6. ANATOMY OF FLOWERING PLANTS
THE
TISSUE SYSTEM
Based on structure and location, tissue systems are 3 types:
o
Epidermal tissue system
o
Ground (fundamental) tissue system
o
Vascular (conducting) tissue system
1. Epidermal Tissue System
-
It forms the outer-most covering of the
whole plant body.
-
It comprises epidermal cells, stomata and
epidermal appendages (trichomes & hairs).
Epidermis
-
It is the outermost layer of the primary
plant body.
-
Epidermis is usually single
layered.
-
It is made up of elongated, compactly arranged parenchymatous cells with small amount of
cytoplasm lining the cell wall and a large vacuole.
-
The outside of the epidermis
is often covered with a waxy thick layer (cuticle). It prevents the loss
of water. Cuticle is absent in roots.
Stomata
-
These are structures present
in the epidermis of leaves.
-
Stomata regulate the transpiration
and gaseous exchange.
-
A stoma is made of two
bean-shaped cells (guard cells).
-
In grasses, the guard cells
are dumbbell shaped.
-
The outer walls of guard
cells (away from the stomatal pore) are thin and the inner walls (towards the
stomatal pore) are highly thickened.
-
The guard cells possess
chloroplasts and regulate the opening and closing of stomata.
-
Sometimes, some epidermal
cells near the guard cells become specialized in shape and size. They are called
subsidiary cells.
-
The stomatal aperture, guard
cells and the surrounding subsidiary cells are together called stomatal
apparatus.
Epidermal appendages
-
Root hairs: Unicellular
elongations of the epidermal cells. They help to absorb water and minerals from
the soil.
-
Trichomes: They
are the epidermal hairs on the stem. They are usually multicellular, branched
or unbranched and soft or stiff. They may be secretory. Trichomes help to
prevent water loss due to transpiration.
2. The Ground Tissue System
- All
tissues except epidermis and vascular bundles constitute the ground tissue.
- It
consists of simple tissues (parenchyma, collenchyma and sclerenchyma).
- Parenchymatous
cells are present in cortex, pericycle, pith and medullary rays, in the primary
stems and roots.
- In
leaves, the ground tissue consists of thin-walled chloroplast containing cells
and is called mesophyll.
3. The Vascular Tissue System
It
consists of complex tissues (xylem and phloem).
Xylem
and Phloem together constitute vascular bundles.
Based
on the presence or absence of cambium, vascular bundles are 2 types:
o
Open type: In
this, cambium is present between phloem and xylem. So vascular bundles can
form secondary xylem and phloem tissues. E.g. dicotyledonous stems.
o
Closed type: In
this, cambium is absent. Hence, they do not form secondary tissues. E.g. monocotyledons.
Based
on the arrangement of xylem and phloem, vascular bundles are 2 types:
o
Radial type: Xylem
and phloem are arranged in an alternate manner on different radii. Seen in
roots.
o Conjoint type: Xylem and phloem are jointly situated at the same radius of vascular bundles. Seen in stems and leaves. Conjoint vascular bundles usually have phloem located only on the outer side of xylem.
AMAZING GR8 EFFORTS SIR
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