Describe Internal Structure of Dicot Stem
Transverse
section of internal structure of Dicotyledonous stem of sunflower shoes the
following structure. It consists of Epidermis, cortex and stele. In the stele
there are numerous vascular bundles in a ring and also large pith.
(1) Epidermis
This forms outer
most layers and consists of single row of cells and fit along their radial walls
with a well defined cuticle over it. It bears some multicellular hairs and few
stomata but no chloroplasts except in the guard cells.
(2) Cortex
It lies in
between epidermis and pericycle and consists of hypodermis externally, general
cortex centrally and endodermis internally.
(a) Hypodermis
(collenchyma): This lies immediately below the epidermis and consists of 4 to 4
layers of collenchymatous cells.
These cells are
thick at the corners against the intercellular spaces oving to deposit of
cellulose and pectin. The cells are living and contain number of chloroplasts.
(b) General
Cortex: This lies internal to hypodermis and consists of few layers of thin
walled large rounded or oval parenchymatous cells. There are clear intercellular
spaces in it.
(c) Endodermis:
This is innermost layer of the cortex consisting of barrel shaped cells and
surrounding the stele. Endodermis is conspicuous outside the hard bast but
often loses its identity on either side. It contains many starch grains and is
also known as starch sheath.
(3) Pericycle
This is region
lying in between the endodermis and the vascular bundles and is represented by
semi lunar patches of sclerenchyma and mass of parenchyma. Each patch
associated with phloem of vascular bundle is called hard bast.
(4) Medullary
rays
A few layers of
big polygonal or radially elongated cells lying in between two vascular bundles
constitute a medullary ray.
(5) Pith
This is large in
sunflower stem and occupies the major portion of it. It extends from below the
vascular bundles to the centre and is composed of rounded or polygonal thin
walled living cells with intercellular spaces in between them.
(6) Vascular
bundles
These are
collateral and open and are arranged in a ring. Each bundle is formed of (a)
Phloem or bast (b) cambium (c) xylem or wood.
(a) Phloem: It
is external and has (i) sieve tubes which are large elements (ii) companion
cells which are smaller cells associated with sieve tubes and (iii) Phloem
parenchyma which is remaining mass of small cells. They are all thin and
living. Phloem is meant to conduct prepared food material from leaves to
storage organs.
(b) Cambium: It
is band of thin walled tissue lying between phloem and xylem. Its cells are in
radial rows and are rectangular in shape, very small in size and very thin
walled. Cambium is responsible for secondary growth,
(c) Xylem or
wood: It is internal and bears (i) wood vessels which are large thick walled
and lie in a few radial rows. Smaller vessels towards centre form protoxylem
and bigger ones lying away from centre form metaxylem. Tracheids and wood
fibres are smaller thick walled and lignified cells lying around metaxylem
vessels. Wood parenchyma is the patch of thin walled parenchyma lying on the
inner side of the bundle around protoxylem. Its cells are living.
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