Q.Describe the regions of a root and its modifications for storage, mechanical support, and special functions such as respiration and nitrogen fixation, giving examples of each.
The root normally develops from the radicle of the germinating seed and grows downward, away from light, showing four successive regions from tip to base. At the very tip is the root cap, a thimble-like covering that protects the delicate growing point as it pushes through the soil. Just behind it lies the region of meristematic activity, made of small, thin-walled, actively dividing cells. Behind this is the region of elongation, where cells enlarge rapidly and push the root tip further into the soil. Finally, the region of maturation shows most cells differentiated, with many epidermal cells extending into fine root hairs that greatly increase the absorptive surface for water and minerals.
Beyond this basic plan, roots are modified in several directions. For storage of food, tap roots may swell into recognisable shapes -- conical in carrot, fusiform (tapering at both ends) in radish, napiform (broad top, sharply tapering below) in turnip, and tuberous (irregularly swollen, no fixed shape) in Mirabilis; where the primary root dies early and adventitious roots take over storage instead, they may become tuberous (sweet potato), fasciculated (occurring in clusters, as in Dahlia and Asparagus), nodulose (swollen only at the tip, as in turmeric), or moniliform (beaded at intervals, as in some grasses).
For mechanical support, prop roots grow downward from aerial branches of banyan, thickening into pillar-like columns that brace its enormous spreading canopy; stilt roots arise obliquely from the lower nodes of the stem in maize and sugarcane, anchoring a comparatively tall, slender stem; and climbing roots, as in betel and Pothos, arise from the nodes and cling to a nearby support.
Finally, roots take on special functions in particular habitats. In mangrove plants that grow in waterlogged, poorly aerated mud, some roots grow upward out of the soil as pneumatophores, bearing pores (lenticels) that let atmospheric oxygen reach the buried root system. In leguminous plants, portions of the root develop into small swellings called root nodules, which house the nitrogen-fixing bacterium Rhizobium in a mutually beneficial (symbiotic) relationship. And in parasitic plants such as Cuscuta (dodder), the root is modified into a haustorium, a peg-like structure that penetrates the host plant's tissue to draw out water and nutrients directly.
Regions: root cap, region of meristematic activity, region of elongation, region of maturation (with root hairs). Modifications -- storage: conical/fusiform/napiform tap roots (carrot/radish/turnip) and tuberous/fasciculated adventitious roots (sweet potato/Dahlia); support: prop roots (banyan), stilt roots (maize); special functions: pneumatophores for respiration (Rhizophora) and nodulated roots for nitrogen fixation via symbiotic Rhizobium (legumes).
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