Sometimes in addition to the normal functions of absorption, conduction and anchorage roots may perform storage of food materials, mechanical support, and vital functions. For this, they get modified structurally/ physiologically which is known as a modification of root. This can be found in both tap root and adventitious root.
Fig:Modification of Taproot
In some plants with tap root system, the extra food materials are stored by the primary root due to which the root gets swollen into different shapes on the basis of which they are categorized into four types:
Root is swollen into a spindle-shaped structure with a broad middle portion that tapers towards upper and lower end. Eg; Raphanus sativus (Radish)
In this type, the root is broad on the upper part that suddenly tapers towards the lower end. Eg: Brassica rapa (Turnip)
The root is swollen into a cone-shaped structure with a broad upper part that gradually tapers towards the lower end. Eg; Daucos carota(carrot).
It is the type of root which is swollen into indefinite shape structure. Eg; Mirabilis jalapa( 4 o'clock plant)
For storage of food materials:
Fig:Modification of Adventitious root
In some plants, the extra food materials are stored by adventitious root due to which they become swollen into thick and fleshy structure arising from different parts of the plant body.
The modified root arises from the node of the prostate stem which occurs singly as a swollen root tube. Eg; Ipoemoeabatatas (Sweet potato).
The modified root occurs as a cluster of swollen root tubers arising on the base of the stem. Eg; Dahlia, Asparagus.
Moniliform (Beaded root)
The modified root has alternate swollen and constricted regions giving it a chain of beads like appearance. Eg: Momordica indica(tete karela)
The swollen root consists of a series of annulis or ring-like structures. Eg: Ipecacuanha
In this type, the tips of roots become swollen into beads like structure.Eg; Curcuma amada.
In the plants like Zea mays(Maize), Saccharum officinarum( Sugarcane) obliquely downward growing structures arise from the lower nodes known as stilt root which provides mechanical support against bending force of the wind.
In the tropical tree like Ficus benghalensis vertically downward growing structures arise from the aerial branches which after reaching the soil surface start to function as the normal root and provide mechanical support to the aerial branches. This is known as prop roots.
In the woody climbers or vines like Piper betel (Betel vine)(Paan), Pothos species(Money plant) etc hanging roots arise from the nodes or internodes or both parts which twine around and clasp the support to provide mechanical strength to the plant.
In the plants like Trapa natans(Water chestnut) (Panifall), Tinospora cordifolia(gurja) etc green hanging roots arise from the aerial parts which due to the presence of chlorophyll pigments can perform photosynthesis and are known as assimilatory roots.
In the epiphytes like orchids, special types of roots arise from the stem which is covered by the spongy tissue known as velamen that can absorb atmospheric moisture. This moisture is used by the plant to carry on photosynthesis process and is known as the epiphytic roots.
Haustorium or sucking roots
In parasitic plants like Cuscuta small peg-like outgrowths are given off which penetrate the host epidermics and get branched. These branches get inserted in the conducting tissue i.e. xylem and phloem. Thus, the parasitic plant can obtain necessary water minerals and food materials from the host plant. This is known as the hostoria or sucking roots.
In the plants growing in saline regions( Mangrove plants) vertically upward growing structure arise from the secondary roots which occur above the soil surface and are known as respiratory roots or pneumatophores. These are provided with minute pores known as pneumathodes through which atmospheric oxygen can be taken in to fulfil the oxygen requirements. Eg; Rhizophora