Anatomy of Primary Dicot Root

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 A dicotyledonous plant with its root
Image source: File:Dicotyledon plant-let showing roots.jpg - Wikimedia Commons

In this post, I am presenting a description of the internal structure of a young dicot root. 

A young dicot root possesses epiblema, cortex, endodermis, pericycle, vascular bundles, conjunctive tissues, and pith (see Figure 12).

Epiblema

Epiblema is the outermost single layer of root. It is made of compactly arranged, thin-walled, radially elongated parenchymatous cells without intercellular spaces. It does not have cuticle and stomata. Some of its cells are elongated and with unicellular tubular outgrowths, called root hairs. Due to presence of root hairs, it is also known as piliferous layer. It is also called rhizodermis. Root hairs help in absorption of water and mineral salts from soil.

Cortex

Cortex lies below epidermis. It is made of a few layers of thin-walled, oval, rounded or angular parenchyma with intercellular spaces. It helps in storage of food. It also conducts water from epiblema to vascular system.

Endodermis

Endodermis is the innermost layer of the cortex. It is made of a single layer of barrel-shaped compactly arranged parenchymatous cells without intercellular spaces. Internal strips of suberin and lignin, called Casparian strips, are present along the radial and tangential walls of young cells lying opposite of phloem. Casparian strips do not allow substances to move from cortex to pericycle. So, these tissues are called biological check posts. Endodermis has passage cells at the opposite of protoxylem. These cells allow substances to enter into vascular system from cortex.

Pericycle

Pericycle lies just below the endodermis. It is made up of thin-walled parenchymatous cells without intercellular spaces. It helps in the formation of vascular cambium, cork cambium, and lateral roots. Pericycle is absent in the roots of some aquatic dicot plants and parasitic plants.

Vascular bundles

Vascular bundles lie inner to pericycle. Xylem and phloem bundles are present in equal number but at different radii. This type of vascular bundle is known as radial vascular bundle. Vascular bundles may be two to six.

Xylem is in exarch condition i.e. protoxylem (the first formed xylem) lies towards periphery but metaxylem (the later formed xylem) lies towards centre of the root. Xylem contains vessels, a few tracheids, xylem parenchyma, and rarely xylem fibres.  Xylem helps in conduction of sap and provides mechanical support.

A phloem bundle is present between two adjacent xylem bundles. It contains sieve tubes, companion cells, phloem parenchyma and rarely phloem fibres. Fibres may occur outside the phloem in some roots (in Gram roots).

Conjunctive tissues

In between xylem and phloem bundles, one or many layers of thin-walled, elongated, polygonal parenchymatous cells without intercellular spaces are present. Such tissues are known as conjunctive tissues. Firstly, they are for the storage of food. Later on, they become meristematic to form the vascular cambium.

Pith

Pith is the central part of a root. It may be absent or reduced in dicot root. If present, it is made of thin-walled, oval, rounded or polygonal parenchymatous cells with or without intercellular spaces. It helps in storage of food.

Figure 12a T. S. of dicot root (diagrammatic)

Figure 12b T. S. of dicot root (detailed view)