Modifications of root

To perform secondary functions such as storage, respiration, mechanical support or absorption, roots often change from their normal cylindrical form - these changes are root modifications, grouped as tap-root modifications and adventitious-root modifications.

Tap root modifications are mostly for storage, and less commonly for breathing. Storage forms include: the conical root - cone-shaped, broad at the base and gradually tapering to the tip, as in carrot (Daucus carota); the fusiform root - swollen in the middle and tapering at both ends, as in radish (Raphanus sativus); and the napiform root - very broad, tapering suddenly like a tail at the apex, as in beetroot (Beta vulgaris). Breathing roots are a different kind of tap-root modification seen in mangrove trees such as Avicennia, Rhizophora and Bruguiera, whose soil is waterlogged and poorly aerated; these roots are negatively geotropic, growing upward out of the mud, and are studded with pneumatophores (breathing pores) that let the plant exchange gases.

Adventitious root modifications serve three broad purposes. For storage: the tuberous root is singly produced and swollen without any fixed shape (Ipomoea batatas); the fasciculated root occurs as a cluster of swollen roots from the stem base (Dahlia, Asparagus, Ruellia); the nodulose root swells only near its tip (Maranta, Curcuma amada, Curcuma longa); the moniliform (beaded) root swells at regular intervals giving a beaded look (Vitis, Portulaca, Momordica, Basella); and the annulated root shows ring-like swellings at regular intervals (Psychotria). For mechanical support: prop (pillar) roots grow straight down from lateral branches into the soil to brace a spreading crown (Ficus benghalensis); stilt (brace) roots grow obliquely from the basal stem nodes (Saccharum officinarum, Zea mays, Pandanus, Rhizophora); climbing (clasping) roots arise from stem nodes, secrete a sticky juice and cling to a support (Epipremnum pinnatum, Piper betel, Ficus pumila); and buttress roots are broad, plank-like outgrowths around the trunk base that brace tall rainforest trees (Bombax ceiba, Ceiba pentandra, Terminalia arjuna, Delonix regia, Pterygota alata). For vital (physiological) functions: epiphytic (velamen) roots hang freely in air and absorb atmospheric moisture through a spongy velamen tissue, as in orchids like Vanda …