The body of a typical flowering plant can be divided into the underground root system and aerial shoot system. The root system is homogeneous and consists of the main root and its lateral branches. The shoot system is heterogeneous and consists of stem, branches, leaves and flowers.
The parts of the plant body which are mainly concerned with important functions of nutrition and growth are called vegetative parts. e.g. root, stem and leaves. The parts which perform the function of sexual reproduction are called floral or reproductive parts. e.g. flower.
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 of it. It elongates to form the primary or the taproot. It gives off lateral branches (secondary and tertiary roots) and thus forms the root-system. The root is positively geotropic (moves towards the soil), positively hydrotropic (moves towards the water) and negatively phototropic (moves away from the light).
Characteristics of Root:
They are cylindrical generally non-green structures.
They are homogeneous because they produce similar organs such as secondary and tertiary roots from the pericycle. i.e. they are endogenous.
They are not differentiated into nodes and internodes.
They so not produce dissimilar organs like leaves.
Types of Root System:
In a majority of the dicotyledonous plants, the direct elongation of the radicle leads to the formation of primary root which grows in the soil. It gives off lateral branches (secondary and tertiary roots) and thus forms the taproot system. All lateral branches are produced in acropetal succession, i.e. the older and longer branches are near the base and younger and shorter ones are near the apex of the main root. The primary roots and its branches constitute the taproot system. e.g. roots in the mustard (Brassica) (सरसों), sunflower (Helianthus) (सूरजमुखी) plant.
This system of roots provides a very strong anchorage as they are able to reach very deep into the soil.
Adventitious or Fibrous Root System:
In monocotyledonous plants, the primary root is short-lived and is replaced by a large number of roots. A cluster of slender, fibre-like roots arises from the base of the radicle and plumule which constitute the fibrous system of roots. e.g. roots in wheat (गेहूँ), maize (मक्का), sugarcane (गन्ना). In some plants, like grass (घास), Monstera (a tropical American vine having roots that hang like cords and cylindrical fruit with a pineapple and banana flavour ) and the banyan (बरगद) tree, roots arise from parts of the plant other than the radicle and are called adventitious roots. Such roots can develop from the base of stem, nodes or from leaves.
They do not branch profusely, are shallow and spread horizontally, do not grow deep in the soil, hence cannot provide strong anchorage to the plant.
Functions of Root:
Its main functions are the absorption of water the and minerals from the soil.
It provides a proper anchorage to the plant parts.
Storing reserve food material and synthesis of plant growth regulators are its other functions.
By undergoing modifications in their structure, roots perform special physiological functions like food storage, assimilation, absorption of atmospheric moisture, sucking food from the host, better gaseous exchange and mechanical functions like floating (buoyancy), stronger anchorage and climbing.
Regions of the Root:
Root Cap Region:
The root is covered at the apex by a thimble-like structure called the root cap. It is produced by produced by meristematic zone. It protects the tender apex of the root as it makes its way through the soil.
As the root grows further down in the soil, root cap wears out but it is constantly renewed. In aquatic plants like Pistia and water hyacinth (Eichornia)(जल कुंभी) root cap is like a loose thimble, called root pocket.
Region of Meristematic Cells or Region of Cell Division:
Meristematic means rapid increasing or rapid growth. A few millimetres above the root cap is the region of meristematic activity. The cells of this region are very small, thin-walled and with dense protoplasm and divide actively. In monocots, the root cap is formed by the independent group of cells known as Calyptrogen.
The apical meristem consists of :
Dermatogen (outermost layer whose cells mature into epiblema and root cap);
Periblem (inner to dermatogen whose cells mature into cortex) and
Plerome (the central region whose cells mature into stele).
Region of Elongation:
The cells proximal to this region undergo rapid elongation and enlargement and are responsible for the growth of the root in length. This region is called the region of elongation.
Region of Maturation:
The cells of the elongation zone gradually differentiate and mature. Hence, this zone, proximal to the region of elongation, is called the region of maturation. Matured cells differentiate into various tissues like root hairs and permanent region.
From this region some of the epidermal cells form very fine and delicate, thread-like structures called root hairs. This region is called piliferous region. The root hairs are elongated, single-celled tubular structures which remain in contact with soil particles. The root hairs increase the surface area of absorption. Root hairs are short-lived and are replaced after every 10 to 15 days.These root hairs absorb water and minerals from the soil.
The permanent region 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. The enlarged cells in this region undergo differentiation to form different types of primary root tissues like cortex, endodermis, xylem, phloem etc.
Modification of Roots:
Roots in some plants change their shape and structure and become modified to perform functions other than absorption and conduction of water and minerals. They are modified for support, storage of food and respiration
Modifications of Tap Roots for Storage of Food:
Taproots of carrot, turnip and adventitious roots of sweet potato, get swollen and store food. The secondary roots remain thin. Hypocotyl, i.e. embyonic region between cotyledons and radicle may also join the tap root in storing food. Stem is reduced and disc shaped in the begining and bears radical leaves. Depending upon their shapes they are further classified into four types
Base is broad and tapers gradually towards the apex.
Carrot (Daucus carota) (गाजर)
Swollen at middle and tapering at both the ends.
Radish (Raphanus sativus) (मूली)
Spherical at the base and sharply tapering towards the apex.
The plants growing in saline, swamps, marshy places and salt lakes are called halophytes. Such plants e.g. Rhizophora growing in swampy areas (mangroves), many roots come out of the ground (negatively geotropic) and grow vertically upwards. Such roots, called pneumatophores. They help to get oxygen for respiration. The roots appear like conical spikes coming out of the water. They occur in large number near the tree trunk. Exposed root tips possess Rhizophora minute pores through which roots respire. e.g. Rhizophora, Avicennia, Sonnerita, Heritiera (सुंद्री found in Sunderbans, Bengal).
Modifications of Adventitious Root for Storage of Food:
Simple Tuberous Roots: These roots become swollen and do not assume any shape. They are always borne singly. These roots arise from nodes of prostrate stem and enter in the soil. e.g. sweet potato (Ipomoea batatus) (शकरकंद).
Fasciculated Tuberous Roots: It is a cluster of adventitious roots for storage of food. e.g. Dahlia, Asparagus (शतावरी)
Nodulose Roots: Only apices of roots become swollen like single beads. e.g. Mango-ginger
Moniliform Roots: Roots alternately swollen and constricted which has beaded or moniliform appearance. e.g. Grasses, sedges
Annulated Roots: Looks like number of disc placed one above the other. e.g. Ipecac
Modifications of Adventitious Roots for Support:
Prop Roots: The hanging structures that support a banyan tree are called prop roots. Roots develop from tree branches hang downwards and ultimately penetrate the ground, thus provide support to heavy branches. A banyan growing in Indian Botanical garden, Owrah (Kolkata) has nearly 1700 such prop roots and has a very large sapread. The tree is about 200 years old.
Stilt Roots: The stems of maize and sugarcane have supporting roots coming out of the lower nodes of the stem. These are called stilt roots. They are mainly found in monocots, shrubs and small trees. They grow obliquely downwards and penetrate the soil. Their primary function is to provide support to the plant. In plants like maize, bajra, sugarcane, jowar, they grow in whorls. In screwpine(केवडा) or Pandanus (a tropical palm-like tree) these roots arise only from the lower surface of the obliquely growing stem to provide support.
Climbing Roots: Such plants produce roots from their nodes, by which they attach themselves to some support and climb over it. Weak climbers twine around and clasp the support with the help of climbing roots arising from their nodes. e.g. Money plant, black pepper (kali mirch), betel (pan).
Clinging Roots: Special clinging roots arise, enter the crevices of support and fix the epiphyte. e.g. epiphytes orchids
Modifications of Adventitious Roots for Special Functions:
Epiphytic Roots: Some plants like orchids grow on horizontal branches of big trees in the forest to get sunlight. They are autotrophic. These plants are called epiphytes. They develop special areal hanging roots called epiphytic roots. These roots are spongy. Due to the presence of velamen tissue are hygroscopic and have a porous wall. They absorb moisture from the atmosphere. e.g. vanda, dendrobium, etc. These roots are also called assimilatory roots due their partial capacity of photosynthesis.
Sucking Roots or Haustoria: These are highly specialized and microscopic roots, developed by parasites to absorb nourishment from the host. In partial parasites penetrate only xylem element of the host and absorb water and minerals. E.g. Viscus album. In total parasites, they establish contact with both xylem and phloem of the host. Thus absorb water, minerals and nutrients. e.g. cuscuta.
Floating Roots:Spongy, floating roots filled with air, arise from nodes of some aquatic plants, and help in floating and respiration. eg. Jussiaea
Photosynthetic Roots:Roots which when exposed to sun develop chlorophyll, turn green and manufacture food. e.g. Tinospora and orchids.