Internal Structure of Root

Introduction

Plants are anatomically complex and are divided into many categories according to several distinguishing features. Although all plants do not have roots, they are significant structures that serve various functions. Mosses and liverworts are plants without roots.

In vascular plants, the roots are the modified organs that serve as the plant’s anchor and as means of absorbing water and nutrients into the plant’s body, enabling plants to grow taller and faster. Roots can be aerial or aerating, that is, growing up above the earth or particularly above water; however, they are most frequently found below the soil’s surface.

Table of Contents

• Roots and Types of Roots
• Internal Structure of Root
  • Internal Structure of Dicot Root (Bean)
  • Internal Structure of Monocot Root (Maize)
• Frequently Asked Questions (FAQs)

Roots and Types of Roots

Roots are the essential underground portion of all vascular plants. This area of the plant is primarily in charge of securing it to the soil and absorbing the vital nutrients, water, and mineral elements from it. It is also used to preserve food.

We could see the different layers inside a root if we cut it down longitudinally. The root cap, a structure unique to roots and distinct from other plant structures, guards the tip of the root. The root cap needs to be replaced often since it is readily damaged as the root penetrates through the soil. The root tip can be divided into three areas: a zone of cell division, a zone of elongation, and a zone of maturation and differentiation.

A cross-section of the root would reveal other characteristics that are not as visible in the longitudinal section. The cortex and pith are two possible divisions of the ground tissue within the root. The pith is located between the centre of the root and the vascular tissue, while the cortex is located between the epidermis and the vascular tissue.

Types of Roots

Although the activities of each root are similar, each root’s structure is different. Thus, the root system is divided into two types based on these criteria:

Tap Root System

A taproot’s primary central root is surrounded by a network of smaller, lateral roots known as root hairs. Examples of taproot systems include mustard, beetroot, carrot, parsley, china rose, and all dicotyledons.

Fibrous Root System

On the other hand, fibrous roots are branched, bushy roots that originate from the stem and have thin, moderately branched roots. Examples of plants with fibrous roots include rice, maize, wheat, marigolds, bananas, and all monocotyledons.

Internal Structure of Root

The inner part of the root includes the vascular tissue (xylem and phloem). Stele is the name of this area which is surrounded by a layer of cells known as the endodermis, thought to be the cortex’s innermost layer. The endodermis is specific to roots and acts as a barrier for substances entering the vascular system of the root.

The walls of the endodermal cells contain a waxy material called suberin. Water and solutes are forced to cross the plasma membranes of endodermal cells by this waxy area, also known as the Casparian strip, rather than slipping between the cells. As a result, infections and poisonous substances are typically kept out of the endodermis, leaving just the materials needed by the root. The pericycle, which is the outermost cell layer of the root’s vascular tissue, can give rise to lateral roots.

Internal Structure of Dicot Root (Bean)

Dicot roots are tap root systems that aid in securing the plant to the soil.

– to draw moisture and nutrients from the ground.

– to preserve food.

– to transport nutrients, liquids, and minerals to the stem and leaves.

The following internal structures are visible in the transverse section (T.S.) of the dicot roots (sunflower, bean, and pea):

Epiblema:

• It is the outermost surface present in the dicot root.
• This layer contains unicellular root hairs, which aid in the absorption of water. It is also referred to as the Piliferous layer or the Rhizodermis.
• The dicot root lacks cuticle, which is a coat of defence found on stems and leaves.
• The stomata, or leaf openings, are also absent.

Cortex:

• The largest area of cells found in multiple layers in the transverse section is referred to as the cortex.
• Parenchymatous cells with thin walls make up these layers.
• These parenchymatous cells, which contain substantial intercellular gaps, aid in the preservation of food and water.

Endodermis:

• The innermost layer of the cortex is referred to as endodermis.
• The cells in this stratum are tightly packed into barrel shapes.
• This layer’s radial walls are frequently thickened, and this thickening may occasionally extend to the inner walls as well. The strip is made of suberin. Later stages result in lignin deposition.
• Casparian strips or bands are created as a result of this deposition. This strip functions as a checkpoint or water dam to prevent the flow of water between the cortex and phloem and to inhibit plasmolysis.
• Against the protoxylem, the endodermis cells lack Casparian strips. They are known as passage cells, and they aid in the movement of water and dissolved salts from the cortex to the xylem.

Stele:

The stele is made up of all tissues found inside the endodermis. A pericycle and vascular bundle are part of it.

(a) Pericycle:

The pericycle, an inner layer of parenchyma cells, is present in the endodermis. This layer serves as the origin of lateral roots.

(b) Vascular Bundle:

It is a vascular bundle of the radial variety. The vascular cambium of dicot root generates from the pericycle tissue situated underneath the phloem bundles. Xylem is an exarch, where protoxylem is aimed towards the periphery, and metaxylem is aimed towards the centre. Also, in a tetrarch, four bundles of xylem and phloem are present alternatively. Conjunctive tissue is the tissue that lies between the xylem and phloem tetrarch bundles. It is composed of parenchyma cells in dicot roots.

Pith:

• It essentially takes up a small portion of the centre and is made up entirely of tightly packed, thin-walled parenchymatous cells.
• The wood vessels merging in the middle can sometimes cause the pith to be almost completely destroyed.

Internal Structure of Monocot Root (Maize)

Fibrous root systems make up monocot roots. Its internal structures are almost identical to that of a dicot root.

The monocot root’s interior structures resemble those of the dicot root in many ways. Both have pericycles, vascular bundles, endodermis, cortex, and epiblema.

A monocot root has more than eight vascular bundles, whereas a dicot root only has four. Additionally, monocot roots have large, developed pith, whereas dicot roots have smaller pith.

Epiblema:

• It is the outermost layer of the monocot root. Thin-walled parenchymatous cells with no intercellular gap constitute this layer.
• This layer contains unicellular root hairs, which assist plants in absorbing water and minerals from the soil.
• It is also referred to as the Piliferous layer or the Rhizodermis.
• The monocot root lacks the cuticle, a protective coating seen on stems and leaves.
• The stomata, or leaf openings, are also absent.

Cortex:

• The cortex refers to the largest cell region with multiple layers.
• These layers are made up of parenchymatous cells, helping in the storage of food and water because of their thin walls and large intercellular spaces.

Endodermis:

• The endodermis refers to the cortex’s innermost layer.
• It is made up of tightly packed, barrel-shaped cells with no spaces between them.
• An internal strip of suberin and lignin, known as the Casparian strip, is present in young endodermal cells.
• The strip is situated near the inner tangential wall.
• The protoxylem vessels are surrounded by a group of thin cells known as passage cells that are used to conduct fluids.
• Controlling both inner and outward flow is the function of the endodermis.

Stele:

The stele is made up of all the tissues inside the endodermis. It contains pith, pericycle, and vascular tissues.

a) Pericycle:

It is made up of single-layered sclerenchymatous cells mixed with parenchyma and is located right below the endodermis.

b) Vascular Tissues:

• The alternate strands of xylem and phloem make up the vascular tissue.
• Along with phloem strands, the xylem creates distinct strands.
• The centre is dominated by the large pith that may be parenchymatous or sclerenchymatous.
• The term “polyarch” refers to vascular bundles with more than six protoxylem poles.
• Protoxylem is found near the outside, and metaxylem is found near the middle, making xylem an exarch structure.
• In contrast to metaxylem, which has large vessels and walls with reticulate and pitted thickenings, protoxylem vessels are narrow and feature annular and spiral thickenings on their walls.
• Sieve tubes, companion cells, and phloem parenchyma are the components of phloem strands.
• Protophloem is found in the periphery while metaphloem is found near the centre in the exarch phloem strands.

c) Pith:

• It is the centre region, typically made up of thin-walled parenchymatous cells which in T.S. appear polygonal or spherical.
• Pith cells may or may not have intercellular gaps between them.
• Pith can occasionally develop thick walls and lignification.
• Food is stored in pith cells.

Related Links:

• Regions of Roots
• Parts of Plants
• MCQs on Internal Structure of Root Stem and Leaf
• Monocot and Dicot Plants – Anatomy
• Preparation and Study of T.S. of Dicot and Monocot Roots and Stems (Primary)