The slides are about the internal structure of stem . And it is presenter by:- Dr. Manoj das

1. Internal structure of stem
Manoj Das

2. Internal structure of stem
• Important anatomical features of stem are as-
• i. The epidermis is covered by a cuticle layer. In herbaceous stem,
stomata are present. It is protective in nature
• ii. The multicellular hairs are present.
• iii. A collenchymatous or sclerenchymatous hypodermis is present.
• iv. The cortex is only present in dicot stem.
• V. The xylem is endarch.

3. • Vi. The VBs are conjoint and collateral or bicollateral.
• Internal structure of dicot stem
• Important anatomical features of dicot stem are as-
• i. The epidermis usually contains multicellular hairs.
• Ii. The hypodermis is collenchymatous.
• Iii. The ground tissue is differentiated into cortex,
endodermis pericycle and pith.

4. • Iv. VBs are arranged in rings.
• V. VBs are conjoint collateral or bicollateral and open type.
• Vi. The bundle sheath is absent.
• Vii. The phloem parenchyma is present.
• Viii. The medullary rays are present between the VBs
• Ix. The secondary growth occurs due to the presence of
cambium.

5. • Internal structure of a typical dicot stem Helianthus shows the
following structure-
• 1. Epidermis- It is the uniseriate, outermost protective, supporting and
rigid layer. It is composed of compactly arranged parenchymatous cells.
It is externally covered with thick cuticle layer. Stomata are present in
young and herbaceous stem .Some epidermal cells produce
multicellular hairs. Both cuticle layer and hairs reduce the rate of
transpiration. The cuticle layer also prevent from the entry of
pathogens.
• 2. Hypodermis- About 4-5 layers of collenchyma occurring just below
the epidermis in form of hypodermis. Chloroplasts are also

6. • present some of these cells. It provides mechanical support and help
in Storage of food materials.
• 3. Cortex- It lies below the hypodermis. It is 2-3 parenchymatous
layers having a large number of intercellular spaces. A few oil ducts
are located in this region. The intercellular spaces help in gaseous
exchange between interior part of the stem and outer atmosphere.
The cortical cells help in storage of food materials.
• 4. Endodermis- It lies below the cortex. It is made of compactly
arranged barrel shaped parenchymatous cells. These cells posses
starch grains and

7. • So, the endodermis is also known as starch sheath.
• 5. Pericycle- It is situated below the endodermis. It is multilayered
and consists of alternate patches of parenchyma and sclerenchyma.
But the sclerenchyma lies above the vascular bundle in form of
bundle cap and also known as hard bast because it provides
mechanical support to the phloem.
• 6. Vascular bundles – They are arranged in a ring. They are separated
with each other by medullary rays. Each VB is conjoint collateral and
open. It consists of
• i. Phloem- It lies just below the sclerenchymatous patch of pericycle
and made of sieve elements, companion cells and phloem
parenchyma.
• Ii. Cambium- It is located between xylem and phloem. It is made up
of 2-3 layers of thin walled meristematic cells. It is responsible for

8. • For secondary growth in dicots
• iii. Xylem- It is endarch and forms the innermost part of VB. It consists
of tracheids vessels, xylem parenchyma and wood fibers. The
protoxylem consists of annular, spiral or scalariform thickenings but
metaxylem shows pitted or reticulated thickening.
• 7 . Pith or medulla :- The centre of the stem is occupied by a large and
well developed pith . It is made of parenchymatous cells with large
inter cellular spaces . It help in the food storage .
• 8 . Medullary rays :- The pith is connected with the cortex is known as
medullary rays . V.Bs. are separated from each other by medullary rays
are thin walled , parenchymatous and radically elongated in position .

10. Internal structure of monocot stem .
• Distinguishing character –
1 . The monocotyledonous stem do not show differentiation of tissues
into cortex and pith . The entire tissue is collectively known as ground
tissue .
2 . Endodermis is also absent .
3. The vascular bundles are scattered throughout the ground tissue
without any arrangement .
4. The vascular bundle are closed , cambium being absent .
5. The bundles are generally surrounded by a bundle starch.
6. Medullary rays are also absent .

11. • T.S. of maize shows following reasons : -
1. Epidermis :- This outermost single layer of cell is cutinized . A few
stomata are also found in epidermis .
2. Hypodermis :- Just below the epidermis , there are about 2 to 3
layers of scelrenchyma . This zone is called hypodermis .
3. Ground tissue : - All the parenchymatous tissue extending from
below the hypodermis up to the center of the section is called
ground tissue . Cortex , endodermis and pericycle are completely
absent .
4. Vascular bundle :- Numerous vascular bundle are scattered irregularly
throughout the ground tissue . The vascular bundles towards the
periphery of the stem are smaller than those situated closer to the
center of stem . Each vascular bundle is conjoint , collateral , endarch
and closed . It is surrounded by a sclerenchymatous bundle sheath .

12. • # Xylem- It is Y shaped. It consists of four distinct vessels surrounded by
many tracheids. Metaxylem consists of two wide and pitted vessels
which form the two arms of Y. Protoxylem elements , generally one or
two with spiral or annular thickenings form the base of Y. In a mature
bundle, the inner protoxylem elements to disintergerate to form a
large lysigenous water cavity.
• # Phloem- It is situated between forked arms of Y shaped xylem. These
represent metaphloem. It is small in amount. The protophloem that
lies near the periphery of the bundle is disfigured protophloem. The
phloem consists of sieve tubes and companion cells. The phloem
parenchyma is absent.