Root is the descending organ of the plant, which grows away from light. It is usually white in colour in young stage and later on, may become brownish. It usually does not bear leaves or buds, with certain exception like the root of sweet potatoes, wood apple, etc, which bear buds for vegetative propagation.
The root bears unicellular hairs, with the help of which it can absorb water from the soil. In roots, nodes and internodes are not differentiated. It usually possesses a protective covering at the tip, known as root cap.
Regions of root:
- Root cap: Each root is covered at its tip by a cap-like structure, called the root cap. It protects the tender apex of the root. The root cap, if injured or worn out, can get repaired and renewed by the growing tissue of the root, lying behind it. However, in the roots of free-floating aquatic plants, usually root caps are not developed.
- Region of cell division: This is the growing region and it lies within and slightly beyond the root cap. It consists of very simple tissue, the cells of this region divide and regarding-divide giving rise to new cells, and hence, this region is otherwise called the meristematic region.
- Region of elongation: This region is an extension of the meristematic region lying just above it. The cells of this region do not divide but undergo rapid elongation and enlargement. The growth and length of the root depends mainly on the activity of this region and hence, this is called the region of elongation.
- Region of maturation: This region lies next to the region of elongation. In this region, the cells of the root mature and at the basal portion, the cells of the peripheral layers produce a cluster of unicellular and delicate thread-like structure known as root hairs. For this reason, this region is also known as zone of root hairs. Above the root of zone hairs, lateral roots are developed. The root hairs and the lateral roots are arranged in acropetal succession. The root hairs are responsible for absorption of water and mineral nutrients from the soil. The matured cells of this region lying towards the base of the root get differentiated into various kinds of primary tissues and gradually merge into the regionof secondary tissues.
Classification of roots:
Roots are broadly classified into normal roots and modified roots. This classification is based on their function, but according to their origin, each of them again fall into two types – tap root and adventitious root.
- Tap root: When the radicle develops into a root, it is known tap root or primary root. The tap root normally grows vertically downward in the soil. It produces the lateral root, also known as secondary roots. The secondary roots further give rise to tertiary roots. All these roots together form the tap roots’ system. This type of root is known as normal tap root.
- Adventitious root: Roots which are not developed directly from the radicle but from any other part of the plant such as stem, branch or leave are known as adventitious roots. Adventitious roots are of two types – fibrous roots and foliar roots.
Fibrous roots: In monocotyledonous plants, the growth of the tap root is checked and a cluster of fiber-like root is developed from the basal portion of the stem. In many grasses such thin, fiber-like roots are developed from the nodes of prostrate stem. These roots are known as fibrous root, examples, Oryza sativa, Cynodon etc.
Foliar roots: Roots which develop from the leaves are known foliar roots. They are usually developed from the basal portion of the petiole, when an isolated leave is kept immersed in water for a few days. Leaves of sweet potato easily produce such foliar roots.
In addition to absorption and conduction of water, both the tap root and adventitious roots serve many other specialized functions. Such roots are called modified roots.
Modified tap root:
The main functions of these roots are to store food material and they are as follows:
- Fusiform root: When the root is narrower at the two ends and swollen in the middle, it is known as fusiform. Example; radish.
- Napiform root: When the root is very much swollen at the basal end so that it becomes almost spherical and suddenly becomes narrower at the terminal end, it is called napiform root. Example, Brassica rapa.
- Conical root: When the root looks like a cone being broad at the base and gradually tapering towards the apex, it is known as conical root, example; carrot.
- Tuberous root: When the root is swollen and irregular in shape it is called tuberous root. Example, Mirabilis jalapa.
Modified adventitious root:
Based on the function, they are classified into the following categories:
- Prop root: In so me trees, adventitious roots are developed from the large, horizontally spreading branches. These roots grow vertically downwards and ultimately penetrate into the soil. Gradually, they become thicker and stouter and give mechanical support to the spreading branches. Such roots are known as prop roots. Example; Ficus benghalensis.
- Stilt root: In case of Pandanus, roots are developed from the trunk of the plant and grow obliquely. They penetrate into the soil and allow the plant to grow vertically. Such roots are called stilt roots.
- Climbing root: Clusters of roots are developed from the nodes of the stems of some climbing plants like Piper betel etc. These roots secrete a sticky juice, which dries up quickly and ensures firm attachment with the support. In some cases, a sort of disc or claw is developed at the apex of these roots, giving a firmer foothold to the climber. This type of root is known as climbing or clinging root.
Storage of food:
- Tuberous root: When a root becomes swollen without having any definite shape, it is known as tuberous or tubercular root. It is produced singly from either the node or internode of the stem. Example; sweet potato.
- Fasciculated root: When more number of tuberous roots are produced from the basal region of the stem and form a cluster or fascicle, they are said to be fasciculated root. Example; Dahlia.
- Nodulous root: When root becomes suddenly swollen at its apex due to accumulation of reserve food materials, it is known as nodulous root. Example; Curcuma amada.
- Moniliform or beaded roots:In some roots, swellings are found at regular intervals just like the beads in a string. Such are roots are called moniliform roots. Example; Momordica etc.
- Annulated root: When the root has continuous series of ring-like swellings, it is called annulated root. Example; ipecac.
- Sucking roots or haustoria: Parasitic plants develop a special type of root known as haustorium which penetrates into the tissues of the host plant and suck food materials from it. This type of root is called sucking root. Example, Cuscuta.
- Respiratory roots: Some aquatic plants develop adventitious roots from the floating branches. These roots are spongy, soft, light and are capable of storing air for respiration. They usually grow on the surface of the water and simultaneously act as floating organs. Such roots are called respiratory roots. Example, Jussiaea.
- Assimilatory root: These roots remain hanging in the air and become green by developing cholorphyll pigments in the tissue. These roots are capable of absorbing carbon dioxide from the air and manufacturing food materials. Hence, they are called assimilatory roots. Example Tinospora.
- Epiphytic roots: Plants which grow upon other plants but do not suck foot materials from the supporting plants are known as epiphytes. These Epiphytes develop some kind of aerial roots which are thick and posses a spongy tissue known as velamen. With the help of velamen, they absorb moisture from the atmosphere for the synthesis of food materials. Such roots are called epiphytic roots. Example, Vanda.