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Roots:The primary root, or radicle, is the first organ to appear
when a seed germinates. It grows downward into the soil,
anchoring the seedling. In gymnosperms and dicotyledons, the
radicle becomes a taproot. It grows downward, and branch, or
secondary, roots grow laterally from it. This type of system is
called a taproot system. In some plants, such as carrots and
turnips, the taproot serves as a storage organ and becomes
swollen with foodstuffs.
Grasses and other monocotyledons have a fibrous root system,
characterized by a mass of roots of about equal diameter. This
network of roots does not arise as branches of the
primary root but consists of many branching roots that
emerge from the base of the stem.
Roots grow in length only from their ends. The very tip of the
root is covered by a protective, thimble-shaped root cap. Just
behind the root cap lies the apical meristem, a tissue of
actively dividing cells. Some of the cells produced by the apical
meristem are added to the root cap, but most of them are
added to the region of elongation, which lies just above the
meristematic region. It is in the region of elongation that
growth in length occurs. Above this elongation zone lies the
region of maturation, where the primary tissues of the root
mature, completing the process of cell differentiation that
actually begins in the upper portion of the meristematic
region.
The primary tissues of the root are, from outermost to
innermost, the epidermis, the cortex, and the vascular
cylinder. The epidermis is composed of thin-walled cells and is
usually only one cell layer thick. The absorption of water and
dissolved minerals occurs through the epidermis, a process
greatly enhanced in most land plants by the presence of root
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hairs—slender, tubular extensions of the epidermal cell wall
that are found only in the region of maturation. The
absorption of water is chiefly via osmosis, which occurs
because (1) water is present in higher concentrations in the soil
than within the epidermal cells (where it contains salts,
sugars, and other dissolved organic products) and (2) the
membrane of the epidermal cells is permeable to water but not
to many of the substances dissolved in the internal fluid. These
conditions create an osmotic gradient, whereby water flows
into the epidermal cells. This flow exerts a force, called root
pressure that helps drive the water through the roots. Root
pressure is partially responsible for the rise of water in plants,
but it cannot alone account for the transport of water to the
top of tall trees.
The cortex conducts water and dissolved minerals across the
root from the epidermis to the vascular cylinder, whence it is
transported to the rest of the plant. The cortex also stores food
transported downward from the leaves through the vascular
tissues. The innermost layer of the cortex usually consists of a
tightly packed layer of cells, called the endodermis, which
regulates the flow of materials between the cortex and
the vascular tissues.
The vascular cylinder is interior to the endodermis and is
surrounded by the pericycle, a layer of cells that gives rise to
branch roots. The conductive tissues of the vascular cylinder
are usually arranged in a star-shaped pattern. The xylem
tissue, which carries water and dissolved minerals, comprises
the core of the star; the phloem tissue, which carries food, is
located in small groups between the points of the star.
The older roots of woody plants form secondary tissues, which
lead to an increase in girth. These secondary tissues are
produced by the vascular cambium and the cork cambium.
The former arises from meristematic cells that lie between the
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primary xylem and phloem. As it develops, the vascular
cambium forms a ring around the primary vascular cylinder.
Cell divisions in the vascular cambium produce secondary
xylem (wood) to the inside of the ring and secondary phloem
to the outside. The growth of these secondary vascular tissues
pushes the pericycle outward and splits the cortex and
epidermis. The pericycle becomes the cork cambium,
producing cork cells (outer bark) that replace the cortex and
epidermis.
Some roots, called adventitious roots, arise from an organ
other than the root—usually a stem, sometimes a leaf. They
are especially numerous on underground stems. The formation
of adventitious roots makes it possible to vegetatively
propagate many plants from stem or leaf cuttings.
Roots are not always underground. When they arise from the
stem and either pass for some distance through the air before
reaching the soil or remain hanging in the air, they are
called aerial. They are seen well in corn (maize), screw pine,
and banyan, where they eventually assist in supporting the
plant.
Root Cross-section:Epidermis: absorbs water and nutrients into the root
Root Hairs: grow on the root surface to aid in the absorption
of nutrients and water
Cortex: storage of sugars and starches
Xylem: transports water through the root
Phloem: transports water through the root
Endodermis: separates vascular bundle from cortex
Casparian Strips: water-proof strips around endodermis cells
that prevent water from moving around the endodermis, so
that water flows through them.
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Root Zones:Root
Zone
Zone
Zone
Cap- protects growing tip
of Cell Division- new cells are produced
of Cell Elongation- cells grow longer
of Maturation- cells mature and root hairs grow
Tap Root and Fibrous Root:1. Taproot system – a strongly developed main root which
grows downwards bearing lateral roots much smaller
than itself.
a. In most dicots, the radicle enlarges to form a
prominent taproot that persists throughout the life
of the plant.
b. Many progressively smaller branch roots grow from
the taproot.
c. This system is called a taproot system; common
in dicots and conifers.
d. In plants such as carrots and sugar beets, fleshy
taproots store large reserves of food, usu. as
carbohydrates.
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e. Taproots are modified for reaching deep water in
the ground: e.g. the long taproots of poison ivy
(Rhus toxicodendron), dandelion (Taraxacum sp.)
and mesquite (Prosopis sp.).
2.Fibrous root system – has several to many roots of the
same size that develop from the end of the stem, with
smaller lateral roots branching off of them.
a. Most monocots (including grasses and onions) have
a fibrous root system.
b. In these plants, the radicle is short-lived and is
replaced by a mass of adventitious roots (from the
Latin, adventicius, meaning “not belonging to”),
which are roots that form on organs other than
roots. Because these roots arise not from preexisting
roots, but from the stem, they are said to be
adventitious.
c. The adventitious roots of monocots are very
extensive and cling tenaciously to soil particles.
These plants are excellent for preventing erosion.
d. The fibrous root of a few plants is edible – sweet
potatoes (Ipomoea batatas) are the fleshy part of a
fibrous root system.
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Functions and structure of roots:1. Absorption – roots absorb large amounts of water and
dissolved minerals (nitrates, phosphates, and sulfates) from the
soil.
2. Anchorage – to locate water and minerals, roots permeate
the soil. In doing so, they anchor the plant in one place for
its entire life.
3. Storage – roots store large amounts of energy reserves,
initially produced in the leaves of plants via photosynthesis,
and transported in the phloem, as sugar, to the roots for
storage, used as sugar or starch, until they are needed.
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1. Absorption – most water and nutrients are absorbed by
roots hairs (in zone of maturation). Root hairs extend the
absorptive surface of roots that is in contact with moist
soil several thousand-fold. Root hairs are short-lived,
single-celled extensions of the epidermal cells near the
growing root tip. Root hairs form only in the maturing,
non-elongating region of the root. They are fragile
extensions of epidermal cells and are easily broken off.
a. Mycorrhizae – the roots of most plant species form a
mutually beneficial relationship with certain soil fungi.
b. Mycorrhizae enable plants to absorb adequate amounts of
certain essential minerals (such as phosphorus) form the soil.
c. Minerals absorbed from the soil by the fungus, travels to
the roots, and carbohydrates produced by photosynthesis in
the plant travel to the fungus.
d. Mycorrhizae often enhance plant growth, and
when mycorrhizae are not present, neither the fungus nor
the plant grows as well.
e. Roots of some plants, like legumes (peas, beans, mesquites)
form an association with nitrogen-fixing
bacteria (Rhizobium, Frankia). Swellings, called nodules,
develop on roots and house millions of the bacteria.
f. Like mycorrhizae, the association between nitrogen-fixing
bacteria and roots is mutually beneficial.
g. The bacteria receive products of photosynthesis from the
plants while helping the plant to meet its nitrogen
requirement.
h. Cortical cells are those infected with the Rhizobium, which
infects the roots through the root hairs, and then forms
infection threads that permeate the root.
2. Anchorage – relatively little absorption occurs past a
few centimeters beyond the root tip, because these parts
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of the root lack root hairs and have a
heavilysuberized endodermis (innermost layer of the
cortex). (Suberin – waxy substance that occurs in both
cork cells and in cells of underground plant parts.
Consists of hydroxylated fatty acids. Impervious to
water). These non-absorptive regions of roots anchor
plants and may later produce branch roots.
3. Conduction and storage – water and dissolved minerals
absorbed by roots move to the shoot in xylary elements.
4. Movement – Each root tip has a root cap, a protective
thimble-like layer of many sells that covers the delicate
root apical meristem. The root cap also appears to be
involved in orienting the root so that it grows downward.
It can sense light, pressure and, perhaps, gravity. It
produces and secretes mucigel, which protects and
lubricates roots.
Specialized roots:1. Prop roots - are adventitious roots that develop from
branches or from vertical stem and grow downward into the
soil to help support the plant in an upright position.
2. Pneumatophores – “breathing roots”
a. In swampy or tidal environments where the soil is
flooded or water-logged, roots often grow upwards
until they are above the high-tide level. Even though
roots live in the soil, they still require oxygen for
aerobic respiration. A flooded soil is depleted of
oxygen, so these aerial, “breathing roots” may assist
in getting O2 to the submerged roots.
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b. Plants such as black mangrove
(Avicennia germinans) avoid suffocation by
producing these roots that impor oxygen from the
atmosphere. Thepneumatophores contain as much
as 80% aerenchyma, grow up in the air, and
function much like snorkels, thru which oxygen
diffuses to submerged roots.
3. Epiphytes – plants that grow attached to other plants.
a. Epiphytes and climbing plants have aerial roots that
anchor the plant to the bark, branch, or other
surface on which it grows.
b. Aerial roots of some epiphytes are specialized not
only for anchorage, but some have photosynthetic
roots (some epiphytic orchids), some absorb water.
c. Some parasitic epiphytes, such
as mistleltoe (Phorodendron sp, etc.), have roots
that penetrate the host plant tissues and absorb
nutrients.
4. Suckers – aboveground stems that develop from
adventitious buds on the roots.
a. Each sucker develops additional roots and becomes
an independent plant when the parent dies.
b. Plants that form suckers include cottonwood,
Lombardy poplar, pear, apple, cherry, blackberry,
etc.
Roots generally grow away from the light, and light inhibits
root growth in corn, wheat, peas, and rice. Light is sensed by
the root cap and inhibits growth by slowing the rates of
cellular division and elongation.
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Hibiscus- a dicot plant:-
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Hibiscus is one of the most sought after flowering plants. The
Hibiscus flower is both huge and delicate, as if they were
made out of airy crepe paper. There are many incredible color
variations, which range from hot pink to plum. Others are
bicolored or have attractive dark veins.
Hibiscus plants are divided into two main categories, the
tropical flowering hibiscus and the hardy flowering hibiscus
family. The tropical hibiscus is not winter hardy in areas
outside of climates such as the state of Florida. The Hardy
Hibiscus is reliably hardy in zone 4 and, with extra
protection, marginally hardy in zone 3. Hardy hibiscus can be
grown and enjoyed as far north as Minnesota and New York.
Hardy hibiscus need very little care over the winter; they are
root hardy to about zone 5 with no protection. They die back
to the ground each year. Hardy hibiscuses grow very quickly
once they break ground in late spring.
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The tropical hibiscus will not tolerate more than a night or
two of light frost. One hard frost, below 25 degrees could kill
the plant. These plants are native to sunny, warm and usually
humid tropical places. They would need to be brought inside in
zones outside of zone 9 before temperatures drop below 45
degrees Fahrenheit at night to avoid damage.
Wheat- a monocot plant:-
Wheat (Triticum aestivum) is the most important crop in the
temperate zone. The oldest traces of wheat cultivation are
from the seventh pre-Christian millennium in the Middle East.
With its subsequent spread to Europe, North Africa and Asia,
wheat became an important crop for ancient cultures and
civilisations.
“Einkorn” wheat (T. monococcum) is the oldest form of
cultivated wheat; some wild forms still exist today.
Wheat is an annual plant sown in spring or in autumn.
Wheat flowers from the end of May to the beginning of June.
The flowers on one plant do not open simultaneously. That’s
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why the flowering time of a wheat cultivar can last more
than a week.
Normally, self-pollination occurs, which means wheat plants
fertilize themselves with their own pollen before flowers even
open. Nevertheless – depending on genotype and climatic
conditions – cross-pollination with other wheat plants is
possible. It usually occurs at a rate of approximately one to
two percent. The rate can increase up to 9.7 percent when
weather conditions are dry and warm.
Wheat pollen is carried by wind. Dissemination is limited by
its relatively high weight and small quantities. Furthermore,
wheat pollen only remains viable for a very short period of
time (a few minutes to three hours).
The genome structure of modern wheat is much different than
its wild ancestors. Its set of chromosomes has multiplied sixfold in the case of bread wheat (T. aestivum) and spelt (T.
spelta). These two forms can interbreed and produce fertile
offspring. The fertility of hybrids between plants with
different numbers of chromosome sets is very limited. Durum
wheat (T. durum, T. trugidium) and emmer wheat
(T. dicoccum) have four sets of chromosomes, which makes
them very unlikely to form fertile hybrids with bread wheat.
The offspring of crosses with T. monococcum (two sets of
chromosomes) are usually sterile.
Some cases of wheat crossing with wild relatives have been
reported. Possibilities include quack grass
(Agropyron), rye (Secale cereale), and several others
(e.g. Elymus, Hordeum, Leymus, Setaria, Sorghum). Most of
the time, such crosses are only possible using artificial
methods.
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Remarks:-