The Root

The root is, in most flowering plants, the underground, non-green organ that develops directly from the radicle of the germinating seed and grows down into the soil, away from light and towards moisture and gravity. Its two chief jobs are to anchor the plant firmly in place and to absorb water and dissolved minerals from the soil, and in many species it also stores food, offers extra mechanical support, or takes on other special roles.

Regions of the root. A young, actively growing root shows four distinct regions from tip to base. At the very apex sits the root cap, a thimble-shaped mass of parenchyma cells that covers and protects the delicate growing point as it is pushed through often-abrasive soil; cells at its outer surface are constantly worn away and replaced from within. Just behind the root cap lies the region of meristematic activity: small, thin-walled cells, densely packed with cytoplasm, dividing repeatedly to produce new cells for the root's growth. Above this is the region of elongation, where the newly formed cells enlarge rapidly along the root's length, and it is this elongation -- not further cell division -- that is chiefly responsible for pushing the root tip deeper into the soil. Finally, the region of maturation lies furthest from the tip; most cells here have differentiated into their mature, specialised forms, and many epidermal cells have grown out into fine, thread-like extensions called root hairs, which dramatically increase the root's surface area for absorbing water and minerals.

Tap root and adventitious root systems. Two basic root systems occur in flowering plants. In a tap root system, typical of most dicotyledons, the radicle itself persists and grows directly into a stout primary root, from which thinner lateral (secondary and tertiary) roots branch off. In an adventitious root system, typical of most monocotyledons, the short-lived primary root is soon replaced by a cluster of roots that arise instead from the base of the stem, not from the radicle.

Modifications for storage. Both tap roots and adventitious roots can be modified to store reserve food, usually changing shape in the process. A tap root may become conical, broad at the top and gradually tapering to a point, as in carrot; fusiform, tapering at both the upper and lower ends with the middle portion swollen, as in radish; napiform, very broad and almost spherical at the top and then narrowing abruptly, as in turnip; or tuberous, irregularly swollen with no fixed, describable shape, as in Mirabilis (four-o'clock plant). Where adventitious roots take over storage instead, they may become tuberous, swelling irregularly along their length, as in sweet potato; fasciculated, occurring in storage clusters at the base of the stem, as in Dahlia and Asparagus; nodulose, swollen only near the tip, as in turmeric; or moniliform, swollen at intervals along their length like a string of beads, as seen in some grasses.

Modifications for mechanical support. Some plants develop extra roots purely to brace an otherwise unstable stem. Prop roots grow downward from the horizontal branches of banyan, thickening over time into stout, pillar-like columns that reach the ground and help support the tree's enormous, wide-spreading canopy. Stilt roots arise obliquely from the lower nodes of the stem in plants such as maize and sugarcane, anchoring a comparatively tall and slender stem firmly against being blown or knocked over. Climbing roots, seen in betel and Pothos, arise from the stem nodes of a climbing plant and cling onto a nearby support such as a wall or tree trunk.

Modifications for special functions. Roots also take on unusual, highly specific roles in particular habitats. Mangrove plants such as Rhizophora grow in waterlogged, poorly aerated coastal mud, so many of their roots grow upward, emerging above the mud surface as erect, conical structures called pneumatophores, studded with small pores (lenticels) that allow atmospheric oxygen to diffuse down to the submerged root system, solving the plant's oxygen-supply problem in an anoxic habitat. In leguminous plants such as pea and gram, parts of the root develop small swellings called root nodules, which house the nitrogen-fixing bacterium Rhizobium in a mutually beneficial symbiotic relationship: the bacterium converts atmospheric nitrogen into a form the plant can use, while the plant supplies the bacterium with food and shelter -- a relationship of major agricultural importance since it naturally enriches soil fertility. Some plants, such as Tinospora and the aquatic Trapa, develop green, chlorophyll-bearing assimilatory roots that carry out photosynthesis. And in the parasitic plant Cuscuta (dodder), which has no chlorophyll of its own, the root is modified into a peg-like haustorium that penetrates directly into a host plant's tissue to draw out water and nutrients.