parts of a Root
The Root Tip. A root
grows in length from an area at its apex
(tip). This growth area is called the apical
meristem. A meristem is any part of a plant
where the cells divide rapidly, forming new cells
continually. The apical meristem is covered by
the root cap, a thimble-shaped group of
cells. The root cap protects the delicate root
tip from damage as the root grows in length and
the tip pushes through the soil.
The cells produced by the apical meristem are
all small and nearly identical. In the region
of elongation, the cells rapidly grow
longer. Farther back lies the region of
maturation, where the cells differentiate
-- that is, take on a different structure and
appearance according to their functions in the
The Outer Tissues.
The outer layer of cells of a root is called the epidermis.
It serves as a sort of skin and protects the
tissues beneath. Tiny, hairlike extensions called
root hairs grow from the epidermis. The
root hairs absorb most of the water and minerals
that a plant takes in from the soil.
A thick layer of rounded cells called the cortex
lies just inside the epidermis. These cells
contain stored food and water. The inner layer of
cells of the cortex make up the endodermis.
Core, or stele, is the
central portion of the root. Its outer layer of
cells is called the pericycle. Inside
the pericycle are two kinds of tissues, xylem
and phloem. Xylem includes rows of dead,
tubular cells called vessels, which
conduct water and minerals up to the stem and leaves. Phloem consists
largely of rows of long, living cells called sieve
tubes. These cells transport food down from
the leaves for use or storage by the root.
All the tissues described above are known as primary
tissues. Many plants that live just one year
have only primary tissues in their roots. But
other plants, especially those that live more
than one year, have secondary tissues in
their roots in addition to primary tissues. The
growth of primary tissue adds to the length of a
root. The development of secondary tissue adds to
its thickness. Secondary-tissue growth produces
the large, brown, woody roots in trees, shrubs,
and other plants that live for many years.
Secondary tissues develop from two meristems.
One, called the cork cambium, originates
beneath the epidermis, generally in the
pericycle. It produces cork cells and pushes them
toward the outside of the root. As the cork
expands outward, the endodermis, cortex, and
epidermis die and peel off. The cork replaces
them and becomes the outer covering of the root.
The other secondary meristem, the cambium,
lies between the primary xylem and the primary
phloem. It produces secondary xylem cells toward
the center of the root, and secondary phloem
cells toward the outside.
World Book Encyclopedia
Chicago: World Book-Childcraft International,
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