Modifications of Root

When roots have to perform some special function in addition to, or instead of, their normal role, they undergo structural change; such roots are called metamorphosed (modified) roots.

Modifications of the tap root

a. Food storage. When a tap root stores food it becomes swollen, fleshy and takes on a definite shape. Usually the main (primary) root is the chief storage organ, though sometimes the hypocotyl part of the embryo axis also joins in; the secondary roots stay thin, and the stem in such plants is reduced and discoid, bearing radicle (rosette) leaves. By shape, swollen tap roots are grouped as:

• Fusiform — swollen in the middle and tapering towards both ends, giving a spindle shape, e.g. Radish (Raphanus sativus).
• Conical — broad at its base and narrowing steadily towards the apex, e.g. Carrot (Daucus carota).
• Napiform — the base of the root is highly swollen, almost spherical, and narrows abruptly towards its apex, e.g. Beet (Beta vulgaris).

b. For respiration — pneumatophores/lenticellate roots. Roots of certain mangrove halophytes (e.g. Rhizophora, Avicennia, Sonneratia, Heritiera) show lenticels — minute pores for gaseous exchange (pneumatic = hollow, phore = stalk).

Modifications of adventitious roots

a. Food storage. Fibrous roots also store food like tap roots do, but they usually do not take on one definite shape. They are further classified as:

• Simple tuberous roots — become swollen without a fixed shape and are produced singly; they arise from stem nodes and penetrate the soil, e.g. sweet potato/shakarkand (Ipomoea batatas).
• Fasciculated tuberous roots — a cluster of roots arising from one point become thick and fleshy from food storage; these clusters occur at the base of the stem, e.g. Dahlia, Asparagus.
• Beaded (moniliform) roots — show swellings at regular intervals along their length, like the beads of a necklace, e.g. Spinacia oleracea (Indian spinach).
• Nodulose roots — a cluster of long, slender roots that become enlarged only at their tips, e.g. Maranta (arrow root), Curcuma amada (Amhaldi).

b. For respiration — Pneumatophores/breathing roots. Halophytes growing in saline swamps, marshy places and salt lakes cannot get enough air for respiration through their main root system because the soil is waterlogged, which also affects mineral absorption. To overcome this, underground roots develop special negatively geotropic roots that grow vertically upward as conical projections around the main trunk of the plant.

c. For mechanical support.

• Prop roots/columnar roots — arise from the horizontal branches of trees such as the Banyan (Ficus benghalensis) and grow straight down until they reach the soil; they later show secondary growth, thicken, and act like pillars supporting the heavy branches above.
• Stilt roots — arise from a few of the lower nodes of a weak stem in some monocot shrubs and small trees, growing obliquely downward into the soil for mechanical support; in members of family Poaceae such as Maize, Jowar and Sugarcane, stilt roots are produced in a whorl around the node.
• Climbing roots — different climbers with weak stems produce roots at their nodes, by which they attach to a support and raise themselves above the ground, e.g. betel leaf/Pan, black pepper (Piper nigrum), Pothos/money plant.
• Clinging roots — tiny roots that develop along the internodes, showing a disc at the tip that exudes a sticky substance, allowing the plant to attach itself to walls of buildings without damaging the surface, e.g. English Ivy (Hedera helix).
• Plank roots/buttresses — often develop at the base of large trees, forming plank-like extensions around the stem, e.g. Silk cotton, Pipal. …