Anatomy of Monocot & Dicot Stem | Biology for JAMB PDF Download

Internal Structure of Dicot Stem

Internal structure of a typical dicot stem shows following features :-

1. Epidermis

Epidermis is the outermost layer of the stem. It is single layered and lack of chloroplast. Stomata multicellular hairs(trichomes) are present on epidermis. Cuticle is present on epidermis.

 Epidermis plays a significant role in protection.

2. Cortex

In dicotyledon stem cortex divided into three parts :

(a) Hypodermis (b) General Cortex (c) Endodermis

(a) Hypodermis : It is present just below the epidermis. It provides additional support to epidermis. It  is thick multicellular layer. This layer is composed of collenchyma and their cells also contain chloroplast.

(b) General Cortex : This part is composed of parenchyma. Storage of food is the main function of the cortex. Resin canal/mucilage canal are present in it. These are schizogenous in origin. The innermost layer of the cortex is called endodermis.

(c) Endodermis : It is single celled thick layer. The cells of endodermis are barrel shaped. These cells accumulate more starch in stem of dicot. Thus , it is known as "starch sheath".

3. Pericycle   

This layer situated in between the endodermis and vascular bundles. The pericycle of stem is multilayered and made up of sclerenchyma. Sclerenchymatous pericyle is also known as Hard bast.

Note : In sunflower stem, pericycle is made of alternate bands of parenchymatous and sclerenchymatous cells. The part of pericycle which is present in front of the vascular bundle is made up of sclerenchyma and remaining part is composed of parenchyma. Part of pericycle which is situated in front of vascular bundle is known as Bundle cap. In sunflower stem, pericycle is heterogenous in nature.

4. Vascular Bundle 

The vascular bundles (wedge shaped) are arranged in a ring. Each vascular bundle is conjoint, collateral and open and xylem is endarch.

5. Pith

This is well developed region, spreading from ring of vascular bundle to the centre. The cells of this region mainly made up of parenchyma.

Note: The part of pith which is radially arranged between the vascular bundles, called pith rays or medullary rays.
The main function of pith rays is radial conduction of food and water.

Internal Structure of Monocot Stem

1. Epidermis

Epidermis is the outer most single celled thick layer. It is covered with thick cuticle. Multicellular hairs are absent & stomata are also less.

2. Hypodermis 

Hypodermis of monocotyledon stem is made up of sclerenchyma. It is 2-3 layered. In monocot stem rigidity is more in hypodermis where as in dicot stem elasticity is more. It provides mechanical support to plant.

3. Ground Tissue

The entire mass of parenchyma cells next to hypodermis and extending to the centre is called ground tissue. There is no differentiation of ground tissue in monocotyledon stem. It means ground tissue is not differentiated into endodermis, cortex, Pericycle etc.

Note : Sometimes in some grasses, wheat etc. the central portion of ground tissue becomes  hollow and is called pith cavity.

4. Vascular Bundle

Many vascular bundles are scattered in the ground tissue and V.B. are generally oval shape.
Vascular bundles lies towards the centre are large in size and less in number.
Vascular bundles situated towards the periphery are small in size but more in number.
Each vascular bundle is conjoint collateral and closed and xylem is endarch. Each Vascular bundle is surrounded by fibrous sclerenchymatous bundle sheath. So vascular bundles are called fibro vascular bundles.

(a) Xylem :- In xylem number of vessels is less. In metaxylem there occur two large vessels while in protoxylem there occur one or two small vessels. Vessels are arranged in V or Y shape. Just beneath protoxylem vessels, there occur a water cavity which is schizolysigenous in origin but major part of water cavity is lysigenous. This cavity is formed by disintegration of the element present below the protoxylem and neighbouring parenchyma.
Exception : In Asparagus water cavity & bundle sheath are absent.

(b) Phloem :- It consists of sieve tube elements and companion cells. Phloem parenchyma is absent.

 5. Pith

Pith is undifferentiated  in monocotyledon stems.

Old NCERT Syllabus

CUCURBITA STEM :-

•  It contains five ridges and five furrows. The vascular bundles arranged in two rows. Each ring has five vascular bundles. In this way the total 10 vascular bundles are present.

• The vascular bundles of outer ring are small in size and situated in front of ridges while the vascular bundles of inner rings are large in size and located below the furrows.

• Vascular bundles are conjoint, bicollateral and open and xylem is endarch.

• Hypodermis is absent or less developed in furrows region and general cortex contains chloroplast.

• In Cucurbita stem pericycle is sclerenchymatous.