Anatomy of Monocot Stem

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 A maize stem bearing a newly sprout fruit

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

A typical monocot stem consists of epidermis, hypodermis, ground tissues, and vascular bundles (see Figure 15).

Epidermis

Epidermis is the outermost layer of monocot stem. It is composed of compactly arranged, elongated, barrel-shaped parenchymatous cells without intercellular spaces. It contains stomata. Each stoma has a pair of specialized dumb-bell shaped cells, called guard cells. It helps in the protection of internal tissues, exchange of gases through stomata, minimize the surface transpiration by cuticle, and prevent the entry of harmful organisms.

Hypodermis

Hypodermis lies below epidermis. It is made up of 2- to 3-layered thick-walled dead lignified sclerenchyma. It helps in mechanical support.

Ground tissues

Ground tissue lies below the hypodermis. It is homogenous mass of tissue that is not differentiated into cortex, endodermis, pericycle, pith, and medullary rays. It is made up of oval or rounded parenchymatous cells with intercellular spaces. It helps in storage of food.

Vascular bundles

Vascular bundles in monocot stem are atactostele i.e. vascular bundles are scattered throughout the ground tissue. They are small and numerous towards the periphery but they are large and few towards the centre. They are oval or rounded.

Each vascular bundle contains both phloem and xylem. Xylem lies towards the centre and phloem lies towards the periphery. Cambium is absent. This type of vascular bundle is called conjoint, collateral, and closed. Each vascular bundle is surrounded by sheath of parenchyma, called bundle sheath.

Phloem

Phloem consists of sieve tubes, companion cells, and a few phloem fibres. Phloem parenchyma is absent. Sieve tubes conduct organic matters.

Xylem consists of vessels, tracheids, xylem parenchyma, and a few xylem fibres. Protoxylem is smaller than metaxylem. Protoxylem is first formed xylem whereas metaxylem is later formed xylem. Protoxylem lies at the centre while metaxylem lies towards periphery. This type of xylem is called endarch xylem. Vessels are arranged to form ‘V’ or ‘Y’. Some of the protoxylem vessels and xylem parenchyma cells are separated during rapid growth of stem and form a cavity, called protoxylem cavity (schizo-lysigenous cavity). Protoxylem cavity stores water. Xylem helps in conduction of sap and provides mechanical support.

Figure 15a T. S. of monocot stem (diagrammatic)

Figure 15b. T. S. of monocot stem (detailed view)