Anatomy of stem | Internal anatomy of Dicots and Monocots

The stem is the visible part of a plant that hold holds it above the ground. This stem develops from the shoot. In the seed, the embryonic stem is called a hypocotyl.

anatomy of stem

Transverse section of a Stem of large plant

The stem is a significant organ in a plant as it is responsible for the conduction of water, mineral salts, food, etc to different parts of the plant from the roots. It is also responsible for providing support to the other structures of the plant. It helps to hold the leaves towards sunlight such that they can perform photosynthesis.

Anatomy of primary dicot stem

The  structures present in a young Dicot stem from outside to inside our epidermis, hypodermis, cortex, endodermis, pericycle, vascular bundles, medullary rays, and pith. Hypodermis and endodermis are considered to be a part of the cortex.

Epidermis

It is the outermost layer of the stem and consists of a single layer of parenchyma cells with no intercellular spaces. The epidermal cells are elongated, rectangular, or barrel-shaped. The outer walls of the cells are thickened due to a layer of cuticle, while the inner walls are thin. Epidermal cells are devoid of chloroplast and are thus transparent.  The cuticle layer reduces the rate of transpiration. The epidermis also has stomata and hair(covered by cuticle). The stomata here, like in leaves, are surrounded by guard cells, which are kidney-shaped containing chloroplast.

Hypodermis

This is the outer layer of the cortex and lies below the epidermis. It is formed by collenchymatous cells. This layer has thickenings along the tangential walls. Small intercellular spaces are present among the cells. The collenchyma cells contain a few chloroplasts(participates in photosynthesis). The hypodermis is absent or thin below the stomata region. This layer provides support to the stem and helps in elongation. The cells of the epidermis also store food.

Cortex

The cortex layer of stem is made up of multiple layers of thin-walled parenchyma cells that lie between the hypodermis and the endodermis. The cells may be oval,rounded or polygonal. Intercellular spaces are present between the cells. Some of the outer cells of the cortex contain chloroplasts and take part in photosynthesis. All the cortical cells store food. In some places the cortex contains oil ducts, resin ducts, latex vessels, or large air cavities.

Endodermis

This is the innermost layer of the cortex, and the cells are oval or barrel-shaped containing multiple starch grains. The Casparian strip is absent. The endodermis of the Dicot stem is called a starch sheath. The cells of the endodermis are used for storage.

Pericycle

Pericycle consists of multiple layers. The pericycle may be homogenous and completely sclerenchymatous, or in other cases, it is heterogeneous with semilunar patches of sclerenchyma over vascular bundles and parenchyma in between. These patches are called bundle caps as they cover the phloem of vascular bundles. The parenchyma cells store food and help in conduction.The sclerenchyma provides support.

Vascular strands

Ring of vascular bundles is present with adjacent bundles separated by medullary rays.Vascular bundles are wedge-shaped with their broader side to the pericycle and narrow roadside towards the center. The vascular bundles are conjoint, have both phloem and xylem with the phloem on the outer side. A strip of intrafascicular cambium is present between the phloem and xylem, which helps in secondary growth. Protophloem and metaphloem are not distinct, but protoxylem(present towards the center) and metaxylem are well marked. This type of xylem is called endarch.

Medullary rays

Nonvascular areas made of parenchyma present between the vascular bundles which connect pith with the pericycle and cortex. They take part in radial conduction.

Pith

A central part of the stem made up of rounded or polygonal parenchyma cells with intercellular spaces. The cells store food and waste products.

Anatomy of primary monocot stem

Monocot stem has only the primary structure. The ground tissue components are not differentiated, and the vascular bundles are scattered. The layers from outside to inside are; epidermis, hypodermis, ground tissue, and vascular strand.

Epidermis

It is a single-layered outer protective covering made of rectangular parenchymatous cells. The outer walls have cutin and silica thickenings. The epidermis has stomata which are surrounded by two dumbbell-shaped guard cells each. While other epidermal cells do not have chloroplasts, the guard cell contains chloroplasts. The silica protects from microbes, and the cuticle reduces transpiration rate.

Hypodermis

It is the outer layer of the ground tissue and lies inner to the epidermis. The hypodermis consists of two to four layers of sclerenchyma fibers. This layer is thin or absent below the stomata. Hypodermis provides strength.  The dead lignified cells of hypodermis protect from heat.

Ground tissue

It entirely occupies the interior of the stem, and there is no distinction between cortex, endodermis, pericycle, pith, and medullary rays.  The ground tissue is made up of thin-walled parenchymatous cells. The cells are small and polygonal in shape in the outer areas, while the inner regions have cells that are oval and large. There are many intercellular spaces. The outer cells contain chloroplast and can thus photosynthesize. All the cells in the ground tissue can store food. In some plants like Maize, the ground tissue remains intact in the center, while in other plants the central part of the ground tissue may degenerate to produce a central cavity. The ground tissue provides turgidity to the soft stem.

Vascular Strand

The vascular bundles remain scattered throughout the ground tissue. The vascular bundles may be oval or rounded in shape. The inner vascular bundles are smaller compared to the peripheral ones. Each vascular bundle is surrounded by a sheet of sclerenchyma called the bundle sheath. This sclerenchyma is thicker on the outer and inner sides. The vascular bundles are  of a conjoint type and possess both phloems, which lie on the outer side and xylem, which lie on the inner side. Thus the vascular bundles are collateral. No cambium strip is present between the xylem and phloem, and they are thus of closed type.

Phloem lacks parenchyma. The protophloem is present towards the outer side. Protoxylem and metaxylem are well differentiated. The protoxylem is towards centre while the metaxylem lies in the outer side. The protoxylem often has a cavity.

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