1. Root is the descending or underground part of the plant axis.
2. Root is usually positively geotropic (i.e. grows downward into the soil) and positively hydrotropic (i.e. grows towards the source of water) but negatively phototropic (i.e. grows away from sunlight).
3. Root is usually cylindrical and non-green (i.e. lack chlorophylls), but sometimes green as in Trapa and Taeniophylum.
4. Root does not bear nodes, internodes, leaves or buds (exceptions are sweet potato, wood apple etc.)
5. The growing point of root tip is sub-terminal and protected by a root cap or calyptra.
6. Unicellular root hairs present just behind the root caps which increase the absorptive surface area of roots,
7. Lateral roots are endogenous in origin i.e. arise from pericycle of the main root.
Many plants growing in aquatic habitats do not possess roots because there is little requirement for absorption of water and mineral salts, e.g., Wolffia, Utricularia, Myriophyllum, Ceratophyllum. In other aquatic plants, roots develop only for balancing (e.g., Lemna, Pistia) and fixation (e.g., Hydrilla).
Types of Roots:
On the basis of their origin, roots are of two types – tap root and adventitious root.
(a) Tap root:
On germination of a seed, the radicle elongates into primary root or true root or tap root. In dicot plants, the tap root is persistent and produces lateral roots such as secondary’ roots, tertiary roots etc. All lateral roots arise in acropetal succession i.e. younger roots towards apex and older roots towards base. The tap root and its branches constitute the tap root system.
(b) Adventitious root:
These are the roots that grow from any part of the plant other than radicle. In monocot plants, the tap root is short lived and soon replaced by adventitious roots. A group of adventitious roots and their branches constitute adventitious root system.
On the basis of their origin, the adventitious roots are of following three types:
i. Fibrous roots:
These are a cluster of equally prominent thread-like roots that develop either from the base of stem (e.g., rice, wheat, maize, onion etc.) or from the nodes of horizontal stem (e.g., grass, wood sorrel etc.)
ii. Foliar roots:
They arise from petiole (e.g., Pogostemon, rubber plant etc.) or veins of leaf due to some injury. These can also be induced by application of hormones. Some foliar buds can produce foliar roots, e.g., Bryophyllum, Begonia etc.
iii. True adventitious roots:
They arise from the nodes and internodes of the stem, e.g., Prop roots of banyan, stilt roots of sugarcane, clasping roots of money plant and roots from the stem cuttings.
A typical root can be differentiated into five regions. From apex to base they are:
(a) Root Cap (Calyptra):
It is a cap like protective structure of the growing root tip. In Pandanus (screwpine) multiple root caps present while in aquatic plants (Pistia, Eichhornia, Lemna) root pockets present instead of root cap.
(i) Protects root meristem,
(ii) Secrete mucilage that help tender root to penetrate the hard soil,
(iii) Helps in perception of gravity (Darwin, 1880),
(iv) Root packet s functions as balances.
(b) Growing point or Meristematic Zone:
It is about 0.25-1.0 mm long, lies just behind the root cap and thus sub-terminal in position. Its shape is like an inverted concave dome of cells. The central rarely dividing cells are called quiescent centre.
Root meristem adds cells to root cap and the basal region of the root.
(c) Zone of elongation:
It is about 1-10 mm long and lies just behind the meristematic zone. As the name implies, it is the site of rapid and extensive cell elongation. This zone increases length of the root. The external cells can absorb water and minerals from the soil.
(d) Root hair Zone or Zone of differentiation:
It is about 1 -6 cm long. It is the zone where cell differentiate to form epiblema, cortex, endodermis, pericycle, xylem and phloem. Many cells of epiblema elongate to form unicellular root hairs. As the root grows, new root hairs develop and older one shrivel and sloughed off.
Root hairs increase the absorptive surface area of root.
(e) Zone of maturation:
In constitute the major portion of the root. The cells attain maturity when they reach this zone.
(i) Lateral roots may emerge from pericycle
(ii) Radial differentiation of tissues causes’ secondary growth in dicots.
Functions of Roots:
Roots perform two kinds of functions — Primary and Secondary. The primary functions are performed by all kinds of roots, and they are structurally adapted to per-form these functions. The secondary functions are specialized one and are performed only by those roots which are modified accordingly.
The primary functions of roots are:
1. Anchorage or fixing the plant firmly to the soil so that they are not easily uprooted.
2. Absorption and translocation of water and minerals from the soil to the aerial parts of the plant.
3. Prevent soil erosion by holding the soil particles.
In many plants, roots are modified to serve many secondary functions like food storage, mechanical support and various physiological activities other than absorption.