Definition of Root

Root Definition

Root is an important vegetative part of the plant mainly responsible for nutrition and support. It is the descending organ of a plant originating from radicle of seed and growing towards centre of earth, away from sunlight.

Characteristics of Root

1. Roots are positively geotropic, positively hydrotropic and negatively phototropic.
2. They are colourless at young stage but become brown or deep brown on maturity.
3. Root is protected at its tip by a thimble like protective structure called root cap. But in aquatic plants, there are root pockets for balancing. In screwpine (Pandanus racemosus), there is multiple root cap. An injured root pocket is not replaced. 
4. Root is not differentiated into nodes and internodes.
5. Root does not bear buds, leaves, flower, fruits, etc. However, radical bud for vegetative reproduction is found in sweet potato (Ipomoea batatas), wood apple (Aegle marmelos), pointed gourd (Trichosanthes dioica), Indian redwood tree (Dalbergia sissoo), ipecac (Cephaelis ipicacuanha), etc.
6. Root bears unicellular root hair occurring in cluster for the absorption of water and minerals.
7. Lateral roots are endogenous in origin.

Also Read: Soil

Regions of the root

The root shows following four regions successively from the apex to base:

1. Root cap region: The apical region of root is covered by a protective structure of thin walled cells known as root cap. The cells of root cap secrete mucilage which lubricates the soil through which the root is to grow. Root cap is the region of graviperception (Darwin and Darwin, 1880). An injured or broken root cap is replaced. 
2. Growing region: Just behind root cap, there is growing region. It is sub divided into two sub regions:
3. Sub region of cell division: It is about 1mm in length. Here cells divide to produce new cells for the root cap and basal region of the root.
4. Sub region of cell elongation: It lies behind the sub region of cell division and is about 4-8 mm in length. Here cells elongate rapidly and increase length of the root.
5. Region of root hair: It is 1-6 cm in length. It consists of unicellular root hairs, which increase the exposed surface of the root for absorption. The growth of the root keeps the size of the root hair zone nearly constant, getting new cells from the zone of elongation and passing on its own older cells into mature or permanent region. 
6. Mature or permanent region: The cells of this region do not undergo any further change. i.e. differentiated into permanent tissues depending upon the functions they have to perform. In some cells special type of thickening develops. e.g., xylem tracheids and trachea. Lateral roots arise endogenously in an acropetal succession.

Types of Roots

On the basis of place of origin, there are two types of roots, normal and adventitious.

Normal root: The root arising from the radicle of seed is called normal root. When a normal root becomes thick and persistent, then it is called tap root. It is deep feeder. 

Adventitious root: The root arising from any other part of plant, except radicle of seed is called adventitious root. It may arise from nodes, internodes, base of stem and leaves. 

In the members of family Poaceae (Gramineae), the normal root is of very short duration. It soon perishes down. From the base of stem many thin, fibrous roots arise. Such roots are surface feeders. 

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Modifications of tap root

To perform functions other than normal, i.e., assimilation, storage of food, etc., tap root gets modified into following types: 

Fleshy tap root: Tap root with or without hypocotyl becomes swollen due to storage of food. The secondary roots borne over it remain thin. Such roots are of four types :

• Fusiform root: Such root becomes thicker in the middle and tapering on both ends. It assumes a spindle shape, e.g. radish (Raphanus sativus).
• Conical root: It is broad at the base and gradually tapers towards the apex giving it the shape of a cone, e.g., carrot (Daucus carota).
• Napiform root: It is swollen at the base and tapering abruptly in the apical region giving rise to a top-shaped structure. Here hypocotyl is large, e.g., turnip (Brassica rapa) and sugarbeet (Beta vulgaris).
• Tuberous root : It is swollen irregularly, e.g. 4 O’clock plant (Mirabilis jalapa). 

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      Fusiform root              Conical root                                  Napiform root                             Tuberous root

• Branched tap root: Sometimes the tap root gets branched and branches modify to perform various functions. Such roots are of the following types :
• Nodulated roots: In this type, the tap root gets variously branched and at the junctions of branches many small irregular swellings called root nodules or tubercles arise. They contain a large number of symbiotic N2 fixing bacteria, e.g., Rhizobium leguminosarum. Such root occurs in legumes. 
• Respiratory roots: These are upright aerial roots arising from horizontal cable roots of mangrove plants, each being provided with numerous pores, called pneumathodes or lenticels, towards the upper ends for exchange of air. Such roots are also called pneumatophores or aerophores, e.g., Rhizophora, Xylophora, Avicennia, Sonneratia, Heritiera, etc.
• Reproductive tap roots : In some plants, tap roots or their branches develop adventitious buds for formation of new plants, e.g., Dalbergia sissoo, Populus, etc. 

Modifications of adventitious roots

To perform functions like storage of food, mechanical support and some special or vital functions, adventitious roots get modified into following types:

For Storage of Food

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For Mechanical Support

For Vital Functions

Functions of roots

NORMAL FUNCTIONS

1. Fixation: Root provides anchorage to the plants. It also provides mechanical support to the aerial parts against gravity and wind. 
2. Absorption: Roots absorb water and minerals from soil by its root hair.
3. Conduction: It is also concerned with upward transport or conduction of water and minerals to the stem.
4. Prevention of soil erosion: Roots hold the soil particles firmly to prevent soil erosion.

ACCESSORY FUNCTIONS

1. Storage: Fleshy roots store food materials.
2. Mechanical support: Several types of modified adventitious roots provide mechanical support to the plants.
3. N2-fixation: Nodulated roots of legumes fix nitrogen.
4. Reproduction: Vegetative reproduction takes place by the radical buds.
5. Photosynthesis: Assimilatory roots manufacture food.
6. Balancing: Free floating plants are balanced in water by a cluster of adventitious roots.
7. Aeration: Pneumatophores of mangrove plants help in respiration.