Modification of Roots - Tap and Adventitious root Modification and Examples

The part of the plant which develops from the radical part of the seed and does not bear any leaf, bud or flower is known as root. it is non-green and grows downward in the soil. In addition to the normal functions of fixation, absorption and conduction performed by the roots, the root may perform additional functions of storage, additional support or vital functions of assimilation, respiration and others. In order to perform these specific

functions tap root and adventitious root undergo various modifications.

i) Modification of root for storage of food: The food prepared by leaves pass into the roots and stored in various forms. Some roots store large quantities of reserve food to be
used during the season of active growth. But tap roots and adventitious roots are modified as storage organs.

Tap root modification: A variety of tap root are swollen due to the food contained in them. They are of various shapes.

a) Fusiform: The modified root is spindle shaped as it is swollen in the middle and gradually tapers towards the apex and the base, e.g. radish.

b) Napiform: The root is very much swollen and almost rounded at the base and tapers abruptly at the lower end. The modified root becomes almost spherical in shape, e.g. turnip, beet root.

c) Conical: In some plants, the root assumes a conical shape. It is broad at the base and tapers gradually towards the lower end, e.g. carrot.

d) Tuberous: The root, in this case is thick and fleshy, but does not take a definite shape, e.g. mirabills.

Adventitious root modification: There are various kinds of modification of adventitious root.

a) Tuberous form: In this type, the fibrous adventitious root swells up without assuming any definite shape and occur singly, e.g. sweet potato.

b) Fasciculated form: When several tubercular roots occur in a cluster at the base of the stem, they are said to be fasciculated, e.g. dahlia, asparagus.

c) Nodulose form: This type is characterized by the fibrous adventitious roots abruptly swelling at the tips, e.g. ginger.

d) Moniliform: The root acquires a beaded appearance as the root shows swellings at frequent intervals, e.g. spinach, momordica.

e) Annulated form: The root is characterized by a series of ring like swellings separated by grooves, e.g. ipecac.

ii) For mechanical support: Adventitious roots may get modified to provide additional support or to perform mechanical functions into the following categories:

a) Prop roots: There are also aerial Adventitious roots which grow vertically downwards from the horizontal branches of large tropical trees and eventually pen-etrate into the soil. They look like pillars or columns and are often referred to as columnar roots, e.g. the banyan tree.

b) Stilt roots: The fibrous root system of certain grasses does not provide adequate support. Hence these plants develop slender aerial roots from their lower or basal nodes, which grow obliquely into the soil and act as stilts. These are known as stilt roots, e.g. maize, wheat, bamboo, etc.

c) Climbing roots: Certain plants having weak stem develop a cluster of aerial adventitious roots from their nodes and often from the internodes. These roots ensure a foothold on neighboring objects and enable the plant to clasp and climb up a support like tree or wall to reach maximum sunlight. These roots secrete a sticky juice which on drying helps the root to adhere to the substratum (support), e.g. money plant, betel, black pepper.

d) Clinging roots: in epiphytic plants such as orchids, special develop which grow into the bark of the tree trunks (of the supporting plant). Orchids cling and fix themselves to their support with these roots, e.g. orchids.

e) Floating roots: Some floating aquatic plants have spongy, soft and light adventitious roots with air spaces to keep the plant afloat. Such roots are called floating roots, e.g. jussiaea roots.

f) Buttress roots: The roots at the base of the trunk of some large forest trees show abnormal growth. they first grow obliquely downwards from the base of the trunk and then radiate horizontally outwards. They become stout and help the huge trunks to maintain an upright position, e.g. silk cotton tree (bombax), white cotton tree (kapok).

iii) For vital function: There are certain types of roots which perform vital functions in a plant.

a) Nitrogen fixing roots: underground roots of leguminous plants for fix atmospheric nitrogen with the help of symbiotic bacteria n their root nodules, e.g. beans, peas, grams, etc.

b) Respiratory roots: sometimes in marshy areas roots develop as conical spikes which grow upward to take oxygen from surroundings and are called respiratory roots, e.g. rhizophora, mangrove plants.

c) Parasitic roots: Parasitic plants develop special adventitious roots in order to obtain nourishment from the host tissue. The parasite develops roots from the stem or its branches at the point of contact with the host plant. The roots penetrate the host, tissue and absorb the nutrients from the host plant, e.g. cuscuta, mistletoe.

d) Epiphytic roots: in plants like orchids which grow as epiphytes on the branches of trees, two types of roots are developed clinging roots and aerial roots. Clinging roots provide support by growing into tree trunks whereas the aerial roots hang downwards in the air to absorb moisture from the air, e.g. orchids like vanda, dendrobium.

e) Assimilatory roots: these are the roots containing chloroplasts and they can manufacture food just like leaves. Such leaves are called, e.g. tinospora, trapanatans.