Internal structure of stem

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 .

9. 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 .

10. T.S. of maize shows following reasons : - Epidermis :- This outermost single layer of cell is cutinized . A few stomata are also found in epidermis . Hypodermis :- Just below the epidermis , there are about 2 to 3 layers of scelrenchyma . This zone is called hypodermis . 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 . 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 .

11. # 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.