6. ANATOMY OF FLOWERING PLANTS
ANATOMY OF DICOTYLEDONOUS & MONOCOTYLEDONOUS PLANTS
Dicotyledonous (Dicot) Root
o
Epidermis (epiblema): The
outermost layer. Many cells of epiblema protrude as unicellular root
hairs.
o
Cortex: It
consists of several layers of thin-walled parenchyma cells with intercellular
spaces.
o
Endodermis: Innermost
layer of the cortex. It comprises a single layer of barrel-shaped cells without
intercellular spaces.
The tangential and radial walls of the endodermal
cells have a deposition of suberin (water impermeable, waxy material) in
the form of casparian strips.
o
Stele: All
tissues on the inner side of the endodermis together constitute stele. They
include
§ Pericycle:
A few layers of thick-walled parenchyomatous
cells next to endodermis. Initiation of lateral roots and vascular cambium
during the secondary growth takes place in these cells.
§ Pith: Innermost
region of the stele. It is small or inconspicuous.
§ Conjunctive
tissue: The parenchymatous cells which lie between the xylem
and the phloem.
§ Vascular
bundles: 2-4 xylem & phloem patches. Later, a cambium ring
develops between the xylem & phloem.
Monocotyledonous (Monocot) Root
- It has epidermis, cortex, endodermis, pericycle, vascular bundles and pith.
- There are usually more than six (polyarch) xylem bundles.- Pith is large and well developed.
- Monocot roots have no any secondary growth.
Dicotyledonous (Dicot) Stem
o
Epidermis: Outermost
protective layer. Covered with a thin layer of cuticle, it may bear trichomes &
few stomata.
o
Cortex: Multiple
layers of the cells arranged in between epidermis and pericycle. It consists of
3 sub-zones:
§ Hypodermis:
Outer
zone. It consists of a few layers of collenchymatous cells just below the epidermis.
It provides mechanical strength to the young stem.
§ Cortical
layers: Below hypodermis. They consist of rounded thin walled
parenchymatous cells with conspicuous intercellular spaces.
§ Endodermis: Innermost
layer. Cells are rich in starch grains. So it is also called starch
sheath.
o
Stele:
Consists of pericycle, vascular bundles, medullary rays & pith.
§ Pericycle:
It
is present on the inner side of the endodermis and above the phloem in the form
of semi-lunar patches of sclerenchyma.
§ Medullary rays:
These are few layers of radially placed parenchymatous cells in
between vascular bundles.
§ Vascular
bundles: Large in number. Their ring arrangement is a
characteristic of dicot stem. Each vascular bundle is conjoint & open. Protoxylem
is endarch.
§ Pith: Central
portion of the stem. It has many rounded, parenchymatous cells with large
intercellular spaces.
Monocotyledonous (Monocot) Stem
- It
has a sclerenchymatous hypodermis, many scattered vascular bundles,
each surrounded by a sclerenchymatous bundle sheath, and a large,
conspicuous parenchymatous ground tissue.
- Vascular
bundles are conjoint & closed. Peripheral vascular
bundles are smaller than centrally located ones.
- The
phloem parenchyma is absent, and water-containing cavities are present within
the vascular bundles.
Dicotyledonous (Dorsiventral) Leaf
The
vertical section of a dicot leaf through lamina shows 3 main parts: Epidermis,
mesophyll & vascular system.
o
Epidermis: It
covers both the upper surface (adaxial epidermis) and lower surface (abaxial
epidermis) of the leaf. It has a conspicuous cuticle.
Abaxial epidermis generally bears more stomata.
In adaxial epidermis, stomata are fewer or absent.
o
§ Palisade
parenchyma: It is adaxially placed. Made up of
elongated cells arranged vertically and parallel to each other.
§ Spongy
parenchyma: The oval or round and loosely arranged. It
is situated below the palisade cells and extends to the lower epidermis. There
are numerous large spaces and air cavities between these cells.
o
Vascular system: It
includes vascular bundles. They can be seen in the veins and midrib.
Size of vascular bundles is dependent on the size of
the veins. The veins vary in thickness in the reticulate venation of dicot leaves. Vascular bundles are
surrounded by a layer of thick walled bundle sheath cells.
Monocotyledonous (Isobilateral) Leaf
o
Stomata are present on both surfaces of
the epidermis.
o
Mesophyll is not differentiated into palisade
and spongy parenchyma.
o
In grasses, certain
adaxial epidermal cells along the veins modify into large, empty, colourless
cells. These are called bulliform cells. When they have absorbed water
and are turgid, the leaf surface is exposed. When they are flaccid due to water
stress, they make the leaves curl inwards to minimise water loss.
o Parallel venation is reflected in the near similar sizes of vascular bundles (except in main veins).
