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The root system is the descending (growing downwards) portion of the plant axis. When a seed germinates, radicle is the first organ to come out. It elongates to form primary or the tap root. It gives off lateral branches (secondary and tertiary roots) and thus forms the root system. It branches through large and deep areas in the soil and anchors the plant very firmly. It also plays another vital role in absorbing water and mineral salts from the soil and transporting them upwards.
CHARACTERISTICS OF ROOTS
- Non-green due to absence of chlorophyll
- Not divided into nodes and internodesA
- Absence of leaves and buds
- Positively geotropic (grow towards gravity)
- Positively hydrotropic (grow towards water)
- Negatively phototropic (grow away from light).
TYPES OF ROOT SYSTEMS
Root systems are mainly of two types:
- Tap root system — It is the root system that develops from the radicle and continues as the primary root (tap root) which gives off lateral roots. They provide very strong anchorage as they are able to reach very deep into the soil. It is the main root system of dicots e.g. gram, china rose, neem (Fig).
- Fibrous root system — In this root system, the primary root is short lived. A cluster of slender, fiber-like roots arises from the base of the radicle and plumule which constitute the fibrous root system. They do not branch profusely, are shallow and spread horizontally, hence cannot provide strong anchorage. Fibrous root system is the main root system of monocots, e.g. maize, grasses, wheat (Fig).
TYPES OF ROOTS
- Tap root – It is the primary and the main root that develops from the radicle, bears many branches and remains underground. It is usually found in dicots e.g. sunflower, mustard, carrot, mango.
- Adventitious root – These are roots that develop from any part of the plant except the radicle. They may be aerial or underground. They may grow from node (money plant, bamboo), stem cutting (rose), tree branch ( banyan)or stem base (fibrous roots in monocots).
REGIONS OF ROOT
Apical region of roots of any root system shows the same zones or regions as can be seen in Fig. A longitudinal section of root apex shows the following structure
- Root cap region — It is a thimble-like structure produced by meristematic (rapidly dividing) zone and protects the tender apex (apical meristem) from harsh soil particles. As the root grows further down in the soil, root cap wears out but it is constantly renewed. In aquatic plants (Pistia and water hyacinth) root cap is like a loose thimble called root pocket.
- Region of meristematic cells — is a small region of actively dividing cells called the apical meristem. It consists of :
(i) Dermatogen (outermost layer whose cells mature into epiblema and root cap);
(ii) Periblem (inner to dermatogen whose cells mature into cortex) and
(iii) Plerome (central region whose cells mature into stele). In monocots, cap is formed by independent group of cells known as Calyptrogen.
- Region of elongation — Lies next to the meristematic region, the cells elongate and enlarge to make the root grow in length.
- Region of maturation — Lies next to the region of elongation. The cells mature and differentiate into various tissues constituting (i) Root hair or piliferous region having unicellular hairs which absorb water and mineral salts from soil and (ii) Permanent region which lies behind the root hair zone and is without hairs. It produces lateral roots, anchors the plant in soil and conducts water and minerals upwards.
In maize root tip, Clowes (1958) discovered a central cup like reservoir of inactive cells lying between the root cap and the active meristematic region, called the Quiescent Centre. These cells become active whenever the previously active meristematic cells are damaged.
MODIFICATIONS OF ROOTS
Tap roots and adventitious roots can get modified into a variety of forms to perform
various functions as can be seen from the following chart and tables
- Tap roots modification:
- Conical root
- Fusiform root
- Napiform root
- Tuberous root
- Adventitious roots modification :
- Tuberous root
- Fasciculated root
- Nodulose roots
- Annulated roots
- Assimilatory roots
- Epiphytic roots
- Sucking roots or haustoria
- Prop roots
- Stilt roots
- Climbing roots
- Clinging roots
- Floating roots
A. Tap root modifications
Tap roots become fleshy for storage of food
|1. Conical||Base is broad and tapers gradually towards apex||Carrot|
|2. Fusiform||Swollen in middle tapering towards both ends||Radish|
|3. Napiform||Spherical at base tapering sharply towards the tip||Turnip|
|4. Tuberous||Thick and fleshy with no definite shape||4 O’clock plant|
B. Adventitious root modifications
Adventitious roots get modified for various functions
|(i) Modifications for food storage|
|1. Tuberous||Swollen roots developing from nodes of prostrate stem||Sweet Potato|
|2. Fasciculated||Swollen roots developing in a cluster from the stem||Dahlia|
|3. Nodulose||Only apices of roots become swollen like single beads||Mango-ginger|
|4. Moniliform |
|Roots alternately swollen and constricted presenting a beaded|
or moniliform appearance
|5. Annulated||Look as if formed by a number of discs placed one above the|
|(ii) Modification for photosynthesis|
|Assimilatory roots |
|Roots which when exposed to sun develop chlorophyll, turn green and manufacture food||Tinospora(aerial root), orchid|
|(iii) Modification for absorbing atmospheric moisture|
|Epiphytic roots||Aerial roots of epiphytes are greenish and covered with spongy tissue (Velamen) with which they|
absorb atmospheric moisture
|(iv) Modification for better gaseous exchange|
|Pneumatophores or respiratory roots||Some roots grow vertically up (negatively geotropic) into air Exposed root tips possess minute pores through which roots respire, appear like conical spikes coming out of water||Mangroves (marshy plants) Rhizophora|
|(v) Modification for sucking nutrition from host|
|Sucking roots or haustoria||Parasitic plants give out sucking roots or haustoria which penetrate living host plant and suck food||Cuscuta|
|(vi) Modification for strong support|
|1. Prop roots|
|Roots develop from tree branches, hang downwards and ultimately penetrate the ground, thus support heavy branches||Banyan|
|2. Stilt roots |
|Extra roots developing from nodes near the base of stem, grow obliquely and penetrate the soil giving strong anchorage||Sugarcane, Screwpine|
|3. Climbing roots |
|Weak climbers twine around and clasp the support with the help of climbing roots arising from their nodes||money plant, betel|
|4. Clinging roots |
|Special clinging roots arise, enter the crevices of support and fix the epiphyte||Epiphytes, orchids|
|(vii) Modification for buoyancy & respiration|
|Floating roots |
|Spongy, floating roots filled with air, arise from nodes of some aquatic plants and help in floating and respiration||Jussiaea|
The great Banyan tree in Sibpur, Kolkata is more than 200 years old, forming a crown of over 404 meters in circumference and has about 1600 prop roots.
FUNCTIONS OF ROOTS
(i) Anchorage – Roots anchor the plant firmly in the soil (mechanical function).
(ii) Absorption – Roots absorb water and mineral salts and conduct them upwards (physiological function).
(iii) Special functions – By undergoing modifications in their structure, roots perform special physiological functions like food storage, assimilation, absorption of atmospheric moisture, sucking food from host, better gaseous exchange and mechanical functions like floating (buoyancy), stronger anchorage and climbing.