Root Anatomy

Root Anatomy

Root systems are responsible for the following functions:

• absorption of water and organic compounds;
• anchoring of the plant body to the ground; and
• storage of food and nutrients.

When a seed germinates, the first structure to appear is the root or the radicle. This becomes the primary root. Other roots that branch out of the primary root are called secondary roots (see the figure below). The growing root tip is protected by the root cap as it moves through the coarse soil. The root cap is slimy in nature to facilitate easy movement. Above the root cap is the apical meristem. In this meristematic region, cells divide continuously by mitosis to produce new cells. In addition to mitosis, newly divided cells undergo elongation in the same direction of root lengthening.

Above the region of cell elongation, thousands of tiny root hairs are found in the root hair region. The function of the root hairs is to absorb water and dissolved mineral salts from the soil. As the root grows, it thickens and may produce lateral roots in the mature region as shown in the figure below.

There are two major types of root system.

1. Taproot system: root system comprising one primary root and many secondary roots branching off the primary root. Examples of taproots include carrot and beetroots where the roots serve a storage function. Taproots are found in dicotyledonous plants.
2. Fibrous root system: system with no dominant primary root but many secondary roots of similar size. Fibrous (adventitious) root systems are common in monocotyledons. Examples include coconuts and grasses.

Tissue distribution in root

The different tissues in the root have a distribution which is common to all dicotyledonous plants and is shown in the figure below.

The epidermis is a single layer of cells on the outside that protects the inner tissues. The epidermal layer of the root has no waterproof cuticle as this would prevent the absorption of water. Structurally, the cells of the root hair (shown in the figure below) have large central vacuoles and cover a large surface area which allows water to enter these cells readily via osmosis.

The cortex consists of parenchyma cells. These large, thin-walled cells have leucoplasts to store starch and large vacuoles to store water and dissolved sugars. Intercellular spaces between parenchyma cells facilitate the movement of water from the root hair cells on the outside of the plant to the xylem on the inside of the plant.

The endodermis forms the innermost layer of the cortex. It is a layer of tightly-packed, modified parenchyma cells. The radial and transverse cell walls are thickened with a water-impermeable, waxy suberin layer, known as the Casparian strip. This layer helps to regulate the flow of water from the cortex into the stele, rather than allowing the water to spread to all the root cells. To aid in directing water, there are also thin-walled passage cells in the endodermis, directly opposite the xylem, allowing water to move into the xylem rapidly.

The stele, or vascular cylinder (responsible for transporting water and minerals), consists of the pericycle, phloem, cambium and xylem. The pericycle is the outermost layer of the stele, and consists of one or more rows of thin-walled meristematic parenchyma cells. It is in close contact with the xylem and phloem tissues of the root. It functions in the formation of lateral roots.

The phloem tissue is responsible for transporting food from the leaves of the plant to the cells of the root. The cambium separates the xylem and phloem tissues from each other. This is the area where secondary growth of xylem and phloem tissues occur. Xylem tissue is responsible for transporting water and dissolved mineral salts to the xylem tissue of the stem and leaves. These cells are strengthened with lignin for support. The pits in the cell walls allow for the lateral movement of water. The figure below shows stained root tissues visualised by confocal microscopy. It shows the internal structure of root cells, including the epidermis, cortex, endodermis and pericycle.