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In this essay we will learn about root. After reading this essay you will learn about: 1. Introduction to Root Systems 2. Regions of the Root 3. Kinds 4. Modifications 5. Functions.
Contents:
- Essay on the Introduction to Root System
- Essay on Regions of the Root
- Essay on the Kinds of Root
- Essay on the Modifications of Root
- Essay on the Functions of Root
Essay # 1. Introduction to Root Systems:
The whole extent of the roots of a plant is called the root system.
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The development of the root system differs fundamentally in vascular plants and may be classified in two categories:
1. The tap root system and
2. The fibrous root system.
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The tap root system is normally found in dicotyledons and gymnosperms whereas the fibrous root system is commonly met with in monocotyledons. The direct prolongation of the radicle forms the primary root. If it persists and continues to grow it is called the tap root.
i. Tap Root System:
The tap root produces lateral branches which are known as the secondary roots, and these in turn produce the tertiary roots, and so on. All these lateral roots are produced in acropetal succession, i.e., the older and longer roots are away from the tip, and the younger and shorter ones are towards it.
The tap root normally grows vertically downwards to a shorter or longer depth while the secondary or tertiary roots grow obliquely downwards or in many cases horizontally outwards. The tap roots absorb water and mineral salts from the soil and give proper anchorage to the plant. The tap root system may be regarded as characteristic of dicotyledons.
ii. Fibrous Root System:
In monocotyledons the radicle also gives rise to the primary root, but this does not develop any further and soon perishes and is replaced by many thin roots developed from the base of the stem. These are known as the fibrous roots. Such roots also develop from nodes of stems, as in sugarcane, bamboo and other grasses. Thus fibrous root system may be regarded as characteristic of monocotyledons.
Essay # 2. Regions of the Root:
The tip of each root is covered by a protective root cap, a thimble-shaped covering of cells which fits over the rapidly growing meristematic region (the calyptrogen). The outer part of the root cap is rough and uneven because its cells are constantly being worn away as the root pushes through the soil. The growing point consists of actively dividing meristematic cells from which all the other tissues of the root are formed.
The growing point also gives rise to new root cap cells to replace the ones worn away. Immediately behind the growing point is the zone of elongation, here the cells remain undifferentiated but grow rapidly in length by taking in large amounts of water.
The growing point is about 1 mm. in length and the zone of elongation is 3 to 5 mm. long; these two are the only parts of the root that account for the continued elongation of the root.
Above the zone of elongation is the zone of maturation characterized externally by a downy covering of whitish root hairs. In this zone the cells differentiate into the permanent tissues of the roots. Each root hair is a slender, elongated, lateral projection from a single epidermal cell, through which most of water and minerals are absorbed.
Root hairs are delicate and short-lived; new hairs are constantly formed just behind the zone of elongation and old hairs farther back wither and die as the root elongates. Only a short segment of the root, perhaps one to six cm. long has root hairs.
Essay # 3. Kinds of Root:
On the basis of place of origin there are two types of roots:
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a. Tap roots and
b. Adventitious roots.
a. Tap Roots:
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The root that develops directly from the radicle is known as primary root. In most of the cases primary root persists and becomes stronger to form tap root. The tap root normally grows vertically downwards in a shorter or longer depth. Tap root usually produces lateral branches known as secondary roots.
The secondary roots may further branch to give rise to tertiary roots. The primary root may be sparingly or profusely branched according to the need of the plant.
b. Adventitious Roots:
The roots that develop from any part of the plant other than the radicle are known as adventitious roots. They may develop from the base of the stem replacing the primary root or in addition to it, or from any node and internode of the stem or the branch or even from the leaf.
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On the basis of the nature of their development the adventitious roots may be categorized as follows:
(i) Fibrous Roots:
These roots may be given off in clusters from the base of the stem, e.g., in onion, or from the nodes of creeping branches of grasses, or from the lower nodes of the stem, e.g., in maize, sugarcane, etc. Fibrous roots of monocotyledons are all adventitious roots.
(ii) Foliar Roots:
The roots that develop from the leaf known as foliar roots, e.g., in Bryophyllum, Begonia, Pogostemon, etc.
(iii) True Adventitious Roots:
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Such roots are given off by many plants from their nodes and sometimes from the internodes as they creep on the ground, e.g., in Indian pennywort. The adventitious roots are also produced from branch cuttings when they are put into the soil, e.g., in rose, sugarcane, tapioca, etc.
Essay # 4. Modifications of Roots:
Many roots of peculiar form and function are known among angiosperms. All these are derived from normal type by the specialized development of one or other of its characteristic functions or structures, in a manner suitable to special circumstances.
When the ‘modification’ of the roots are being considered, it is implied that the simple type of absorbing root in the soil was the earliest type to be evolved and that specialized forms have been derived from it.
a. Modifications of Tap Root:
For storage of food:
(i) Fusiform root:
The primary root is swollen in the middle and gradually tapering towards the apex and the base, (e.g., Raphaiuis = radish).
(ii) Napiform root:
The primary root becomes considerably swollen at the upper part (hypocotyl). It becomes almost spherical, and sharply tapering towards lower end, it is called napiform root, e.g., turnip and beet.
(iii) Conical root:
In this case the root is broad at the base and it gradually tapers towards the apex like a cone, e.g., carrot.
(iv) Tuberous or tubercular root:
Here the root becomes thick and fleshy but does not maintain any specific shape. It is known as tuberous or tubercular root, e.g., in four ‘o’ clock plant (Mirabilis).
(v) Branched root modification- Pneumatophores:
Many plants growing in marshy places especially in the tidal swamp forests produce special roots which grow up into the air from beneath the slime are known pneumatophores or beathing roots. They arise from long horizontal roots, and as they are negatively geotropic they grow vertically upwards, projecting several inches into the air. They often occur in large numbers around the trunk of the tree.
There is a corky layer over each root, which covers even the apex, and the portion in the air bears numerous lenticels. Root hairs are not produced either by the pneumatophores themselves or by the horizontal roots from which they arise, but the pneumatophores form short absorbing branches, from their lower portions and hairs are borne on these.
They are the centres of active respiration, and their vertical growth may be necessary to place the absorbing roots at the most favourable level. Examples are seen in Rhizophora, Sonneratia, Heritiera, Avicennia, etc.
b. Modifications of Adventitious Roots:
(a) For storage of food:
(i) Tuberous or tubercular roots:
Such adventitious roots are swollen without any definite shape, e.g., sweet potato.
(ii) Fasciculated roots:
In this type of modification of roots several tubercular roots occur in a cluster at the base of the stem, they are known as fasciculated roots, e.g., in Asparagus Dahlia, etc.
(iii) Nodulose roots:
In such modification the slender root becomes suddenly swollen near the apex, and is known as nodulose root, e.g., in mango ginger.
(iv) Moniliform or beaded roots:
In such modification of root there are some swellings at frequent intervals, and the roots are termed moniliform or beaded, e.g., in Momordica.
(v) Annulated roots:
When the root possesses a series of ring like swellings on its body, it is known as annulated root, e.g., in ipecacuanha.
(b) For mechanical support:
(vi) Prop roots:
Such roots are those which develop from the main branches of a tree. The best example of this is Ficus benghalensis, the Banyan tree. The prop roots grow straight downwards to the soil, which they enter and form underground branch roots. As the roots grow old they increase in thickness due to secondary growth.
As they become thickened they give support to the branches which are thus able to continue their horizontal growth, producing more prop roots at intervals.
(vii) Stilt roots:
Adventitious roots sometimes form supporting stilts. The maize (Zea mays) plant has a tall slender stem. It develops a cluster of roots from the first one or two nodes above ground level. They grow obliquely downwards into the soil and give added support to the stem. Stilt roots are also well known in Pandanus.
The stilt roots are less commonly found in Mangroves such as Rhizophora, Sonneratia and Avicennia. Besides the functions of absorption such roots provide additional support to the stem.
(viii) Climbing roots:
Many tropical climbers like Piper betle, P. longum, P. nigrum, Pothos, etc., produce roots from their nodes and often from the internodes, by means of which such plants attach themselves to their support and climb it. These roots cling closely to the bark of the supporting tree. To ensure a foothold such roots secrete a kind of sticky juice which quickly dries up in the air as seen in Hedera helix and Ficus pumila.
In Hedera adventitious roots are formed in great numbers on the side of the stem next to the support. They attach themselves to the support by the formation of mucilage from the surface cells of the apex. Often the roots form at their apex a disc like structure or a sort of claw for firm foothold. Such roots are also called clinging roots.
(c) For vital functions:
(ix) Sucking roots or haustoria:
Parasites among the flowering plants make use of modified roots as a means of penetrating the tissues of the host plant. In Orobanche, the primary root of the seedling performs this function. It bears no root cap and is the only root formed by the parasite, which serves only for fresh attachments and may be modified roots. Cuscuta has only a temporary root, in the seedling state.
The stem of Cuscuta twines around the stem of the host and obtains its nourishment by means of numerous penetrating suckers which spring from the surface of its stem nearest to the host. A strand of xylem and phloem now differentiates in the sucker and makes connection between the vascular tissues of the host and corresponding tissues in the Cuscuta. Such sucking roots are known as haustoria.
(x) Respiratory roots:
In some aquatic plants such as Jussiaea the floating branches develop a kind of adventitious roots which are soft, light, spongy and colourless. They usually develop above the level of water and serve to store up air. Such respiratory roots facilitate respiration.
(xi) Epiphytic roots:
In many orchids, which grow on branches of trees, the epiphytic roots are found. Such roots do not absorb any food from the supporting plant. The epiphytic roots are aerial and hang freely in the air. Each hanging root is surrounded by a spongy tissue, known as velamen. With the help of velamen the hanging root absorbs moisture from the atmosphere, e.g., Vanda (a common orchid).
(xii) Assimilatory roots:
In some plants, the aerial adventitious roots form chlorophyll in them and become assimilatory in function. They synthesize food by photosynthesis. The common examples are: Tinospora, Trapa, etc.
Essay # 5. Functions of the Root:
The root performs many functions. They may be mechanical, e.g., fixation, and physiological, e.g., absorption, conduction and storage.
The details are as follows:
i. Fixation:
This is mechanical function. Here the main root goes deep into the soil and the lateral roots spread out in different directions, with the result the root system as a whole firmly anchors the plant. In monocots this is done by the fibrous roots.
ii. Absorption:
The absorption of water and minerals from the soil is done with the help of root hairs which develop in a cluster at a little distance behind the root apex. These root hairs adhere to the soil particles and absorb water and soluble mineral salts from them.
iii. Conduction:
The root is also concerned in the conduction of water and mineral salts sending them upwards into the stem and the leaves.
iv. Storage:
There is a certain amount of food stored up in the root, usually in its mature region. As the root grows further the stored food is utilized.
v. Assimilation:
On branches of Tinospora, long, slender, hanging roots are developed, which form chlorophyll in them and turn green in colour. These roots are assimilatory in function.
vi. Hygroscopic roots:
The hygroscopic roots are found in epiphytic orchids, e.g., in Vanda. They absorb moisture directly from the atmosphere.
vii. Symbiotic root nodules:
In most of the genera of Papilionaceae (Fabaceae) (e.g., in gram, pea, many other pulses, etc.), the symbiotic root nodules are present. They contain nitrogen fixing bacteria, and help in fixing the atmospheric nitrogen in the soil to increase its fertility.
viii. Assimilatory roots:
The assimilatory roots are green and prepare food by photosynthesis. Such roots are commonly found in Tinospora, water chestnut, certain orchids, etc.
ix. Some roots help in vegetative propagation, e.g., in sweet potato.
x. Root floats:
In some aquatic plants, e.g., in Jussiaea, roots are spongy and store much air in them. They are buoyant and act as root floats.
xi. Pneumatophores:
The pneumatophores of mangrove vegetation (e.g., in Avecinnia, Sonneratia, etc.) are negatively geotropic and possess lenticels on them for exchange of gases.
xii. Mycorrhiza:
Roots of several plants develop mutually beneficial partnership with some fungal hyphae to form mycorrhiza. e.g., in Pinus, Quercus, etc.
