Roots of angiosperms

The root
• The prolongation of the radicle (first root axis arises from
cells laid down in the seed)of the embryo is ROOT.
• Principally, the underground organ of the plant body which
absorbs water and minerals from the soil transporting them
to other parts of the plant body.
• Due to this, root tends to grow downwards, away from light
and towards water.
• As a general rule, they bear neither leaves nor buds.
• The primary roles are anchorage, absorption and transport.
• However, it has adapted to fulfil a variety of other functions
including storage, support and aeration.
• Compared with stem, root is relatively simple and uniform in
structure.

Root Structure
• The main root, primary root or Tap root is
formed from the radicle. The lateral branches
of main root are called secondary roots which
are further branched to form tertiary roots.
Roots are absent in some angiosperms, e.g.
genus utricularia ( free-
floating, aquatic, canivorous plants that trap
and digest very thin animal).

Four zones or regions are commonly recognized
in developing roots namely:
1. the root cap,
2. the meristematic zone (zone of cell division),
3. the elongation zone, and
4. the maturation zone.

Zone of elongation: This root is covered by point occurring
Meristematic tip of
Root cap: The zone: the zone is a growing a cap (root
Zone of maturation:
Newsthat is shaped likemitosiscap.specific cell meristems
cap) cells produced by arootinto the is composed of two
Elongated cells differentiate by It primary
immediately behind the thimble. Meristematic region
elongate of meristematiccolumella (they look likeby the root
consists rapidly and become several times longer
types (e.g. xylem and phloem) in the zone of than wide.
types,of cells, the inner tissue and is protected columns)
Thus, their width also increases slightly. The small vacuoles
cap. and theThe the apical rootlateral that are
cells It consists ofcells of the andcap cellsmeristems. Most of
maturation. outer, lateral root surface
present merge and grow until they occupy 90% or more of the
the activity in this zone of cell divisionapical place toward
continuously replenished by thecells, which meristem. As
mature into epidermal root takes
cylinderof each cell. No further increase in cell size occurs
volume
the edges ofcells dome, Just abovecells a strong protective
theseaouter thin cuticle. where the form divide every 12 to
have the zone of disintegrateand the mature parts of the root,
the they the
above very elongation,
36 hours, often rhythmically, remain stationaryof division of
cover which protects in is a part offrom a peak and damage as
elongation zone, there girth, reaching injury for the life
except for an increase the root tip the
once or twice causes the calleddifferentiate to form more
the plant.pushes day. Thethroughelongate and root provides a
maturation zoneits way root topiliferous The
the root This a with is cells the soil.
specialized supply of the soil in search cells are provided
continuous root tissues and additionalfor water andthat liefor
region. deeper into expendale parenchyma cells mineral
penetrate is where the epidermal cells produce many
Thiszone meristematic tissues.
over region elongation.
the the of
salts. unicellular outgrowth called hair roots. Root
tubular,
hairs, which can number over 35,000 per square
centimeter of root surface and many billions per
plant, greatly increase the surface area and therefore the
absorptive capacity of the root. Thus, Water absorption
mostly takes place through this area. The root hairs usually
are alive and functional for only a few days before they are
sloughed off at the older part of the zone of
maturation, while new ones are being produced toward
the zone of elongation. Above the root hair region, the root
becomes thicker and secondary or lateral roots are
developed. The secondary roots in turn rebranch to
form tertiary roots. Each lateral branch has its own
cap, root hairs, meristematic, elongation and mature
regions. The roots in this region are covered by a protective

Types of roots
Tap root
• Generally found in dicotyledons
• Main or primary root develop
from the radicle.
• Grows vertically down into the
soil
• Later lateral or secondary
roots grow from this at an acute
angle outwards and
downwards, and from Lateral
these other branches may arise roots
for absorption of water and
nutrients.
• When tap root is associated with
many branched roots, it forms
the tap root system. e.g.
pastinaca sativa (parsnip) and
taraxacum officinale (Dandelion)

Adventitious roots
• Commonly found in monocots
• Growth of the radicle is usually
arrested at an early stage and is
replaced by numerous
roots that develop from the
stem.
• These adventitious roots are
slender and equal in size.
• Adventitious roots associated
with branched or lateral roots
and form the adventitious root
system.
• Also known as fibrous roots.

Modifications in roots
• Root modification occurs when there is a
permanent change in the structure of tap or
adventitious roots. This is to perform
additional specific functions to those of
anchorage and absorption for adaptation to
their surrounding environment.

Roots modified for the storage of food:
• Conical roots: modified tap root
of conical shape throughout which
they store food. It is broad at the
base and gradually tapers towards
its apex - e.g. Carrot (Daucas
carota).
• Napiform roots: modified tap
root, fleshy with the upper
portion inflated or swollen and
abrupt narrowing of the base into
a tail-like portion. - e.g. Turnip
(Brassica rapa) and Beetroot (Beta
vulgaris) .
• Fusiform roots modified tap root
with thickened middle portion
containing food and tapered
towards both ends – e.g. radish
(Raphanus sativus)

Turnip

• Tuberous/storage root is • Nodulated roots are
modified fibrous root with fibrous/adventitious roots
irregular shape. Many single of the plants of the
roots are modified to form Leguminosae family.
several similar swollen roots Nodules like structures are
for the storage of food. They present on branches of root
occur in a bunch - e.g. in which nitrogen fixing
Mirabilis jalapa bacteria can be found.The
apex of these roots become
swollen because of the
accumulation of food e.g.
Curcuma amada, Ginger.

Roots modified for mechanical support:
• Prop roots – the name is related
to the pillar like appearance. They
are aerial adventitious roots that Maize
develop from branches and give roots
support to the branches. They
grow vertically downward,
penetrate the soil and become
thick to provide additional support
to the plant. they brace the plant
against wind - e.g. Ficus Screw pine
bengalensis, banyan tree (Ficus roots
macrophylla) and maize.
• Stilt roots - roots develop from
nodes of the lowermost portion of
the stem and provide mechanical
support to the plant by fixing it in
soil firmly. e.g. sugarcane,
Pandanus Tectorius (screw pine). Decumaria
barbara
• Climbing roots - in some weak
stemmed plants roots develop
from nodes which are useful to
climb on the hard object.

• Contractile roots widely distributed
among monocotyledons and
herbaceous perennial dicotyledons. Bulbs and
The roots from the bulbs of lilies roots of
and of several other plants such as lilies
dandelions and colocassia contract
by spiraling to pull the plant a little
deeper into the soil each year until
they reach an area of relatively
stable temperatures. The roots may
contract to a third of their original
length as they spiral like a corkscrew
due to cellular thickening and
constricting.
• Floating roots - in some aquatic
plants, the roots will float on the
surface of the water. These roots
store air, become inflated and Water
spongy, project above the level of primrose
water and make the plant light. (Jussiaea )
They also help in exchange of gases.
E.g. Jussiaea

Roots modified for vital function:
• Pneumatophores or Respiratory
roots - The roots of some aquatic
plants and plants which grow on
marshy areas, such as
mangroves, develop outgrowths
(pneumatophores). This type of
roots arises from underground
branches of tap root, grows in
upwards direction and usually
extends several centimeters
above water, facilitating the
oxygen supply to the roots
beneath. In aquatic plants,
floating roots are acting as
respiratory roots. In marshy area,
the roots are not getting
sufficient oxygen and hence they
grow upwards from the ground.

• Epiphytic roots are aerial roots of
plants such as orchids. A distinguishing
feature is that these roots absorb
moisture from atmosphere with the
help of velamen tissue (outer layer of Orchids roots
dead cells). Most of the cells are water
absorbing while the others are filled
with air and thus facilitate the exchange
of gases with the inner cortex. Epiphytic
roots have only physical contact and
cause no harm to the host plant.
Taeniophyllum
• Photosynthetic or Assimilatory roots:
The roots are green, flattened, and
ribbon like. The root tip cells contain
chloroplasts and thus, perform
photosynthesis. In some cases such as
in the case of leafless orchids of the
genera Taeniophyllum and
Chiloschista, they are the only
Trapa
photosynthetic tissues. E.g. Tinospora (-
aerial roots) , Trapa (-Hydrophyte is
with submerged green roots)

• Parasitic roots: The stems
of certain plants that lack
chlorophyll, such as dodder
(Cuscuta), produce peg-like
Cuscuta
roots called haustoria that on host
penetrate the host plants plant
around which they are
twined. The haustoria
establish contact with the
conducting tissues of the
host and effectively
parasitize their host. Thus, Electromacrograph
they are food sucking roots. to show the
• Root thorns: roots of some haustoria.
plants arise from the stem
and change into thorns
performing the protective
function e.g. Pothos (money
plant)
Thorns of Pothos

Other modifications of roots:
• Fasciculated roots: from the bas or
lower nodes of the stem, these
tuberous roots arise in groups. E.g • Buttress roots. Certain
Dahlia species of fig and other
• Moniliform roots: these are also tropical trees produce
called beaded roots because of their huge buttress roots
bead-like appearance e.g.
Momordica (bitter gourd) toward the base of the
• Annulated roots: these thickened trunk, which provide
roots look as if formed by a number considerable stability.
of discs placed on above another.
Ipecac (Cephalis)
• Water storage roots. Some
members of the pumpkin family
(Cucurbitaceae), especially those
that grow in arid regions, may
produce water-storage roots
weighing 50 or more kilograms.

Root growth
• Most individual root growth can be divided into two
main phases; the indeterminate growth phase and the
termination growth phase. The indeterminate growth
phase is one where growth is maintained for an
undefined period of time. Growth is usually
accomplished by division of RAM while the terminate
phase is a phase where growth stops usually after a
certain period of time, size or length of roots or when
appropriated conditions are unavailable. According to
Wilcox 1962, the RAM can become dormant for
example during droughts but can restart its division
and growth after some time.

Primary growth

• Primary growth is a longitudinal growth
occurring in all vascular plants and
involves all elongation of the roots.
Primary growth takes place only in the
apical meristem.

Secondary growth
• Secondary growth occurs mainly in dicots but
rarely in monocots. It is the result of division
in the lateral meristems, the cork cambium
and the vascular cambium. Secondary growth
takes place in woody as well as in non woody
plants. It involves all growth in diameter of the
roots.
• 1.
Vascular
cambium

3. One of the new cells remains
2. The vascular cambium cells vascular cambium and the other
divides longitudinally. becomes xylem.

Vascular
cambium

4 & 5. The cells can be seen enlarging in
this diagram

Vascular
Vascular cambium
cambium

6. Occasionally, the inner cells remain vascular
cambium and the outer ones become phloem.

Vascular phloem
cambium

Root meristem
• Meristems are areas in plants where
mitosis occurs, and due to this cell
division, it is also where growth
occurs. Apical meristems are
responsible for vertical growth and
they can be found at the root tips.
• The planes of cell division in the root
meristem are strictly ordered, and are
primarily transverse divisions that
provide growth of the root in length.
• A primary root meristem generates
two tissues simultaneously, the main
root axis extending proximally
towards the shoot, and the root cap
pushing relentlessly forward into the
soil. Primary roots arise through
controlled cell divisions in the apical
meristem and subsequent expansion
and differentiation of these cells.

Apical meristem and development
• Divisions apical meristem is covered
• The root can be in any of three planes,
either anticlinal which provides root
by the root cap (normal to the a
axis), periclinal lubricative function as
protective and (tangential to the root
axis) or takes place through soil
growth radial to the axis. These
divisions will give rise, respectively, to
particles. Root caps advance at a
increasedspeed: a rootincreased root
dramatic root length, might
thicknessby 5 cm per day cells new
elongate (more layers of and through
the root), or increased root in
root cap cells can be pushed
circumference. apex of the primary
advance of the
• The apical meristem supplies all the
axis at about the same rate.
• cells for the primary root axisapices is
A remarkable feature of root and the
consequencescentre,planes of cell the
the quiescent of the a paradox at
division are evident long after
heart of the meristem. The quiescent
meristematic activity ceases.inactive,
centre is a zone of relatively
• Separate cell divisions at the leading
slowly dividing cells, numbering
about 500–600 meristem maize
edge of the rootin a maturegenerate a Diagram showing longitudinal
root.
root cap which extends forward as a section of a maise (Zea mays)
protective structure. root tip. The quiescent centre
is shaded dark green.

• The Picture shows the Root Cap
(Thimble-like covering which
protects the delicate apical
meristem), the Apical Meristem
(Region of rapid cell division of
undifferentiated cells), the
Quiescent Center (Populations of
cells in apical meristem which
reproduce much more slowly
than other meristematic cells),
the Zone of Cell Division -
Primary Meristems (Three areas
just above the apical meristem
that continue to divide for some
time), the Zone of Elongation
(Cells elongate up to ten times
their original length )and the
Zone of Maturation (Region of
the root where completely
functional cells are found) of a
root.

Lateral meristem and development
• Lateral root formation is a major determinant of root
systems architecture. The degree of root branching impacts
the efficiency of water uptake, acquisition of nutrients and
anchorage by plants.
• Lateral roots extend horizontally from the primary root and
serve to anchor the plant securely into the soil. This
branching of roots also contributes to water uptake, and
facilitates the extraction of nutrients required for the
growth and development of the plant.
• Many different factors are involved in the formation of
lateral roots. Regulation of root formation is tightly
controlled by plant hormones such as auxin, and by the
precise control of aspects of the cell cycle. Such control can
be particularly useful: increased auxin levels, which help to
promote lateral root development, occur when young leaf
primordia form and are able to synthesise the hormone.
This allows coordination of root development with leaf
development, enabling a balance between carbon and
nitrogen metabolism to be established.

• Lateral root primordia originate
After the lateral root primordium
is formed, it becomes a mature
from the mature pericycle of the
lateral root. One of stage
parent root by a twothe first events
process. First, the primodium
is a periclinal division that
emerges through the overlaying
generates a double layer of
tissues by cell expansion. The
pericycle-derived cells. Cells of the
increase in cell size is particularly
primordia begin to differentiate
apparent in cells near the base of
almost immediately after initiation,
the primordium, while cell
as evidenced by differential gene
number remains inner and
expression in therelatively outer
unchanged. Second, the new
layers.
• lateral root begins to elongate,
Lateral root primordia develop
and cell numbers increase at the of
through a characteristic program
rootdivisions and expansions to
cell tip. This is characteristic of
maturearoot elongation via
create fully patterned structure
division of cells the primaryapical
that resembles in the root root
meristem.
tip.

• It was found that lateral root formation can be divided
into four stages:
1) Differentiation of pericycle cells
2) First morphological event is series of asymmetric
transverse divisions of pericycle cells in three cell files
positioned opposite the xylem pole. Although all
pericycle cells are morphologically identical, these
three files are already fated differently from their
neighbors.
Followed by ordered cell divisions and differentiation that
generates a lateral root primordium
3) Emergence via cell expansion
4) 'Activation' of lateral root primordia to form a
functional root.

Root-stem transition
• The root transition zone concept states that root cells leaving the
apical meristem need to accomplish a transitional stage of cyto-
architectural rearrangement, especially of the actin cytoskeleton, in
order to perform rapid cell elongation. Cells of this zone also have
unique functional and sensorial properties.
• The root and the stem make a continuous structure called the axis of
the plant. The vascular bundles are continuous from the root to the
stem, but the arrangement of vascular bundles is quite different in
the two organs; the stems possess collateral bundles with endarch
xylem, whereas the roots possess radial bundles with exarch xylem.
The change of position involving inversion and twisting of xylem
strands from exarch to endarch type is referred to as vascular
transition, and the part of the axis where these changes occur is
called transition region.

There exist four types of root-stem transition.
1. In Fumaria, Mirabilis and Dipsacus,
and others, each xylem strand of
the root divides by radial division
forming branches, they swing in
their lateral direction; one towards
right and the other goes to the left.
These branches join the phloem
strands on the inside. The phloem
strands, do not change their
position and also remain unchanged
in their orientation. They remain in
the form of straight strands
continuously from the root into the
stem. In this type as many primary
bundles are formed in the stem as
many phloem strands are formed in
the root.
Dipsacus

2. In Cucurbita, Phaseolus, Acer and
Trapaeolum and others, the
xylem and phloem strands
fork, the branches of the strands
of both swing in lateral direction
and join in pairs. After joining in
the pairs they remain in the
alternate position of the strands
in the root. The xylem strands
become inverted in their position
and the phloem strands do not
change their orientation. This
way, in the stem, the number of
bundles becomes double of the
phloem strands found in the root.
This type of transition is more
commonly found. Phaseolus vulgaris

3. In Lathyrus, Medicago and
Phoenix, the xylem strands do
not fork and continue their
direct course into the stem.
These strands twist through
180 degrees. The phloem
strands divide soon and the
resulting halves swing in the
lateral direction to the xylem
positions. The phloem strands
join the xylem strands on the
outside. In this type as many
bundles are formed as there
are phloem strands in the root
(as in type 1). Lathyrus
tuberosus

4. This type is rarely found and is
known in only a few
monocotyledons
(e.g., Anemarrhena). In this type
half of the xylem strands fork and
the branches swing in their
lateral direction to join the other
undivided strands of xylem which
become inverted. The phloem
strands do not divide but they
become united in pairs. These
united phloem strands unite with
the triple strands of the xylem.
Thus, a single bundle of the stem
consists of five united strands. In
this type half as many bundles
are formed in the stem as there
are phloem strands in the root. Zhi-Mu (Anemarrhena
asphodeloides)

• The transition zone of root apices is a unique part
of the whole plant body. Apart from tip-growing
cells, such as root hairs and pollen tubes, the cells
of the transition zone have the highest rate of
vesicle recycling activity, and their auxin transport
shows the highest degree of activity. In this root
apex zone synchronized electrical activity has
been reported. The activity of auxin-secreting
domains of the transition zone is sensitive not
only to internal developmental cues but,
importantly, also to environmental inputs such as
light and gravity.

Dicot roots:
• The parenchyma cells of the cortex
• The Cortex contains ground tissue
store starch and other substances.
which stores spaces between the
They have air photosynthetic
cells which are active in the uptake
products. It is essential for aeration
of the root tissue as they are non
of water and minerals.
photosynthetic.
• The vascular tissue, i.e.cylinder once
• The Endodermis is a the xylem and
the thick that forms a boundary
cell phloem forms a central cylinder
through the root and is surrounded by
between the cortex and the cells
the pericycle which is a ring of stele.
It contains lateral roots arise.
from which the casparian strip.
• The primary xylem of Dicot roots
• The Pericycle is found just inside
forms a star shape in the center of the
of the endodermis. It may become
vascular cylinder with usually 3 or 4
meristematic. It is responsible no
points, unlike monocots, there is for
central pith of parenchyma cells.
the formation of lateral roots.
• The Epidermis of the dicot root
• Vasculardermal tissue and the
contains Tissue contains acts
mostly and Phloem and forms an
Xylem to protect the root Figure showing
X-shaped pattern in very center of the cross section
root of a dicot root

Monocot roots
• In the Fibrous rootform bulbs, such as
Many monocots system of Monocots,
the primary root is and tulips. These are not
onion, gladiolus, almost non-existent.
The secondary roots are important in
root structures, but rather modified
absorption, but are not as deep as the
primarymade of compact leaves.
stems, root of most Dicot.
• Because many monocots have shallow
• The Epidermis containssystem has manyand
root systems (the fibrous dermal tissue
it protects the that spread more on top
secondary roots root.
than they grow deep into the ground),
• The Cortex contains ground tissue which
secondary or adventitious roots will be
stores photosynthetic products. It is also
produced.
• Most monocotyledon plants such as grass
active in the uptake of water and minerals
and onions have fibrous foot systems. The
• The Endodermis is a cylinder once cell
actual root structure differs from the
dicotsthat forms a tend to have parallel the
thick as Monocots boundary between
vein systems in their stems.
cortex and the stele. It is even more
• In monocots, the first root to emerge from
distinct thanoff, andcounterpart. central
the seed dies Dicot so no strong, It also
tap root forms. Instead, monocots sprout
contains the casparian strip.
roots from shoot tissue near the base,
• The Vascular Tissue contains the Xylem
called adventitious roots. The familiar
fibrous root systemIt forms a ring near
and the Phloem. of grasses is an
example of this rooting pattern. Figure showing the
center of plant
• The Pith is the center of most region of cross section of a
root. monocot

References
• http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20102/bio%20102%20lectures/plant%20structure/plant%20structure.htm [Accessed on 21.01.12]

• http://plantsinaction.science.uq.edu.au/edition1/?q=content/7-1-1-root-apical-meristems [Accessed on 21.01.12]

• http://www.cls.zju.edu.cn/sub/fulab/plant_Antomy/plant/mitosis/root1/038344.html [Accessed on 21.01.12]

• http://www.public.iastate.edu/~bot.512/lectures/Roots.htm [Accessed on 21.01.12]

• http://www.aensionline.com/jasr/jasr/2011/706-713.pdf [Accessed on 22.01.12]

• http://hortsci.ashspublications.org/content/28/5/545.1 [Accessed on 22.01.12]

• http://www.cell.com/trends/plant-science/abstract/S1360-1385(09)00146-0 [Accessed on 22.01.12]

• http://www.gmi.oeaw.ac.at/documents/extranet-documents/publications/Sanchezetal.pdf [Accessed on 22.01.12]

• Biology, Raven et al., 6th edition

•  introduction to plant structure and development, 2005, Charles Beck, Cambridge University Press
An

• http://www.northlakebiology.com/1411/lab/Botany%20Lab%20Unit%202%20-%20Root%20Structure%20and%20Function.PDF [Accessed on the 22.01.12]

• http://www.northlakebiology.com/1411/lab/Botany%20Lab%20Unit%202%20-%20Root%20Structure%20and%20Function.PDF [Accessed on the 22.01.12]

• http://prakashamarasooriya.wikispaces.com/file/view/Root+Structure.pdf [Accessed on the 22.01.12]

• http://www.biologyreference.com/Mo-Nu/Monocots.html#b [Accessed on the 22.01.12]

• http://people.bridgewater.edu/~lhill/Monocotdicot.htm [Accessed on the 22.01.12]

•  http://www.northlakebiology.com/1411/lab/Botany%20Lab%20Unit%202%20-%20Root%20Structure%20and%20Function.PDF [Accessed on the 22.01.12]
• http://www.tutorvista.com/content/biology/biology-iii/angiosperm-morphology/roots-modifications.php
• http://www.beta-theta.com/Biology/Root.html
• http://www.mhhe.com/biosci/genbio/tlw3/eBridge/Chp17/17_3.pdf
• http://www.sakshieducation.com/(S(2iwaqj3wm3ymsemwhqzaud55))/Inter(New)/..%5CEAMCET%5CQR%5CBotany%5Cjr_botanysynopsis%5C04Root.pdf
• http://edudel.nic.in/PAHAL/biology_260309/biology_dt_260309.pdf

Roots of angiosperms

Recomendados

Recomendados

Más contenido relacionado

Destacado

Destacado (19)

Último

Último (20)

Roots of angiosperms