Roots: Definition,characteristics, types and regions of roots-digieduco

The root

Definition: The root is the descending axis of the plant. It develops normally from the radicle of the embryo, but may sometimes develop from any other part of the plant body.

Characteristics of the Root: Roots are characterised by some features of their own which differentiate them from stem, these are :

(a) Descending part of the axis, negatively phototropic ie, roots grow away from light, positively hydrotropic i.e. grow towards water regions and positively geotropic ie growing within the soil towards gravity.

A- a germinating seed with radicle,
B-Taproots and its branches, C- Adventitious fibrous root

(b) Usually root surface is non-green in colour [exceptions : submerged roots of
Trapa bispinosa (Onagraceae), Tinospora sp. (Menispermaceae), roots when exposed to light etc.].
(c) Normally roots do not bear buds. in some cases only vegetative buds develop e.g. Trichosanthes sp. (Cucurbitaceae); Dalbergia sissoo (Leguminosae). Root bears branches only of similar types.
(d) There are no leaves, hence nodes and intemodes are absent.
(e) Development of lateral roots takes place endogenously i.e. from the inner layer (pericycle). '
(f) Root-ends are protected either by root caps or root pockets.
(g) Root hairs are always unicellular and their wall is composed of cellulose only.

Types of Roots : Roots are of two types viz. (a) Tap roots(normal or main) and
(b) Adventitious roots.

(a) TAP ROOT-Here the radicle prolongs directly to form a main pnmary roots when the primary root becomes stronger and persistent, it forms the tap root e.g. roots of dicotyledonous plants.
Primary root forms tap root, its lateral branches are secondary roots; branches of secondary roots in turn produce tertiary roots and so on. The tap root with its branches forms the tap root system.

(b) ADVENTITIOUS ROOTS-When roots develop from any other region instead of their normal place of origin i.e. the radicle, the roots are called adventitious and the
resulting root system is known as adventitious root system.

On the basis of the nature of development and function, adventitious roots are of 
following types : 
(l) Fibrous roots of monocotyledons (Gramineae)-Here radicle grows into primary root in the beginning but does not form the tap root system, instead cluster of slender thread-like roots arise from the base of the plumule or from its lower nodes-these are called seminal roots. Seminal roots grow and behave in the same manner as the primary root-later some more roots grow from the base of plumule or its lower part and form fibrous root, e.g. most species of the Family Poaceae(oryza sativa, Zea sp., Triticum sp. etc.)
(2) Foliar roots-Roots develop directly from the leaves i.e., either from petiole or
lamina e. g. Bryophyllum sp(Crassulaceae). (3) True adventitious roots-Here roots develop from the nodes and internodes of the stem e. g. Centella asiatica, Oxalis sp. (Oxalidaceae), Ficus benghalensis (Moraceae) etc.
Some aquatic plants like Utricularia, Wolffia arrhiza, Lemna sp. are rootless. On the other hand in Podostemon the main vegetative body is thread-like root which bears at its apex root cap.

Regions or Parts of the Root - The root, whether belonging to a tap root system or an adventitious root system, shows some distinct regions or zones. These regions of a root from the tip to the base are as follows :

A-Simple root cap, B- Multiple root cap of pandanus sp. ,C- Root pockets of pistia sp. 

(a) Root cap region-The apex of root is protected by a thin cushion or cap of tissue known as root cap. Main function of root cap is to protect delicate root tip during soil penetration. Root cap may be simple (common for most plants) or multiple (Pandanus sp.-Pandanaceae).

In aquatic plants e.g. species of Lemna (Lemnaceae), Pistia (Araceae), Eichhornia (Pontederiaceae) root caps are absent, instead root tips are protected by loose thimble-like sockets known as root pockets.

(b) Growing region or Zone of elongation-This region lies just behind root cap region. In this region,growth of roots actually takes place by mitotic cell divisions and cell elongation. According to some authors this region consists of two zones viz,(a) zone of cell division and (b) zone of cell elongation.

(c) Root hair region or Piliferous region-This region lies just above the growing region and is covered by fine unicellular root hairs, the function of which is absorption of water and solutes from the soil and a little of anchorage. Aquatic plants do not have root hairs as they do not need.

A- a mature root, B- showing different regions of root

(d) Permanent region-All the remaining portions of the root beyond the root hair zone come under this region-it includes all the branches and the regions upto the base of the stem. Growth of this region is stopped. Function of permanent region is to conduct the substances absorbed by root hair and also anchorage.