In vascular plants , the root is
the organ of a plant that
typically lies below the surface of the soil. This is not always the case,
however, since a root can also be aerial (growing
above the ground) or aerating (growing up above the ground or especially above
water). Furthermore, a stem normally occurring below ground is not exceptional
either (see rhizome ).
So, it is better to definerootas a part of a plant body that bears
no leaves, and therefore also lacks nodes . There are also important internal structural
differences between stems and roots.
The first root that comes from a plant is called the radicle .
The four major functions of roots are
1) absorption of water and inorganic
2) anchoring of the plant body to the ground and
3) storage of food
and nutrients and
4) to prevent soil erosion.
In response to the concentration
of nutrients, roots also synthesise cytokinin ,
which acts as a signal as to how fast the shoots can grow. Roots often function
in storage of food and nutrients. The roots of most vascular plant species
enter into symbiosis with certain fungi to form mycorrhizas , and a large range of other organisms including bacteria also
closely associate with roots.
Early root growth is one of the
functions of the apical
meristem located near the tip of the
root. The meristem cells more or less continuously divide, producing more
meristem, root cap cells
(these are sacrificed to protect the meristem), and undifferentiated root
cells. The latter become the primary tissues of the root, first undergoing
elongation, a process that pushes the root tip forward in the growing medium.
Gradually these cells differentiate and mature into specialized cells of the
Roots will generally grow in any
direction where the correct environment of air , mineral nutrients and water exists
to meet the plant's needs. Roots will not grow in dry soil. Over time, given
the right conditions, roots can crack foundations, snap water lines, and lift
sidewalks. At germination , roots grow downward due to gravitropism , the growth mechanism of plants
that also causes the shoot to grow upward. In some plants (such as ivy ),
the "root" actually clings to walls and structures.
Growth from apical meristems is known as primary growth , which encompasses all
growth encompasses all growth in
diameter, a major component of woody plant tissues
and many nonwoody plants. For example, storage roots of sweet potato have
secondary growth but are not woody. Secondary growth occurs at the lateral meristems , namely the vascular cambium and cork cambium . The former forms secondary xylem and secondary phloem , while the latter forms the periderm .
In plants with secondary growth, the
vascular cambium, originating between the xylem and the phloem, forms a cylinder of
tissue along the stem and
root. The vascular cambium forms new cells on both the inside and outside of
the cambium cylinder, with those on the inside forming secondary xylem cells,
and those on the outside forming secondary phloem cells. As secondary xylem
accumulates, the "girth" (lateral dimensions) of the stem and root
increases. As a result, tissues beyond the secondary phloem (including the
epidermis and cortex, in many cases) tend to be pushed outward and are
eventually "sloughed off" (shed).
At this point, the cork cambium begins
to form the periderm, consisting of protective cork cells
containing suberin. In roots, the cork cambium originates in the pericycle , a component of the vascular
The vascular cambium produces new layers
of secondary xylem annually. The xylem vessels are dead at maturity but are
responsible for most water transport through the vascular tissue in stems and