|The root system of a plant
constantly provides the stems and leaves with water and dissolved minerals. In order
to accomplish this the roots must grow into new regions of the soil. The growth and
metabolism of the plant root system is supported by the process of photosynthesis
occurring in the leaves. Photosynthate from the leaves is transported via the
phloem to the root system. Root structure aids in this process. This section
will review the different kinds of root systems an look at some specialized roots, as well
as describe the anatomy of the roots in monocots and dicots.
Characterized by having one main root (the taproot) from which smaller branch roots
emerge. When a seed germinates, the first root to emerge is the radicle, or primary
root. In conifers and most dicots, this radicle develops into the taproot.
Taproots can be modified for use in storage (usually carbohydrates) such as those found in
sugar beet or carrot. Taproots are also important adaptations for searching for
water, as those long taproots found in mesquite and poison ivy.
Top of Page
Fibrous Root System:
Characterized by having a mass of similarly sized roots. In this case the
radicle from a germinating seed is short lived and is replaced by adventitious roots.
Adventitious roots are roots that form on plant organs other than roots. Most
monocots have fibrous root systems. Some fibrous roots are used as storage; for
example sweet potatoes form on fibrous roots. Plants with fibrous roots systems are
excellent for erosion control, because the mass of roots cling to soil particles.
Top of Page
Root Structures and Their Functions:
Root Tip: the end 1 cm of a root
contains young tissues that are divided into the root cap, quiescent center, and the
Root Cap: root tips are covered and protected by the root cap.
The root cap cells are derived from the rootcap meristem that pushes cells forward
into the cap region. Root cap cells differentiate first into columella cells.
Columella cells contain amylopasts that are responsible for gravity detection.
These cells can also respond to light and pressure from soil particles. Once
columella cells are pushed to the periphery of the root cap, they differentiate into
peripheral cells. These cells secrete mucigel, a hydrated polysaccharide formed in
the dictyosomes that contains sugars, organic acids, vitamins, enzymes, and amino acids.
Mucigel aids in protection of the root by preventing desiccation. In some
plants the mucigel contains inhibitors that prevent the growth of roots from competing
plants. Mucigel also lubricates the root so that it can easily penetrate the soil.
Mucigel also aids in water and nutrient absorption by increasing soil:root contact.
Mucigel can act as a chelator, freeing up ions to be absorbed by the root.
Nutrients in mucigel can aid in the establishment of mycorrhizae and symbiotic bacteria.
Quiescent Center: behind the root cap is the
quiescent center, a region of inactive cells. They function to replace the
meristematic cells of the rootcap meristem. The quiescent center is also important
in organizing the patterns of primary growth in the root.
Subapical Region: this region, behind the quiescent center
is divided into three zones. Zone of Cell Division - this is the location of the
apical meristem (~0.5 -1.5 mm behind the root tip). Cells derived from the apical
meristem add to the primary growth of the root. Zone of Cellular Elongation - the
cells derived from the apical meristem increase in length in this region. Elongation
occurs through water uptake into the vacuoles. This elongation process shoves the
root tip into the soil. Zone of Cellular Maturation - the cells begin
differentiation. In this region one finds root hairs which function to increase
water and nutrient absorption. In this region the xylem cells are the first of the
vascular tissues to differentiate.
Top of Page
Mature Root: the primary tissues
of the root begin to form within or just behind the Zone of Cellular Maturation in the
root tip. The root apical meristem gives rise to three primary meristems: protoderm,
ground meristem, and procambium.
Epidermis: the epidermis is derived from the protoderm and
surrounds the young root one cell layer thick. Epidermal cells are not covered by
cuticle so that they can absorb water and mineral nutrients. As roots mature the
epidermis is replaced by the periderm.
Cortex: interior to the epidermis is the cortex which is derived
from the ground meristem. The cortex is divided into three layers: the hypodermis,
storage parenchyma cells, and the endodermis. The hypodermis is the suberinized
protective layer of cells just below the epidermis. The suberin in these cells aids
in water retention. Storage parenchyma cells are thin-walled and often store
starch. The endodermis is the innermost layer of the cortex. Endodermal cells
are closely packed and lack intercellular spaces. Their radial and transverse walls
are impregnated with lignin an suberin to form the structure called the Casparian Strip.
The Casparian Strip forces water and dissolved nutrients to pass through the
symplast (living portion of the cell), thus allowing the cell membrane to control
absorption by the root.
Stele: all tissues inside the endodermis compose the stele.
The stele includes the outer most layer, pericycle, and the vascular tissues. The
pericycle is a meristematic layer important in production of branch roots. The
vascular tissues are made up of the xylem and phloem. In dicots the xylem is found
as a star shape in the center of the root with the phloem located between the arms of the
xylem star. New xylem and phloem is added by the vascular cambium located between
the xylem and phloem. In monocots the xylem and phloem form in a ring with s the
central portion of the root made up of a parenchymatous pith.
Top of Page
Fibrous Root System
Root Structures & Their Functions
Root Tip: young root tissues
Back to Main Page