The root can be generally
defined as the descending
axis of a
plant which
serves to
anchor the plant and to
conduct water and
minerals to the
stem and
leaves. Roots are also specially
adapted for other purposes in several common plant species (see Modifications).
General description and appearance
As a plant begins its
life cycle, the primary root is produced in the
embryo. In
gymnosperms and
dicotyledons, this root becomes the
taproot, which grows
directly downwards and gives rise to
lateral roots, which branch off the
taproot. In
monocotyledons, the primary root is shorter-lived, and is used to
initially anchor the young plant to the ground. The root system, instead of
dividing from a primary root, derives from the stem in a
fibrous pattern. If you
unearth a plant of each type, and clean the
soil away from the roots, the
taproot system looks like an
inverted
tree (where the taproot is the trunk),
while the
fibrous root system looks like a
head of hair.
Roots can penetrate to great depths in some species, if the soil conditions
permit. For example, mesquite roots have been found to penetrate up to 53 meters
underground. In other circumstances, the root system may be constrained to very
shallow depths. Pine trees growing on the Canadian shield may have roots that lie
directly on the rocks' surface, and gather soil around them making their depth no
more than 15 centimeters.
Morphology and physiology
The tip of a root is covered by a
root cap, which is a small mass of cells in
the general shape of a
bowl. Its primary purpose is to
protect the root as it
penetrates deeper in to the soil. The root cap is covered by a slimy
gel (the
mucigel) which
lubricates the tip of the root as it forces its way through the
substrate. Immediately behind the root cap is the
apical meristem, from which
new
cells are produced as the root grows.
Above the apical meristem are the regions of cell division and elongation, in
which cells produced by the apical meristem divide rapidly to produce more length
and increase the girth of the root. Beyond these two regions lies the region of
maturation, at which the newly produced cells grow, mature and begin to perform
their absorbing and transport functions. In this region, root hairs (tubular
extentions of epidermal cells) are also produced. In most plants, the each root
may have staggering numbers of root hairs: in one four-month old rye plant,
there were approximately 14 billion root hairs, with a surface area of roughly
401 square meters. Root hairs die off as a section of root thickens and ages; root hairs are mostly confined to the youngest parts of a root.
The histology and morphology of the region of maturation, which comprises the majority of the root, is remarkably simple when compared with the stems and leaves. There are three tissue types in a mature root, from the outside in: the epidermis, the cortex and the vascular cylinder.
The outer part of a root is the epidermis. This layer of cells is, structurally, the simplest in the root. The epidermis of the younger parts of a root and the root hairs absorbs water and minerals. As a section of root ages, however, the absorbing function ceases and the epidermis toughens and thickens. At this point is plays a protective role only, shielding the root from mechanical damage.
The cortex of the root comprises the greatest volume of the root. The cells of
the cortex store starches and sugars, but do not have any chloroplasts. In
larger and longer-lived plants, this region is shed as the roots age, but in
annual plants roots generally retain this region throughout the life of the
root. The cells in the cortex are normally organized in a semi-haphazard manner,
but in such a way as to leave air spaces between the cells. Tbese spaces are
essential for the aeration of the root. Without these spaces, the cells of the
root would be deprived of oxygen and would die rapidly. Despite these large and abundant air spaces, the cortical cells do touch one another, allowing water and mineral transport by diffusion between cells. They are also connected by plasmodesmata (little cytoplasmic bridges). As such, these cells are capable of rapidly transporting water and minerals from the epidermis to the central transport cells. The inner layer of
the cortex, which is tightly packed and lacks air spaces, is called the
endodermis. All materials to be transported from the roots to the stems and
leaves must pass through this tightly packed wall of cells, and may do so
through direct transport through cells walls or via plasmodesmata.
The inner core of the root is reserved for the transport cells, and is called
the vascular cylinder. The vascular tissues in this cylinder are surrounded by
the pericycle, which is used to produce the cork cambium, as the root ages, and lateral roots. Inside the pericycle lie the main transporting tissues, the primary xylem and phloem. As the root ages, secondary xylem and phloem are produced and a cork cambium forms.
Modifications
Some plants never touch the soil, and thus their roots
dangle in the air (these
plants are called
epiphytes). Their roots are very fine, and are occasionally
photosynthetic. The flower pot plant (
Dischidia rafflesiana) in
particular has a fascinating
body plan. Some of its leaves are formed like
hollowed containers that collect debris and rainwater.
Ant colonies live in
these pots, and contribue
nitrogen to the plant by
defecating in these bowls. The roots are produced from the stem, in the
node above the bowl, and grow into
the bowl to absorb these minerals and water. In essence, it fertilizes and
waters itself!
While all roots contain some storage cells (in the cortex, see above), some
plants have specially modified root structures used to store great quantities of
energy. Some common examples can be found on your dinner plate: the carrot,
sweet potato, beet1. While externally similar in appearance, these swollen structures may be histologically quite different. For example, the beet aquires its girth through the production of extra cambium, around the original vascular
cambium, while the carrot simply produces an overabundance of parynchemal cells
in the cortex and the secondary xylem and phloem.
1Note that the potato, despite similarities in appearances with the carrot and beet, is not infact the result of modified root tissues. It is a tuber, a specially modified stem.
Composed with help from Raven, P. H., Evert, R. F. and S. E. Eichorn. (1992) Biology of Plants, 5th Ed. Worth Publishers.