Mangrove forests only grow at tropical and subtropical latitudes near the equator in coastal saline or brackish water. Mangroves are a group of salt-tolerant trees, generally called halophytes. The anoxic sediments under mangroves act as sinks for a variety of heavy metals, which have been scavenged from the water by the colloidal particles in the sediments. Black Mangroves (e.g. Avicennia germinans) are easily identified by the presence of pneumatophores, which are tubular bristles like a root system that sticks out vertically and trap oxygen for its oxygen-starved roots. The Black Mangrove is adapted to high saline conditions and these trees grow in isolated groups or in woodland formations. Individual trees are fairly large and may grow up to 20-25 meters in height. Red Mangroves (e.g. Rhizophora mangle) are evergreen trees, which are immediately recognized by their elaborate prop and aerial root systems that stabilize the trees. The roots contain a waxy substance to prevent salt intake. When salt gets through, it is deposited in older leaves and the tree then sheds them. The tree also creates a propagule resembling an elongated seed pod that is, in reality, a living tree. The fully-grown propagule on the mangrove is capable of rooting and producing a new tree even after floating over a year in brackish water. White Mangroves (e.g. Laguncularia racemose) normally grow in the back portion of mangrove swamps and remain unaffected by tidal inundation except during spring tides. The tree grows up to 18 meters. The leaves are adapted to the saline environment by developing special openings (glands) to expel salt. Mangrove zonation is the predictable and discrete ordering of mangrove species caused by a unique intertidal environment. The red mangrove is closest to the water, while the buttonwood mangrove is found farthest from the water. Their positions depend on land elevation, water and soil salt levels, and tidal changes.