Root: Types and Structure | Plants

After reading this article you will learn about the types and structure of roots of plants.

Types of Root:

Terrestrial plants produce roots aerial and storage types. In most plants, the emergence of a root is the first sign of germination. The first root axis arises from cells laid down in the seed and for dicotyledonous plants is called the tap root.

1. Storage Roots:

Parts of roots of plants such as carrot (Daucus carota), sugar beet (Beta vulgaris), and sweet potato (Ipomea batatas) are specially adapted to store products photosynthesized in the shoot. The products are synthesized above ground and transported to the root in the phloem where they residue until completion of life-cycle.

2. Aerial Roots:

Aerial or shoot borne roots originate from a range of above ground structure. Many plants like banyan, epiphytes etc. produce-prop roots and also function in the absorption of nutrients and water.

3. Air Roots:

In some trees that live in swamps, such as mangroves, parts of roots develop extensions which grow upward into the air. These air roots or pneumatophores grow above the surface of the water and allow oxygen to be transported to the inner cortex of the root system.

4. Hair Roots:

These are produced by many heathland plants such as the Ericaceae and Epacridaceae and are the finest roots. Root hairs form a dense fibrous root system and when excavated from soil, the roots have a coating of tightly bound soil particles.

5. Cluster Roots:

Cluster roots are bottle brush like clusters of hairy rootlets and may appear as ellipsoidal shaped clusters of roots. Agriculturally important species include white and yellow lupin.

6. Contractile Roots:

Contractile roots are widely distributed among monocotyledonous and herbaceous perennial dicotyledons and serve to pull the shoot closer to the ground or in bulbs, deeper into the soil.

7. Parasitic Roots:

In parasitic associations between higher plants, the connection between two plants is established via haustoria formation by parasite. Sticky hairs develop on this structure resulting in parasite-host adhesion.

Structure of Root:

The anatomy of roots is complex with very variable structures both between and within plant species. There are considerable differences among species, habitats, length of roots etc. There may be primary and secondary root structures.

In the primary stages of plant growth, roots exhibit a clear separation between three types of tissue systems—epidermis (absorbing tissue), cortex (occupying largest volume of roots) and vascular (xylem and phloem) tissues.

Secondary root structure is characteristics of roots of gymnosperms and most dicotyledons but is commonly absent in monocotyledons. Secondary growth consists of (a) formation of secondary vascular tissues dividing and expanding in the radial direction, (b) formation of periderm, composed of cork tissue.

There are some other important aspects of root structures like extension and branching, root hairs and root tip includes root cap and border cells which are important for the absorption of nutrients and water from the soil and such absorption varies with nature of roots, especially root hairs (nutrient absorption) and root tips (liberation of extracellular products).

Roots change their direction and rates of growth in response to a wide range of stimuli including gravity, light, water, touch, nutrients, toxic metals and micro-organisms. Roots are positively gravitropic and grow towards nutrients and water. Border cell appear to play a major role in lubricating the passage of the root cap through the soil and in response of roots to the soil microflora.

The gel-like mucilage secreted by the cells of the root cap contributes too many interactions between the plant and the soil including root penetration, soil aggregate formation, microbial dynamics and nutrient cycling.