Anatomy of Monocot Root

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 Prop root of Maize (Zea mays)

In this post, I am presenting a concise description of the internal structure of monocot root.

In monocot plants, a young and an old root do not have such distinction due to lack of secondary growth. A typical monocot root consists of epiblema, cortex, endodermis, pericycle, vascular bundles, conjunctive tissues, and pith (see Figure 13).

Epiblema
Epiblema is the outermost single layer of the root that is made up of compactly arranged, thin-walled, radially elongated parenchymatous cells without intercellular spaces. It does not have cuticle and stomata. Some cells elongate and make unicellular tubular outgrowths, called root hairs. As it has root hairs, epiblema is also called piliferous layer. Root hairs help in absorption of water and minerals from the soil.

Cortex

Cortex lies below the epiblema. It consists of many layers of thin-walled, oval, rounded or angular parenchyma with intercellular spaces. In older roots, outer one layer (in Smilax) or many layers (in Zea mays) of cortex contain sclerenchymatous tissues, called exodermis. Exodermis gives mechanical support. Cortex helps in conduction of water from the root hairs to the vascular system and storage of food.

Endodermis

Endodermis is the innermost layer of cortex which is made of barrel-shaped compactly arranged parenchymatous cells without intercellular spaces. Young endodermal cells lying opposite to phloem possess an internal strips or bands of suberin and lignin, called Casparian strip. A few thin-walled cells lying opposite to protoxylem are called passage cells. Casparian strips do not allow the movement of substances from cortex to pericycle. However, passage cells conduct fluid inwards and outwards.

Pericycle

Pericycle lies below endodermis. It is made up of thin-walled parenchymatous cells without any intercellular spaces. Pericycle may be uniseriate (single layered in maize) or multiseriate (multilayered in Smilax). It is responsible for the formation of lateral roots.

Vascular bundles

Vascular bundles are made of xylem and phloem. Xylem and phloem are present in different bundles. They are equal in number and lie alternate to each other. So, these vascular bundles are called radial vascular bundles. Vascular bundles are more than six (polyarchy) in monocot root.

Xylem is made of tracheids, vessels, xylem parenchyma, and xylem fibres. Vessels are rounded or oval. Xylem bundles are exarch i.e. protoxylem (the first formed xylem) lies towards periphery while metaxylem (the later formed xylem) lies towards the centre of a root. Xylem helps in conduction of water and minerals. It also provides mechanical support.

Phloem consists of sieve tubes, companion cells, rarely phloem fibres. Phloem transports food.

Conjunctive tissue

One or many layers of thin-walled parenchymatous or thick-walled sclerenchymatous cells are present in between phloem and xylem, called conjunctive tissues. The parenchymatous conjunctive tissues store food whereas sclerenchymatous conjunctive tissues help in mechanical support.

Pith

Pith is the centre of a root. In monocot root, pith is with large area. It is composed of thin-walled, oval, rounded or angular parenchymatous cells with or without intercellular spaces. It stores food.

Figure 13 a. T.s of monocot root (diagrammatic)

Figure 13b. T. S. of monocot root (detailed view)