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Anatomy of stem

Introduction:

  • Stems originates from the plumule of a germinating seed.
  • It can bear leaves, fruits, flowers, etc. A stem is identified by the presence of nodes and internodes.
  • It is used for the support of other parts of the plant and conduction of food, water and minerals.
  • Stem anatomy shall make us understand and know about the various tissues involved in the function of water and food conduction.
  • It also shall help us identify the tissues that are involved in providing mechanical strength to the plant.

Topics covered:

  • Anatomy of Dicot Stem
  • Anatomy of Monocot Stem
  • Comparative study of dicot stem and monocot stem

Anatomy of Dicot Stem
 

Definition and characteristic features:

  • The tissues are found to be arranged in a concentric fashion in the transverse section of a dicot stem.
  • Dicot stem showcases tissues, i.e. Epidermis, hypodermis, cortical layers, endodermis, pericycle, vascular bundles, pith.

Detailed explanation:

ground-plan-sector-enlarged


i) Epidermis

  • The outermost layer is the epidermis which is composed of parenchymatous cells.
  • This layer is often coated with a waxy layer of the cuticle which reduces the rate of transpiration.
  • Stomata and trichomes are present on the epidermis.

ii) Cortex: In dicotyledon stem cortex is divided into three parts:
    (a) Hypodermis
    (b) General cortex
    (c) Endodermis

(a) Hypodermis

  • This layer is found just beneath the epidermis and is composed of collenchyma.
  • There is found added thickening on various parts of the cells, such as tangential walls, angular walls and close to the intercellular spaces.
  • The hypodermis is a supporting tissue for the epidermis and provides it with mechanical strength and flexibility.
  • It can act as food storage as well as photosynthesis sites.

(b) General cortex

  • Cortex is made up of rounded, parenchymatous cells with prominent intercellular spaces.
  • Resin canal/mucilage canal is present in it.
  • These are schizogenous in origin.
  • The main function of the cortex is the storage of food.

(c) Endodermis

  • The endodermis is found at the back of the cortex as its last layer.
  • The cells are in the shape of a barrel and absent of intercellular spaces.
  • Casparian strips are absent.
  • Endodermal cells contain conspicuous starch grains as food reserves. Hence, endodermis is also known as a starch sheath.

iii) Pericycle

  • It lies between the endodermis and vascular bundles.
  • The pericycle of the stem is multi-layered and made up of sclerenchyma.
  • Sclerenchymatous pericycle is also known as Hard bast.
  • The pericycle is heterogeneous and is composed of alternate bands of parenchymatous and sclerenchymatous cells. e.g. sunflower stem.
  • Part of the pericycle which is situated in front of the vascular bundle is known as Bundle cap.
  • In plants like Cucurbits, it is present as a continuous cylinder of sclerenchymatous cells.

iv) Vascular bundle

  • Vascular bundles are conjoint, collateral, open and endarch.
  • The vascular bundles are arranged in a ring (eustelic condition) & are wedge shaped.
  • Each vascular bundle is made of phloem and xylem with cambium in between them.
  • In Cucurbits vascular bundles are of bicollateral and open type.

v) Medullary rays

  • Rows of parenchymatous cells arranged radially from pith towards the periphery which appear like spokes of a wheel are called medullary rays.
  • They provide pathways for radial conduction of food and water.

vi) Pith

  • It constitutes the central portion of the stem.
  • It is formed of a large number of rounded, parenchymatous cells with large intercellular spaces.
  • The pith can act as food storage but in many dicots, they disintegrate with time and form a cavity.

Anatomy of Monocot Stem
 

Introduction:

  • The tissues are not found to be arranged in a concentric fashion in the transverse section of a monocot stem.
  • Monocot stem does not feature secondary growth, hence the stem is made up of only primary permanent tissues.

Detailed explanation:


t-s-monocot-vascular-bundle
 

t-s-stem-monocot

i) Epidermis

  • It is the outermost single celled thick layer covered by thick cuticle.
  • Multicellular hairs are absent & stomata are also less.

ii) Hypodermis

  • This layer is found just beneath the epidermis 2-3 layers thick and is sclerenchymatous.
  • The hypodermis is a supporting tissue for the epidermis and provides it with mechanical strength.

iii) Ground tissue

  • The entire mass of parenchyma cells next to hypodermis and extending to the centre is called ground tissue.
  • In monocots, the ground tissue is not differentiated into endodermis, cortex, pericycle, pith, etc.

iv) Vascular bundles

  • Vascular bundles are scattered in the ground tissue and generally in an oval shape.
  • Vascular bundles are smaller towards the periphery and larger towards the centre.
  • Vascular bundles are conjoint, collateral, closed with endarch xylem.
  • Each vascular bundle is surrounded by a layer of sclerenchymatous tissue that is known as bundle sheath.
  • Atactostele (vascular bundles not arranged in a ring) is found in monocot.
  • Normally secondary growth does not occur in monocotyledonous stem but in Yucca & Dracaena shows anomalous secondary growth.
    (a) Xylem: In xylem, Vessels (metaxylem & protoxylem) are arranged in V or Y shape.There is a water- containing cavity at the end of protoxylem vessels and parenchyma. This cavity is formed by disintegration of the element present below the protoxylem and neighbouring parenchyma.
    (b) Phloem: It consists of sieve tube elements and companion cells. Phloem parenchyma is absent.

Comparative study of dicot stem and monocot stem
 

Detailed explanation:

  • The major difference between dicot and monocot stem is w.r.t. secondary growth.
  • Monocots do not exhibit secondary growth hence possess closed vascular bundles while dicots exhibit secondary growth and possess open vascular bundles.
  • The differences between dicots and monocots are laid down in a tabular form-
     
Sl. No. Monocot stem Dicot stem
1. Hairs are normally absent. Presence of multicellular hairs.
2. Epidermis has larger cells. The epidermis has smaller cells than monocot stems.
3. Hypodermis is of sclerenchymatous nature. The hypodermis is collenchymatous.
4. Cortex is absent normally, but from hypodermis to centre of the stem, ground tissue is present. Cortex is made up of multilayered parenchymatous cells.
5. Absence of endodermis. Poorly developed endodermis present.
6. Absence of pericycle. Multilayered pericycle present.
7. Absence of medullary rays. Medullary rays are present in between the vascular bundles.
8. Absence of pith. Presence of pith.
9. Vascular bundles-  
i. Vascular bundles are scattered. Vascular bundles arranged in a ring formation.
ii. Conjoint, collateral and closed vascular bundles. Conjoint, collateral and open vascular bundles.
iii. Vascular bundles are larger at the centre and smaller towards the periphery. Vascular bundles appear in the uniform size.
iv. Presence of bundle sheath around the vascular bundles. Absence of the bundle sheath cells.
v. Absence of phloem parenchyma. Presence of phloem parenchyma.


Frequently asked questions (FAQ)
 

Q1. Differentiate between dicot and monocot stem?
Ans:

 

Sl. No. Monocot stem Dicot stem
1. Hairs are normally absent. Presence of multicellular hairs.
2. Epidermis has larger cells. The epidermis has smaller cells than monocot stems.
3. Hypodermis is of sclerenchymatous nature. The hypodermis is collenchymatous.
4. Cortex is absent normally, but from hypodermis to centre of the stem, ground tissue is present. Cortex is made up of multilayered parenchymatous cells.
5. Absence of endodermis. Poorly developed endodermis present.
6. Absence of pericycle. Multilayered pericycle present.
7. Absence of medullary rays. Medullary rays are present in between the vascular bundles.
8. Absence of pith. Presence of pith.
9. Vascular bundles-  
i. Vascular bundles are scattered. Vascular bundles arranged in a ring formation.
ii. Conjoint, collateral and closed vascular bundles. Conjoint, collateral and open vascular bundles.
iii. Vascular bundles are larger at the centre and smaller towards the periphery. Vascular bundles appear in the uniform size.
iv. Presence of bundle sheath around the vascular bundles. Absence of the bundle sheath cells.
v. Absence of phloem parenchyma. Presence of phloem parenchyma.


Q2. Why is there no wood formation amongst monocot plants?
Ans:
Monocot stem lacks the presence of cambium ring or vascular cambium which is responsible for the secondary growth or formation of secondary xylem and phloem.

Q3. What is the eustelic condition?
Ans:
This condition is represented by vascular bundles forming a ring around the pith, and the presence of parenchyma cells between the bundles.

Q4. What do you mean by hard bast?
Ans:
This can be defined as a patch of sclerenchyma cells that are found right outside the vascular bundles thus forming a bundle cap. These caps help provide added mechanical support.

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