In this article we will discuss about:- 1. Meaning of Root 2. Regions of the Root 3. Root Systems 4. Normal and Adventitious Roots 5. Normal Functions 6. Modifications.
Meaning of Root:
The root is the descending axis of the plant and the downward prolongation of the radicle. It is usually non-green in colour and grows away from light. The root has no nodes and internodes. It bears only similar members, i.e. only roots.
The main root developing from the radicle is called the primary root which, in turn, produces many secondary and tertiary branches, thus forming a system of roots going inside the soil in different directions.
These branch roots develop from internal tissues of the primary root, so they are endogenous in origin. They are arranged in acropetal succession, i.e. older roots near the base and smaller and smaller ones towards the tip.
Regions of the Root (Fig. 32):
A root has usually four regions, in order, from the apex toward the Fig. 32.
(i) Root Cap Region:
The extreme tip of the root is usually protected by a cap or thimble-shaped body called the root-cap. When the root pushes its way through the soil, the tender and soft tip runs the risk of being injured due to friction with soil particles.
The root-cap, serving as a buffer, protects the tip from that danger. Further, the cap secretes slimy mucilaginous matters which facilitate the course of the root through the soil. The root-cap is absent in many aquatic plants. In screw pine root- caps are multiple (Fig. 33).
(ii) Region of Active Growth and Elongation:
Next to the root-cap is the region of active growth. It is a very short region where cells divide actively. This merges into the region of elongation where the newly formed cells grow in length.
(iii) Root-Hair Region:
This region is found next and here the outer cells of the roots grow out as unicellular hairs. The unicellular root-hairs are responsible for the absorption of water and mineral matters dissolved in water from the soil.
By germinating mustard seeds on a piece of soaked blotting paper the dense root-hairs can be conveniently observed (Fig. 34). During the growth of the root old zone is exhausted and is replaced by new zones of root-hairs more and more towards the tip.
(iv) Permanent Region:
This is located higher up. Here secondary roots develop from the primary root. The branches also have four regions like the primary root.
Root Systems (Fig. 35):
In dicotyledonous plants the radicle produces the primary root which is quite prominent, that is, long and stout. This is called the tap root. The tap root bears many secondary and tertiary branches developing acropetally.
All these branches can be easily distinguished from the main root. The whole system is called the tap root system. In monocotyledons like maize and rice the radicle produces the primary root which soon aborts and is replaced by a tuft of roots developing from the base of the stem. All of them are more or less similar and are called fibrous roots and the whole system is known as fibrous root system.
Normal and Adventitious Roots:
Roots that develop from the radicle (primary root) or branches of the roots from that origin, are called normal or true roots; and roots growing from any position other than the normal point of origin, are called adventitious roots. So the tap root system, usually found in the dicotyledons, is normal and fibrous root system, common in the monocotyledons like grasses, is adventitious.
Normal Functions of the Root:
Roots perform two normal functions. First, they fix the plants to the soil and thus secure anchorage. Secondly, roots are responsible for the absorption of water and dissolved mineral matters from the soil through the delicate root-hairs. Water is ultimately conducted to the leaves.
The primary roots with their large number of secondary and tertiary branches in the dicotyledons, or the fibrous roots in the monocotyledons penetrate into the soil, spread out in different directions and thus effectively carry on the normal functions.
In common land plants the root system, in fact, equals to or even exceeds the aerial portions (shoot system) both in length and volume. The fixation or anchorage is the mechanical function of the root; and absorption and conduction of water and dissolved mineral matters, are the chief physiological functions.
Besides these, roots may perform many special functions which will be discussed in connection with the modified roots.
Modifications of Roots:
Many roots undergo modifications for carrying on functions other than the normal ones. Some of the modified roots are normal, while many of them are adventitious.
1. Normal Roots Modified for Storage of Food:
Some roots become swollen and fleshy due to the storage of food matters and assume different shapes (Fig. 36):
(a) Fusiform:
Here the root with the hypocotyl becomes swollen. The swelling is maximum in the middle. It gradually tapers towards the ends, i.e. base and apex, e.g. radish. Practically the whole swollen portion in radish is the hypocotyl.
(b) Conical:
Here also the root is fleshy; it is broadest at the base and gradually tapers towards the apex like a cone, e.g. carrot.
(c) Napiform:
It is very much swollen at the base (with the hypocotyl), but abruptly tapers towards the apex, as in beet and turnip.
2. Adventitious Roots Modified for Storage of Food:
(d) Tuberous Root:
In plants like sweet potato which grow on the surface of the soil, some of the adventitious roots, developing from the stem, become swollen and fleshy due to the storage of food. They do not have any regular shape. These are called tuberous root (Fig. 37).
(e) In Asparagus (B. Satamuli), Ruellia, etc., quite a good number of tuberous roots are produced from the stem, forming a bundle or fascicle. They are called fasciculated roots (Fig. 38).
(f) In mango-ginger (B. Amada) the adventitious roots suddenly become swollen at the tips. They are called nodulose roots (Fig. 39).
(g) In many grasses the swellings are often found at frequent intervals giving the root a beaded appearance. These peculiar roots are called moniliform (Fig. 39).
Adventitious Roots Modified:
1. Prop Roots:
Roots are normally underground organs. Some roots develop completely above ground and, so, are aerial. In the common banyan trees of our country roots are often found to hang freely in the air from the branches.
In course of time they reach the soil, become stouter and serve as so many extra supports or props. Thus they help the stem in bearing the weight of the heavy crown. These are called the prop roots (Fig.40).
2. Stilt Roots:
These are also supporting roots. In plants like screw pine (B. Keya) quite a good number of adventitious roots develop from the basal part of the stem and go downwards obliquely. These roots help the plant in maintaining the upright position. These are stilt roots (Fig. 41).
3. Climbing Roots:
Weak climbing plants like betel, vine, Scindapsus (B. Gaj-pipul), etc., produce some adventitious roots which cling to the supports and thus help the plants in climbing. They often secrete sticky juices or develop disc-like structures for that purpose (Fig. 42).
4. Epiphytic Roots:
Many epiphytes, like orchids, produce long roots which freely hang in air. They have sponge-like tissues, called velamen, at the exterior by means of which they can absorb moisture from the air (Fig. 43).
5. Sucking Roots or Haustoria:
Sucking roots or haustoria develop in the parasitic plants like Cuscuta or dodder (B. Swarnalata). These roots penetrate into the body of the host and draw nourishment from there without caring to manufacture their own food (Fig. 44, 2 & 3).
6. Breathing Roots:
These roots are produced by the plants like Heritiera (B. Sundri), Rhizophora (B. Bora) growing in saline marshes or on the seashore where the soil is very poor in oxygen. Here some roots with pores at the tips come vertically surface of the soil and carry on the gaseous inter- with the change outer atmosphere. They are also called pneumatophores (Fig. 44—1).
In an aquatic plant fuissiaea (B. Keshar, dam) some peculiar, adventitious roots develop from the floating branches. They are light, soft and spongy due to the presence .of airspaces. Besides facilitating respiration, they give the plants buoyancy for floating on the surface of water (Fig. 45).
7. Assimilatory Roots:
Roots of plants like Tinospora (B. Gulancha) hang freely in the air and. develop green colour. They can manufacture food matters. The sub-merged roots of Trapanatans (water chest nut) coming out in pairs from the nodes are green and assimilatory in function.
Roots are also used for vegetative multiplication. Common vegetable plants like Trichosanthes dioica(B. Patol), sweet potato, etc., are propagated by means of roots. Many garden plants are also multiplied by root cuttings.