The root system of a flowering plant begins its development from the hypocotyl of the embryo of the seed which gives rise to the primary root. Roots generally grow downwards into the soil (positively geotropic) and upwards (negatively geotropic). Roots do not bear leaves and therefore no nodes are present.Two kinds of root systems can be distinguished in flowering plants: tap root systems and adventitious root systems. Usually dicotyledons posses tap root systems and monocotyledons adventitious root systems.
Functions of the Root
Back to Top
If we study the root tip of a flower plant under a dissecting microscope or with a hand lens, the following regions can be distinguished:
The Root Cap
The tip of the root is covered by a cap that is shaped like a thimble. The outer cells of the root cap are continuously being worn away and new cells are added to the inner portion. As these cells disintegrate they form a strong protective cover.
This region occurs immediately behind the root cap. The Meristematic region consists of meristematic tissue and is protected by the root cap. The cells are very active and divide rapidly to form new cells which differentiate later to form more specialized root tissues. The cells of this region also replace the cells rubbed-off from the root cap and provide additional cells for the next region.
The cells formed in the meristematic region undergo rapid growth in length. This causes the root to elongate and penetrate deeper into the soil in search for water and mineral salts. The meristematic and elongation zones are also referred to as the region of growth.
The root hair region occurs a short distance above the region of elongation. Here a large number of fine, hair-like outgrowths are formed. The root hairs arise from the epidermal tissue of the root and are called root hairs. The root hairs are short and short-lived and develop on the primary and secondary roots. A root-hair consists of the following parts: a thin cell wall, a thin lining of cytoplasm which contains the nucleus and a comparatively large vacuole containing cell sap.
The mature region is situated above the root hair region. Here the root becomes thicker and secondary or lateral roots are developed. The secondary roots in turn rebranch to form tertiary roots. Each lateral branch has its own cap, root hairs, meristematic, elongation and mature regions. The roots in this region are covered by a protective cork layer .
A diagrammatic representation of a tap root its tissue systems.
When we study microscopically, a transverse section through the root hair region of a young dicotyledonous root, we are able to identify the following tissues from the outside to the inside:
The epidermis is the outer layer of the cells of the young root. The cells are closely- packed, thin-walled parenchyma cells with no cuticle, chloroplasts or stomata. Root hairs arise from some of the epidermal cells.
On the inner side of the epidermis is the cortex and in many plant types it can be subdivided into an exodermis, a central cortex and endodermis. The cell walls are thickened and contain suberin and lignin. The central cortex usually consists of thin-walled parenchyma cells with numerous intercellular spaces . The endodermis forms the innermost layer of the cortex.These cells are more rectangular in shape, the side walls being thickened with suberin. These thickenings are called casparian strips.
The vascular cylinder comprises all the tissues enclosed by the endodermis. It consists of the pericycle and vascular tissues(xylem and phloem). The pericycle is a single layer of thick-walled, tightly-packed cells without intercellular spaces. Lateral roots arise from the pericycle. The vascular tissue is conducting tissue in the root. It consists of xylem and phloem, which are separated from each other by parenchyma. Only primary xylem is present in young roots, which differentiated into protoxylem, which lies against the pericycle, and the metaxylem lying towards the inside. The primary xylem consists of non-living, thick- walled xylem vessels and tracheids. The phloem alternates between the arms of the xylem and consists of living thin-walled cells (sieve tubes, companion cells, and phloem parenchyma).
Diagram of a transverse section through the young root of a dicotyledon.
| stems | leaves | flowers | index |