Root systems and their modifications in plants
The root system is the most important organ system of the plant body. It plays an important role in all its life activities. This article deals with the two different types of root systems observed in plants and also the various functional activities carried out by them. This article deals with the various modifications of the roots and their utility in serving additional functional needs of the plant. Some frequently asked questions were also included along with the topic.
Introduction
Root system is the underground non-green or non-photosynthetic part of the plant body. It fixes the plant body firmly to the soil and it absorbs water and minerals from the soil. In plants, the first root develops from the radicle of the germinating seed and it is known as the primary root. The thin branches that arise from the primary roots are known as secondary roots. The tiny thin roots that branches out from secondary roots are known as root hairs. Root hairs of the root system are mainly concerned with the absorption of water from the soil.Characteristics of roots
The following are important characteristics features of roots.Types of root systems
In flowering plants we can find two types of root systems-Tap root system and adventitious root system.Tap Root system
In this type of root system, there will be a bulky mother root or taproot and is developed from the radicle of the germinating seed. The mother root always grows vertically downwards in the direction of gravity. This main root is known as the Primary root. On this bulky taproot there develop long thin secondary roots. These roots grow somewhat laterally from the taproot. From the lateral roots, there arises a large number of thin unicellular root hairs. These roots are known as root hairs or tertiary roots. So a root system with a bulky tap root, thin long secondary roots, and finally with numerous tiny root hairs together known as 'Taproot system'. The Taproot system is a characteristic feature of all dicot plants. Taproot system can be found in Bean, Groundnut, Gram plants, etc.Adventitious root system
In adventitious root system, taproot is absent or it degenerates immediately after its formation. A large number of long thin fiber-like roots arise in tufts from the base of the stem. Such a root system with a large number of slender long roots arise from the base of the stem in aggregation is said to be an 'Adventitious root system' or 'Fibrous root system'. The presence of an adventitious root system is a characteristic feature of all monocot plants. The adventitious root system can be found in plants like Rice, Maize, Grass, etc.Chief functions of root system
Root system of a plant will carry some vital functions for the plant. Modification of roots
Roots show various specific modifications to carry out some specific functions to serve the needs of the plant. Modification of Taproots
In some plants, taproots show certain modifications in their normal structure to serve some additional function to the plant body. Storage roots
In some plants, the food prepared in leaves is transported into roots for storage. Due to the accumulation of food in roots, the roots take different shapes. The stored reserved food in the roots will be utilized by the plant in needy times. Storage roots are classified into different types basing on the shape they acquire due to the accumulation of food in them. Conical roots
The tap roots acquire conical form due to accumulation of food in certain plants. Conical roots are broader at the top and gradually narrower towards the tip. Ex: Carrot (Daucus carota).
Storage roots-Conical-Carrot (left) and Napiform-Beet root (right) (Photograph by Author)Napiform roots
In some roots, the roots will be swollen towards the top and sharply tapering towards the lower end. The shape of the root appears to be like a top as the food gets completely accumulated at the base of the root. Ex: Turnip (Brassica rapa) and Beetroot (Beta vulgaris).Fusiform roots
In this type of modified roots, the root will be swollen in the middle but tapering towards the base and apex. The shape of the modified root appears to be like a spindle. Ex: Radish (Raphanus sativus).
Fusiform and Tubercular Storage roots (Diagram by the Author)Tuberous or Tubercular roots
This type of root is thick and fleshy but it doesn't maintain any specific shape. Ex: Mirabilis (Mirabilis jalapa), Tapioca (Manihot utilisima), Cichorium.Modifications of Adventitious roots
In some plants, the adventitious roots that develop from any part of the plant other than taproot will aid to serve in the storage of food, mechanical support, and other vital functions of the plant body.Adventitious roots in the storage of food
Like in taproots, adventitious roots of certain plants also accumulate food in them for their future utility. According to the shape they acquire, they are classified into several types. 
Different types of storage Adventitious roots (Diagram by the Author)Tuberous roots
In some plants, the adventitious roots transform into swollen and fleshy roots to store food. This type of storage roots has no specific shape. Ex: Sweet potato (Ipomea batatas).Fasciculated roots
In certain plants, bunches of tuberous roots develop at the base of the stem and are simply the modification of adventitious roots for the storage of food. Ex:-Asparagus, Dahlia.Nodulose roots
The adventitious roots of certain plants become swollen at their tips due to accumulation of food are known as nodulose roots. Ex:- Turmeric, Mango ginger.Moniliform or Beaded roots
Adventitios roots in certain plants show swellings at frequent intervals for storage of food are known as beaded roots. The swellings of the modified root appear like beads and thus they are known as beaded roots. Ex:-Momordica, Spinach, Dioscorea.Annulated roots
In this type of modified adventitious roots, the roots show serial ring like structures separated by grooves. Ex:-Ipecac.Adventitious roots in Mechanical function
In some plants, the adventitious roots helps in providing support to the plants in different ways.Prop roots
Banyan tree usually grow very large with heavy trunk and it bear heavy branches. To provide support to these heavy branches, long pillar-like roots called prop roots (adventitious roots) develop from the branches of the tree vertically downwards and fix themselves firmly to the soil to give additional mechanical support to the heavy tree. Even in severe cyclonic conditions these prop roots prevent uprooting or overturning of the tree. In Kolkota Botanical garden there present a huge Banyan tree and it has 800 pillar-like prop roots from its branches. Ex:- Banyan (Ficus benghalensis).
Mechanical support providing Prop roots in Banyan tree (Courtesy:-wikipedia.org)Buttress roots
In some very long trees, plank-like structures called root buttresses develop from the base to provide mechanical support to the plant. Ex:- Terminalia, Bombax. 
Plank-like Buttress roots in Termilania tree (Courtesy:- wikipedia.org)Stilt roots
Some plants grow very long with weak stems. In such plants, adventitious roots from the lower base of the stem get transformed into stilt roots and get themselves fixed to the soil firmly to provide mechanical support to the plant. These stilt roots help the plant to stand in an upright position. Ex:-Maize, Sugarcane, Pandanus.
Stilt roots, Climbing roots, and Floating roots in plants (Diagram by the Author) Climbing roots
In some weak stemmed climbing plants, adventitious roots arise from nodes and rarely from internodes to attach themselves to support. Many such climbing roots secrete a sticky substance or develop a disk-like structure to hold on to the support firmly. Ex:-Money plant, black pepper, Piper betel.Floating roots
Some floating aquatic plants develop spongy, soft, and light adventitious roots to float themselves on the surface of the water without submerging. Ex:-Jussiaea repens.Adventitious roots in vital functions of the plantCertain plants develop modified adventitious roots to carry some vital physiological activities of the plant body.
Respiratory roots or Pneumatophores or Breathing roots
Certain plants grown in soils that are very poor in oxygen content like sea-shores, saline marshes, muddy and swampy areas. In order to overcome oxygen deficiency, these plants develop vertically upward growing breathing roots with lenticels (holes). These modified adventitious roots will help in the exchange of respiratory gasses through the holes present on these roots. Pneumatophores are negatively geotropic and possess lenticels on them. Ex: Avicennia, Rhizophora, Heritiera, Sonneratia.
Respiratory roots in Avicennia plant (Courtesy:-wikipedia.org)Parasitic roots or Haustoria
Parasitic plants absorb nourishment by some modified roots called haustoria. Haustoria of partial parasite will be attached to the xylem of the host to absorb water and minerals. Haustoria of complete parasites will be attached to the xylem and phloem of the host to absorb water, minerals, and organic substances. 
A partial Root parasite Striaga (White in Picture) on host roots(Courtesy;-wikipedia.org)
Loranthus, Viscum, and Cassytha are partial stem parasites. Cuscuta, Arceuthobium are complete stem parasites. Striga, Santalum album (Sandalwood tree) are partial root parasites. Rafflesia, Orobanche, and Balanophora are complete root parasites. Interestingly, Santalum album starts its life as a partial root parasite and later leads independent life.Assimilatory or Photosynthetic roots
Usually roots be never photosynthetic in nature but rarely certain plants develop green chlorophyllous roots that can carry photosynthesis. The submerged Trapa natans (water chest plant) develop green photosynthetic roots from nodes in pairs and can carry photosynthesis. The hanging roots of the Orchid plant, Vanda has hanging epiphytic roots and that can transform into assimilatory roots to perform photosynthesis. Similarly in the Tinospora plant also assimilatory roots are present to carry photosynthesis.
Assimilatory roots (Trapa natans), Epiphytic roots (Vanda), and Leaf roots (Bryophyllum) (Diagrams by the Author)Epiphytic roots or Velamen roots
Certain Orchid plants grow on the branches of other trees but they are not parasites. These plants are known as epiphytes. These plants develop some special spongy aerially grown epiphytic roots. These special roots are covered by spongy velamen tissue around them. This tissue helps these roots to absorb moisture from the atmosphere and it helps the plant to carry photosynthesis. Hence the roots are also known as velamen roots. Ex:- Vanda (Orchid plant)Leaf roots
Roots also help in the vegetative propagation of plants. Trichosanthes and sweet potato are propagated by roots. The ornamental garden plant Dahlia is propagated by root cuttings. From the leaf margin of Bryophyllum leaf, vegetative shoots and adventitious roots develop and thus help in vegetative propagation of plants.Most frequently asked questions:-
Ans. Velamen roots are freely hanging spongy light roots developed in Orchid plants like Vanda. These plants grow on the branches of other plants. The surface of these hanging roots is covered by velamen tissue. This tissue is hygroscopic in nature and absorbs moisture from the atmosphere to carry out photosynthesis.
Ans. Root hairs.
Ans. Bryophyllum leaf.
Ans. Haustoria are modified parasitic roots that penetrate into host tissue to get nourishment.
Ans. Certain mangrove plants like Avicennia, Rhizophora, etc. grow in soils with a deficiency of oxygen. These soils can be found in seashores, swamps, and marshy areas. Respiratory roots are modified adventitious roots that grow above the soil surface to breathe oxygen.
And. A partial plant parasite depends on the host for only water while the complete plant parasite depends on the host for both water and food.
And. Roots that develop from any part of the plant body other than on taproot are known as adventitious roots.