Specialized and Modified Roots
Roots often perform functions other than support and absorption. Some store starch (beets and turnips) or water (desert plants). Pneumatophores are roots that grow into the air and are filled with a specialized parenchyma called aerenchyma. The large, intercellular spaces of aerenchyma are filled with oxygen and other gases. The pneumatophores seem to assist in aerobic respiration and gas exchange and are abundant on woody plants like cypress and mangrove, which grow in water‐logged soils.
Some roots produce suckers from adventitious bud-like growths. Suckers grow into aerial shoots capable of independent existence, and serve to propagate the plant. Because they have the same genetic make-up as the parent plant, they are clones of the parent. Aspen groves in the western United States frequently are clones of only one or a few parent trees.
Many flowering plants develop as parasites on other plants. They produce adventitious roots called haustoria that penetrate the tissues of the host and connect to the vascular system, thus becoming part of the host pipelines. Parasitic roots lack most of the tissues of ordinary roots. Dodder and mistletoe are two of the more than 3000 flowering plant parasites.
Mycorrhizal roots are known from 90 percent of plant species and are a mutualistic association of a fungus with plant root tissue. Most plants require specific mycorrhizal fungi without which they are unable to absorb sufficient quantities of P, Zn, and Mn. The fungus takes the place of root hairs and may penetrate the cortex completely (endomycorrhizae) or remain on the surface of the root ( ectomycorrhizae).