TUTORIAL ON ROOTS

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THIS IS A PART OF PART OF FLOWERING PARTS FOCUSSING ON ROOTS. THIS ARTICLE PROVIDES COMPLETE INFORMATION ABOUT THE ROOT AND WILL CATER TO THE NEED OF STUDENTS STUDYING BIOLOGY FROM SIXTH TO TWELVE STANDARD. IT ALSO PROVIDES THOROUGH KNOWLEDGE ABOUT ROOTS TO ANYONE WHO IS INTERESTED IN IT.

Submitted: October 26, 2014

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Submitted: October 26, 2014

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PARTS OF A FLOWERING PLANT I

TUTORIAL ON ROOTS

 

 

Megha Gupta

M.PHARM(Pharmacognosy and Phytochemistry, Jamia Hamdard, New Delhi)

 

 

 

TUTORIAL ON ROOTS

 Root is the descending portion of plant axis developed from the direct prolongation of embryonic root or radicle. During seed germination the radicle grows down wards within the soil and fix the seedling on the ground thereby providing anchorage to the same.

Characteristics of Roots:

  1. Root is the descending part that is it grows downwards from the plant body. Hence, it is positively geotropic (geo-soil, tropic-love for), positively hydrotropic (hydro-water, here subsoil water, tropic-love for) and negatively phototropic (photo-light, tropic - love for).
  2. They are generally colourless due to absence of chlorophyll pigments. However, roots of Tinospora cardifolia, Trapa bispinosa , Orchids, Podostemon have chlorophyll and are known as Assimilatory roots.
  3. Roots are not differentiated into nodes, internodes, leaves and buds. However, in some roots adventitious buds rise for propagation as in Ipomea batatas, Dalhia, Dalbergia and are known as reproductive roots.
  4. Lateral roots arising from main root are endogenous and arranged acropetally (arrangement from base inwards, youngest at apex).
  5. Root hairs are exogenous and unicellular because they are tubular elongations of epiblemma cell wall.
  6. Root apex is covered by root cap in terrestrial plants and root pocket in aquatic plants.
  7. Some plants are rootless like water milfoil, Bladderwort, Wolfia.
  8. In some plants the main body is the root like Rafflesia, Monotrapa. The roots in these cases are either filamentous or thalloid.

Parts of a typical root:

In all roots the five regions are depicted from apex inwards which gradually merge into another. Let us discuss about them one by one.

  1. Root cap region- Each root (main, secondary and tertiary) are covered by thimble like covering called root cap or calyptera. It is multicellular made of calyptrogen (meristimatic cells). The root cap may be lost by friction of roots with the soil but is renewed. Root caps secrete a slimy material that makes root tip slippery and helps in penetration into soil. In acquatic plants root pockets are present which are looser and cannot be regenerated. Examples in Lemna sp., Pistia sp., Eichornnia sp. Root cap is single but is multiple in Screw Pine and Banyan.
  2. Meristimatic region (Region of cell division) - The cells of this region have denser cytoplasm and large nucleus to promote active cell divisions. They add tissue both forward and backward. The anterior cells form root cap region and posterior to region above.
  3. Region of elongation- In this region, the cells increase in volume, enlarge and elongate so root increases in length. The regions have distinct types of tissues, protoderm, ground meristem and procambium. They increase the length of the root.
  4. Root hair region-Beyond region of elongation root hair region is present which can be differentiated as the region is covered by numerous root hairs of unicellular feature. The length of the zone remains constant as old hairs are destroyed by friction and new hair formed. They help in absorption of water and minerals from soil and helps in anchorage of plant in the soil.
  5. Permanent region- The region behind root hair is permanent region that gives rise to lateral roots in acropetal succession. The lateral roots help in anchorage and absorption of water and minerals from the soil.

 

Classification of root system:

On the basis of origin following are the varioustypes of root system. 

True or normal root -These roots develop from radical either directly or indirectly. It contains primary root, secondary root and tertiary root. For example in Mango, Banyan.

Adventitious root- Roots not developing from radical nor from its branches but from any other part of plant body are called adventitious roots. For example Prop root of Banyan, climbers of Piper betel.

Fibrous root (seminal roots)- Primary roots die down early or does not develop strongly and new roots develop from the base of radical in branches. They are also called Cauline roots beacuse they develop from stem. They are solitary in nature or in bunches growing from nodes or internodes of stems. For example Sugarcane, Rose

Foliar roots- Roots that develop from petiole or leaf margin are called foliar roots. For example in Bigonia sp. 

Functions of root:

  1. Normal Function-

Mechanical function- The root with root system provides anchorage of the plant to the soil.

Physiological function-

  1. Absorption- The roots with root hairs favors absorption of water and minerals from the soil to the different parts of shoot system. The synthesized food materials are transported to the root system for food storage.
  2.  Conduction- The water and minerals absorbed from the soil are transferred from the root system to the shoot system. The organic materials synthesized are transferred to the roots for storage.
  3.  Storage of food- The surplus food is stored in the roots. Due to storage the roots become swollen and during plant formation the stored food is utilized. Examples are Sweet potato, Dalhia, Beet root

Special Function-

  1. Mechanical Function- In plants like Banyan prop roots develop that act like pillar like support for the plants. Roots developing from nodes of the plants like betel help in climbing. The clinging roots of Vanda fix the plant with the host. Contractile roots of Onion, Canna keep the plant erect. Floating roots of Jussiaea maintains plant buoyancy.
  2. Physiological Function- Roots of beet, radish and carrot store food. Breathing roots of Rhizophora and respiratory roots of Jussiaea act as additional respiratory organs. The epiphytic roots of Vanda absorb water and nutrients from air and act as assimilatory roots. The sucking roots of Cuscuta draw nourishment from the host. The green assimilatory roots of Tinospora cordifolia photosynthesize organic food.
  3. Vegetative reproduction- Roots of Ipomea batatas produce adventitious buds for vegetative propagation.

Modification of Roots:

Modified true roots

Storage roots- Due to food accumulation tap roots swell and assume various size and shapes. Some of the forms are

  1. Fusiform- Here the roots swell in the middle and gradually taper in the end assuming a spindle shape. Example Raddish.
  2. Conical- The roots swell at the base and gradually taper toward the end assuming inverted dome shaped body. Example carrot.
  3. Napiform- The root swells in the centre to extreme and abruptly taper at end. Example turnip.
  4. Tubercular- The tap root with branches swell and give it a regular but peculiar shape. Example – Dalhia sp., Mirabilis jalapa.

Saprohytic roots- These are also called symbiotic roots. The roots and branches are infested with fungi called mycorhizzae covering it like a mantle. The plant and fungi live under symbiosis. The plant provides shelter and fungi provide   nutrition. Example screw pine.

Breathing roots- These are also called pneumatophoric roots.  The roots in these cases are negatively geotropic that is they grow against gravity above the soil. The tips have minute lenticular pores called pneumathodes through which exchange of gases take place. They are mainly found in mangrove and tidal areas. Example Rhizophora , Ceriops.

Modified Adventitious roots

Storage roots- Adventitious roots are variously modified to act as storage organs. Some forms are

  1. Tuberous roots in plants like Sweet potato the adventitious roots rise from the nodes of prostrate stem become enlarged and swell without forming any definite shape.
  2. Fasiculated roots in plants like Asparagus the roots develop as clusters from the base of stem and swell like tuberous roots.
  3. Nodulose roots in plants like Mangozinger the tips of rhizomatous plants swell like nodule.
  4. Moniliform roots in plants like Momordica charantia (karela) swelling occur at regular intervals on adventitious roots giving it a beaded appearance.
  5. Annulated roots as in Cepahalis ipecacuanha the roots are swollen in a fashion that they give appearance like series of rings or swollen discs placed over the other. 

Mechanical roots-

  1. Prop roots in plants like Banyan the roots arise from aerial part of plants growing vertically downwards which when fixed to soil become woody and thick and act as pillar.
  2. Stilt roots in maize and pine to provide additional anchorage to the soil where the roots system is weak. These roots arise form lower parts of stem, grow obliquely downwards into soil.
  3. Climbing roots in betel produce short adventitious roots from nodes and often internodes which clasp the support and provide footage to climbers on neighbouring support.
  4. Clinging roots are stout and rise form nodes with claw like structures. These enter into cracks and crevices of supporting tree after which they give out fluid which dries up and fixes the plant to the support. They aid in absorption of water and minerals. Example vanda.
  5. Contractile roots arise from different underground stems which contract and keep aerial parts of plant erect and above soil at proper level. Example Onion, garlic.
  6. Root thorns in plants like palms the root arise from lower nodes of stem that are modified into hard pointed like structures called root thorns which protect the plant from animals.
  7. Floating roots in aquatic plants the roots become spongy and provide buoyancy because they are provided with innumerable air spaces that keep the plant floating over water.
  8. Buttress or plank buttress are large plank like structures found in large huge trees radiating out in base in order to provide additional support. They are morphologically partly roots and partly stem.
  9. Haptera or holdfast are root developing from nodes and are green in color hence, photosynthetic in nature. They help the plant to stick to a rock by special disc shape. Example Podostemon sp.

Physiological roots-

  1. Epiphytic roots in plants like orchids grow on branches of trees that hang freely in the air. These roots are greenish and help in absorption of water and mineral from air. They are covered with spongy tissue called velamen. Due to chlorophyll they carry out photosynthesis.
  2. Assimilatory roots in plants like Tinospora cordifolia they are slender long hanging green roots capable of photosynthesis hence, known as assimilatory roots.
  3. Parasitic roots or hausteria in plants like Cuscuta sp. Broom rage, Mistletoe small adventitious roots arise from the stem penetrating deep into the tissues of the host. The roots are sucking channels through which food is sucked by parasite from the host.
  4. Aquatic roots or breathing roots in plants like Jussiaea are colorless and spongy usually developing from the level of water and stores air in the intercellular spaces facilitating respiration.

Reproductive roots- Sometimes adventitious roots produce adventitious buds with roots that help in vegetative production. Example Trichosanthes dioca.

 

 

 

 


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