In warm climates where freezing is not a problem, roots can grow above ground where they assume diverse functions. Above-ground roots are well developed in orchids, fig trees, and mangroves growing throughout the tropics and subtropics.
The orchid family includes climbing orchids and epiphytes that live on tree branches, close to sunlight but away from soil. Roots of the vanilla orchid emerge from the stem and support this climbing vine by twining around sticks and branches of trees. Roots of epiphytic orchids are out in the open where they have access to sunlight and rainwater, but not to soil. Long-term studies at Hummingbird Cay Tropical Field Station in the Bahamas show these orchids grow very slowly but live for decades. Their aerial roots have a spongy multilayered epidermis called velamen that enables roots to store water from rain and bark runoff. To the inside of the velamen the cortex is modified for photosynthesis, performing a function usually restricted to leaves. Photosynthesis by the elaborate green roots of epiphytic orchids in the tropics and Japan compensates for the reduced leaves in these plants.
Ficus trees such as figs and banyans use buttress roots and stilt roots to support large canopies atop shallow tropical soil. Buttress roots resemble rocket fins at the bottom of large tree trunks. They develop from the fusion of the upper side of a horizontal root with the vertical tree trunk in response to tension as the tree leans away from the developing buttress. Trees growing on hillsides show larger buttresses on the uphill side of the trunk than on the downhill side. Buttresses are prominent on Brazilian rubber tree (Hevea), and kapok (Ceiba). Stilt roots develop from horizontal branches and grow down to the soil where they thicken and become long-lived supports for the tree canopy.
Mangroves are trees living in tropical coastal areas. Their roots are of enormous value in stabilizing tropical coast lines against typhoons, hurricanes, and wave action, and they give refuge to young stages of commercially important fish. The global value of such ecosystem services provided by mangroves was estimated in 1997 to exceed $600 billion. To help restore damaged environments, mangroves are being replanted in Vietnam, the Philippines, and Mexico. Mangrove roots allow these coastal trees to grow in shifting sand and oxygen-poor soil. The stilt roots of red mangrove (Rhi-zophora) spread down into sand from dozens of canopy branches, thereby stabilizing the tree. Pores, called lenticels, on the root surface allow oxygen to enter the aerial part of the root and diffuse down to submerged tissues in oxygen-depleted soil. Massive intertwined root systems of red mangrove forests prevent hurricanes from removing acres of land from south Florida and the Caribbean. The root system of black mangrove (Avicennia) has at least three root types. Underground cable roots radiate horizontally from the central tree trunk and stabilize the tree. They produce upward-growing roots (pneumatophores) that grow out of the soil and act as snorkels to bring oxygen into belowground roots. Feeder roots branch from the base of each pneumatophore where their large surface area absorbs minerals from the soil. see also Anatomy of Plants; Compost; Epiphytes; Nitrogen Fix ation; Nutrients; Orchidaceae; Tissues; Tropism; Vascular Tissues; Water Movement.
George S. Ellmore
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