ANATOMY OF FLOWERING PLANTS -NOTES
ANATOMY OF FLOWERING PLANTS
The
tissue
A group of cells having a common
origin and usually performing common function are called tissues.
- Meristematic tissue is a simple tissue composed of
group of similar and immature cells which can divide and form new cells.
The meristem which occurs at tips of roots and shoots are called apical meristem.
- Intercalary meristem occurs
between mature tissues especially in grasses. Both apical meristems and
intercalary meristems are primary meristems because they appear early in
life of a plant and help to form the primary plant body.
- The meristem which occurs on the sides and takes part
in increasing girth of the plants are called Lateral meristem. Intra fascicular cambium in the
primary lateral meristem. Vascular cambium, cork cambium are secondary
meristem.
- The cells that have become structurally and
functionally specialized and lose the ability to divide are called
permanent tissue. Permanent tissues having all cells similar in structure
and function are called simple permanent tissues and
those having different kinds of cells are called complex tissue.
- Parenchyma is
a simple permanent living tissue which is made up of thin-walled
isodiametric cells. Each cell encloses a large central vacuole and
peripheral cytoplasm containing nucleus. They are found in non-woody and
soft areas of stem, root, leaves, fruits and flowers. They store the food
and provide turgidity to softer parts of plant.
- Collenchyma consists
of cells which are much thickened at corner due to cellulose,
hemicellulose and pectin. Oval, spherical or polygonal often contain
chlorophyll. They provide mechanical support to the growing parts of the
plants like young stem.
- Sclerenchymas are supportive
tissue having highly thick walled cells with little or no protoplasm due
to deposition of cellulose or lignin. They are of two types: fibres and
sclereids. They provide mechanical support to mature plant organs to
tolerate bending, shearing, compression etc.
Complex Tissues– Xylem and phloem constitute the complex tissues in plants and work together as a unit.
Xylem | Phloem |
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- Primary xylem is of two types- protoxylem and
metaxylem. In stem, protoxylem lies in centre and metaxylem towards
periphery. This type of primary xylem is called endarch.
- In roots, protoxylem lies in periphery and metaxylem
lies towards the centre. This type of primary xylem is called exarch.
- In gymnosperms, albuminous cells and sieve cells lack
sieve tube and companion cells.
Epidermal
Tissue System
- It forms the outermost covering of whole plant body,
which consists of epidermal cells, stomata, epidermal appendages
(trichomes and hairs).
- Epidermis is single layered, parenchymatous with waxy
thick layers of cuticle to prevent water loss.
- Stomata is present in epidermis of leaves. It regulates
the transpiration and gaseous exchange. In dicots, stomata are bean-shaped
having two guard cells closing the stomatal pore. In monocots, stoma is
dumbbell-shaped. Guard cells contain chloroplasts and help in opening and
closing of stomata.
- Guard cells are surrounded by subsidiary cells. The
stomatal aperture, guard
cells and the surrounding subsidiary cells are together called stomatal apparatus
Dicots (Bean shaped) Monocots
(Dumb-bell shaped)
- Epidermis also contains a number of hairs. Root hairs
are unicellular elongation of epidermal cells. Trichomes are present on
stems, which are multicellular, branched or un-branched preventing water
loss due to transpiration.
The
ground Tissue System
- All the tissue between epidermis and vascular bundle
forms the ground tissues. It consists of simple permanent tissues.
Parenchyma is present in pericycle, cortex, pith and medullary rays in
stem and roots.
- In leaves the mesophyll, chloroplast containing cell,
forms the ground tissues.
The
Vascular Tissue System
- The vascular system consists of complex tissues, xylem
and phloem that together form vascular bundles.
- When xylem and phloem within a vascular bundle are
arranged in alternate manner on different radii, the arrangement are
called radial as in roots. When
xylem and phloem are situated at the same radius of vascular bundle, it is
called conjoint as in stem and
leaves.
Radial
Dicotyledonous
Root
- The outermost layer of dicot root is epidermis containing unicellular root hairs.
- The cortex consists
of several layers of thin-walled parenchyma cells.
- The innermost layer of cortex is called endodermis having waxy material suberin
as casparian strips, which is impermeable to water.
Monocotyledonous
Root
- The anatomy of the monocot root is similar to the dicot
root in many respects.
It has epidermis, cortex, endodermis, pericycle, vascular bundles and pith. As
compared to the dicot root which have fewer xylem bundles
Dicotyledonous
Stem
- Epidermis: is covered with a thin layer of cuticle and
may have Trichomes and stomata.
- Cortex: The cortex is made up of the multiple layers of
cells including hypodermis, middle layer of parenchyma cells and innermost
layer called endodermis.
- Endodermis cells are rich in starch grains and are
called the starch sheath. Pericycle is present on the inner side of
endodermis. Layers of radially placed parenchyma between the vascular
bundles are called medullary rays.
- A large number of vascular bundles are arranged in a
ring. Each vascular bundle is conjoint, open. Protoxylem is endarch
Monocotyledonous
Stem
- The hypodermis is made up of sclerenchyma. Vascular
bundles are conjoint, closed and scattered. Each vascular bundle is
surrounded by a sclerenchymatous bundle sheath.
- Phloem parenchyma is absent. Water-containing cavities
are present within the vascular bundles.
Dorsiventral
(Dicotyledonous) Leaf
- The leaf lamina of a dorsiventral leaf has 3
parts: epidermis, mesophyll and vascular system.
- The upper epidermis is called adaxial epidermis and
lower one is called abaxial epidermis. More number of stomata are present
on the abaxial epidermis.
- There are two types of cells in the mesophyll: palisade
parenchyma and spongy parenchyma. The palisade parenchyma is placed
adaxially.
- The spongy parenchyma is situated below the palisade
parenchyma and extends to the lower epidermis. There are numerous large
spaces and air cavities between the cells of spongy parenchyma.
- Vascular bundles are surrounded by a layer of
thick-walled bundle sheath cells.
Isobilateral
(Monocotyledonous) Leaf
- Stomata are present on both the surfaces of an
isobilateral leaf. The mesophyll is not differentiated into palisade and
spongy parenchyma.
- Some adaxial epidermal cells in grasses are modified
into large, empty cells called bulliform cells. When the bulliform cells
absorb water, they become turgid. So the leaf surface is exposed. During
water stress, when the bulliform cells become flaccid, the leaves curl
inwards to minimize water loss.
SECONDARY
GROWTH
The increase in girth of a plant
body is called secondary growth. The tissues involved in secondary growth are:
vascular cambium and cork cambium.
Vascular Cambium:
In case of young stem vascular cambium is present in patches as a single layer
between the xylem and phloem. It forms a complete ring at a later stage.
Activity
of the Cambial Ring:
- The cambial ring becomes active and begins to cut off
new cells, both towards the inner and the outer sides.
- The cells which are cut off towards pith mature into
secondary xylem. The cells which are cut off towards periphery
mature into secondary phloem.
- The cambium is more active on the inner side than on
the outer. As a result, the amount of secondary xylem produced is more
than secondary phloem. The primary and secondary phloems get gradually
crushed due to the continued formation and accumulation of secondary
xylem.
- At some places, the cambium forms a narrow band of
parenchyma, which passes through the secondary xylem and the secondary
phloem in the radial directions. These are the secondary medullary rays
Spring
wood and autumn wood:
- Cambium is very active during the spring season,
but less active during the winters. Hence, during spring; a large number
of xylem elements are formed having wider vessels. During winter, less
xylem elements are formed having narrow vessels.
- The wood formed during summer is called spring wood.
The wood formed during winter is called autumn wood.
- The two kinds of wood appear as alternate
concentric rings in transverse section of a trunk of a tree. These are
called annual rings and provide information about age of the tree.
Heartwood
and sapwood:
- In old trees, the greater part of secondary xylem is
dark in colour, hard, and resistant to attacks by microorganisms and
insect. This region is made of dead elements with highly lignified walls.
This wood is called heartwood. The heartwood gives mechanical support but
does not conduct water.
- The peripheral part of the secondary xylem is lightly
coloured. This is known as sapwood. It helps in conduction of water and
minerals.
Cork
Cambium
- Meristematic tissue which develops in the cortex
region is called cork cambium or phellogen.
- The phellogen cuts off cells on both sides. The outer
cells differentiate to form cork or phellem while the inner cells
differentiate into secondary cortex or phelloderm.
- Phellogen, phellem and phelloderm are
collectively called periderm.
- Due to activity of the cork cambium, pressure builds up
on the remaining layers peripheral to phellogen. These layers gradually
die and fall off.
Lenticels
- At certain regions, the phellogen cuts off
closely arranged parenchymatous cells on the outer side instead of
cork cells. These parenchymatous cells soon rupture the epidermis, forming
a lens-shaped openings called lenticels.
- Lenticels permit the exchange of gases between the
outer atmosphere and the internal tissue of the stem.
Secondary
Growth in Roots
- The vascular cambium of the dicot root originates from
the tissue located just below the phloem bundles. A portion of pericycle
tissue present above the protoxylem forms a continuous wavy ring. It
gradually becomes circular. Rest of the steps are similar as in dicot
stem.
- Secondary growth takes place in stems and roots of
gymnosperms. No secondary growth occurs in monocots.
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