Anatomy of Dicot Root Primary Structure

Anatomy of a dicot root primary structure can be studied through a Cross Section (CS). The present post discesses the Anatomy of Dicot Root Primary Structure with diagrams.

Anatomy of Dicot Root

Ø  Anatomically, the primary structure in a dicot root is differentiated into the following tissue zones:

(1).   Root cap
(2).   Epidermis
(3).   Cortex
(4).   Endodermis
(5).   Pericycle
(6).  Vascular Tissue
(7).   Conjunctive Tissue
(8).   Pith

(1). Root Cap

Ø  Root cap is a mass of tissue present in the exact tip of the root.

Ø  Root cap is also called as calyptra.

Ø  Root cap composed of only closed packed parenchymatous cells.

Ø  Root cap contains specialized gravity perception cells called statocytes.

Functions of root cap

$  Protection of root meristem.

$  Acts as the site of perception of gravity.

$  Have the capacity to control the activity of meristematic cells in the root apex by producing growth hormones.

(2). Epidermis

Ø  Also called as piliferous layer, epiblema or rizodermis.

Ø  It is the outermost layer of cells derived from dermatogen of the root apex.

Ø  Composed of a single layer of compactly packed parenchymatous cells.

Ø  Cells are barrel shaped, cuticle and stomata are absent.

Ø  Some epidermal cells give off unicellular root hairs.

Ø  Epidermal cells which give rise the root hairs are called Trichoblasts.

Ø  Root hairs are epidermal extensions.

Ø  Root hairs absorb nutrients and water from the soil.

Ø  Root hairs increase the surface area for absorption.

Ø  Root hairs are ephemeral (= short lived) structures.

Ø  Root hairs are absent in the exact tip portion of the root.

Ø  In herbaceous plants, the epidermis is long lived and acts as the chief protective tissue.

Ø  In a majority of dicots, the epidermis is immediately replaced by the bark during secondary growth.

Functions of epidermis

$   Acts as the outermost boundary

$   Provide protection

$   Root hairs absorb water and nutrients from the soil

(3). Cortex

Ø  Cortex is simple, composed of parenchymatous cells.

Ø  Cells are thin walled and loosely packed with plenty of intercellular spaces.

Ø  Cortex is undifferentiated.

Ø  Chlorenchyma is usually absent in the cortex of roots.

Ø  In some plants (hydrophytes) cortex contain a large amount of aerenchyma.

Ø  Cortical cells show distinct pattern of arrangement as distinct rows.

Ø  Cortical cells store large amount of starch grains.

Ø  Plenty of secretory structures and idioblasts are present in the cortex.

Functions of Cortex

$  Aerenchyma in the cortex facilitates air exchange.

$  Thin walled cells allow the transport of water from cortex to xylem.

$  Cortex maintains the root pressure.

$  Cortical cells store food materials as starch grains.

$  Aerial roots can perform photosynthesis.

$  Sclerenchymatous cells in the cortex provide mechanical support.

$  Air cavities in the cortex of aquatic plants provide buoyancy.

$  Vascular cambium during secondary growth is derived from the cortex.

(4). Endodermis

Ø  Endodermis is the innermost layer of cortex.

Ø  Endodermis is very distinct and prominent in dicot root.

Ø  Composed of a single layer of barrel shaped cells.

Ø  Shows special thickening in the radial and inner tangential wall.

Ø  This special type of thickening is Casparian Thickening of Casparian Band.

Ø  Endodermal cells opposite to proto-xylem elements remain thin-walled and these cells lack the Casparian thickening.

Ø  Endodermal cells devoid of Casparian thickening are called Passage Cells.

Ø  Endodermal cells store plenty of starch grains, hence called Starch Sheath.

Functions of Endodermis

$   Regulation of movement of water from cortex to xylem.

$   Endodermal cells can store starch grains.

(5). Pericycle

Ø  A layer of cells present next to the endodermis.

Ø  It is the outermost layer (boundary) of the vascular cylinder.

Ø  Usually composed of thin walled parenchymatous cells.

Ø  Pericycle is usually uniseriate (single layered).

Ø  In some plants (Ficus benghalensis and Morus) the pericycle is multiseriate.

Ø  Pericycle is absent in most of the aquatic plants and in some parasites.

Functions of pericycle

$   Lateral roots originate from the pericycle.

$   In some plants pericycle also give raise the phellogen (cork cambium).

(6). Vascular Tissue

Ø  In roots, vascular bundles show radial arrangement.

Ø  Radial arrangement: xylem and phloem bundles are arranged alternatively in different radii.

Ø  Vascular bundles are limited in number, 2 (diarch) to 6 (hexarch).

Ø  Usually, it is tetrarch (four xylem and phloem strands).

Ø  Xylem is exarch (proto-xylem is oriented towards the exterior and meta-xylem towards the interior).

Ø  Meta-xylem elements are polygonal (angled) in outline (in cross section).

Ø  Phloem usually composed of sieve tubes, companion cells and phloem parenchyma.

Ø  Phloem fibres are generally absent in the primary vascular tissue of dicot root.

Ø  Proto-phloem occupies toward the periphery whereas the meta-phloem towards the centre.

Functions of vascular tissue

$   Conduction of water and minerals (xylem)

$   Conduction of food materials (phloem)

$   Provide mechanical support

(7). Conjunctive tissue

Ø  Parenchymatous tissue present between xylem and phloem are called conjunctive tissue.

Ø   Also called as conjuctive tissue, connective tissue or complementary tissue.

Ø  Inter-fascicular cambium originates from the conjunctive tissue during secondary growth.

(8). Pith

Ø  Pith is usually absent in dicot root

Ø  If the pith is present, very small and centrally located with loosely packed parenchymatous cells.

Practical identification points (Dicot Root Anatomy- Primary, Example: Tinospora, Ficus)

Ø  Single layer of epidermis without cuticle

Ø  Presence of unicellular unbranded epidermal hairs.

Ø  Cortex undifferentiated, chlorenchymatous zone absent in the cortex.

Ø  Very distinct endodermis and pericycle

Ø  Radial arrangement of vascular bundles.

Ø  Exarch xylem

Ø  Prominent Casparian thickening and distinct passage cells.

………………………………….………………………. Root

Ø  Limited number of vascular strands (usually 4).

Ø  Vessel elements of xylem are polygonal in outline in cross section.

Ø  Pith absent, very small if present.

………………………….………………….. Dicot Root

Review Questions

1.  With a neat cellular diagram, explain the anatomy of Dicot root primary structure.
2. What is casparian thickening?
3. What are the functions of endodermis in roots?
4. How the anatomical features of dicot root is different from dicot stem?
5. Describe the structure of vascular tissue in dicot roots.
6. What are the functions of cortex in dicot roots?
7. What are the peculiarities of root cortex in hydrophytes?
8. What are the difference between dicot root and monocot root?

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@. Anatomy of Monocot Root (with PPT)

@. Anatomy of Dicot Stem Primary Structure (with PPT)

@. Anatomy of Monocot Stem (with PPT)

@. Difference between Dicot and Monocot Stem