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What are root's functions?[/SIZE]
The three universal functions of all roots are anchorage, absorption and translocation of water with dissolved mineral nutrients. In many perennial and biennial species, roots are also sites for food storage. These food reserves keep the plant alive through the non-growing season, and are used to resume growth in spring or after cutting or grazing. Some species that store food in their roots are yams, alfalfa and red clover. Food storage organs of some vegetables (carrots, beets, and radishes) are actually a combination of root and stem tissues.
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[SIZE=+1]Types of root systems
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There are two major types of root systems: fibrous and taproot (left). Grasses have fibrous root system. Their roots are adventitious, arising from the lowest nodes of the stems. Species with a fibrous system are more shallowly rooted than plants with a persistent taproot. Most dicots have a taproot system. The taproot originates from the primary root (radicle) of the seed. The taproot may have many branches originating from it. Roots of legumes may also have root nodules, which are sites for
nitrogen fixation .
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Zones of the root
A root can be divided into the mature zone, zone of maturation, zone of cell elongation, and the zone of cell division (the apical meristem) protected by the root cap (right). All of the root cells originate from the divisions of the cells of the
apical meristem. These cells are small, thin-walled, and contain large nuclei. Root meristem is protected by a
root cap.
The root cap is a dynamic, multifunctioning organ. For many years it was believed that the root cap functioned solely to protect the apical meristem of the root. Recently, it was shown that the cells of root cap percieve both light and gravity. Root caps of both dicots and monocots produce large numbers of metabolically active root "border" cells, which are programmed to separate from the root into the surrounding soil. In soil, border cells play important roles in protecting the roots from the soil-borne diseases (Hawes et al, 199
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What are the root tissues?[/SIZE]
The primary root tissues are the
epidermis, the outermost layer of cells covering the root surface, the
cortex that surrounds the stele, and the
vascular tissue or
stele, which occupies a central position.
The root
epidermis (
1 on the cross-sections below) is usually a single cell layer that protects the root. The cells of epidermis can elongate to produce
root hairs. These root hairs have larger surface area and are more efficient in absorbing water. Root hairs are also the sites of
Rhizobium invasion of the legumes.
Right: scanning electron micrograph of soybean root hairs.
above: crossection of a dicot root
below: crossection of a grass root
Let's focus on the micrographs of root crossections (left). The
cortex (2) is composed of thin-walled parenchyma cells, which are frequently arranged in radial rows or concentric circles. The root cortex region frequently functions as a major storage region, its parenchyma cells are packed with starch grains or other compounds. The innermost layer of the cortex is
endodermis (3). The endodermis is a single cellular layer enclosing the vascular cylinder.
The central region of the root (
stele) consists of
xylem (xy),
phloem (ph) and associated parenchyma cells. When xylem occupies the center of the root, it has variable number of extensions projecting outward toward the endodermis. The phloem tissue lies between these radiating arcs of xylem. Such anatomy is typical to the roots of dicots (carrots, beans, etc).
Where a pith is present, the vascular tissue takes the form of discrete strands of
xylem (xy) with alternating strands of
phloem (ph), like in the roots of corn, grains, turf and other grasses.
The main
function of
xylem is the upward transport of water and dissolved nutrients. The
phloem is the tissue through which
photosynthate, manufactured by the leaves and other green parts of the plant is translocated to other regions of the plant. At times of reserve mobilization, the phloem is also the tissue through which organic materials are transported from storage regions (roots and tubers) to support new growth.
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