MEWManaptions


	GENERAL CHARACTERISTICS AND ADAPTATIONS


	Most dicotyledons of mangal form as small trees. In some Old World mangrove swamps, trees may reach 40 meters, but most mangal rarely exceeds 10 meters. Except for epiphytes, which are not restricted to mangal, herbaceous species are absent.


	Zones of mangal are often fairly easy to distinguish because a zone will be dominated by only one or two species. Monocultures are produced when vigorous growth of one species, such as Nypa fruticans or Rhizophora mangle, prevents other species from becoming established.

The major plant species forming the mangrove tangle have aerial roots, commonly prop roots or even stilt roots. These serve, of course, to anchor the plants, but also are important in aeration, because the mangrove mud tends to be anaerobic. Special vertical roots, called pneumatophores, form from lateral roots in the mud, often projecting above water. These are particularly well developed in species of Avicennia, Sonneratia, less so in Laguncularia, and as knee-like structures in Xylocarpus mekongensis, Bruguiera, and Ceriops, permitting some oxygen to reach the oxygen-starved submerged roots. Roots also can exhibit development of air cavities in root tissues, designs that aid oxygenation of the tissues. Aeration occurs also through lenticels in the bark of mangrove species, e.g., species of Rhizophora.




		*This is the growing tip of a thick, aerial stilt root of a Rhizophora* mangrove (Family Rhizophoraceae) at Port Douglas, Queensland, Australia.**


	The aerial stilt roots of red mangrove, Rhizophora mangal (family Rhizophoraceae), originate from the lower branches and grow downward into the mud or sand of open seawater, to be submerged at high tide.



			Avicennia (Family Verbenaceae/Avicenniaceae) pneumatophores are abundant in mangrove swamp mud that is covered at high tide with seawater.


	Pneumatophores of Avicennia (Family Verbenaceae/Avicenniaceae) are pencil-like projections that form in the open mud flat of the mangrove swamp. These structures are thought to be important for oxygenating roots that grow in the anaerobic mud. Amidst the pneumatophores is a seedling of Avicennia eucalyptifolia.




		*The spike-like pneumatophores of Sonneratia* (Family Lythraceae) form along the seaward margin of mangal.**




	Sonneratia alba (Family Lythraceae) forms a thicket of long, cone-shaped pneumatophores. This is a common mangrove species in the seaward zone, and this species ranges from eastern Africa to Micronesia and eastern Australia. Here the species is thriving in northern Queensland.


	Leaves of typical mangrove plants are evergreen, relatively tough, and very similar in size across the species that belong to different families.




		The mangrove swamp is inhabited by tree species with elliptical to ovate evergreen leaves. Many of these have opposite leaf arrangement, so that an observer must look at the flowers, fruits, and bark features for positive identifications.

			*The cannonball mangrove, Xylocarpus granatum* (Family Meliaceae), is one of only a few mangrove trees with pinnately compound leaves. This genus is fairly common from Southeast Asia to Australia.**




Called by some a woody herb, this shrubby or scandant species is the holly-leaved mangrove, Acanthus ilicifolius (Family Acanthaceae), which typically has spinose margins on its evergreen leaves and stipular spines at stem nodes.

Mangrove plants have salt resistance. This may involve cytoplasmic tolerance of high solute concentrations, and many of the common species have in leaves sodium and chloride ion levels that each exceed 250 millimoles, about half that of sea water. In addition, the most salt-resistant species also tend to show avoidance to salt stress. Avoidance can be achieved by excretion of crystalline salt from glands or hairs of leaves. Examples of salt excretion are Avicennia, Aegialitis annulata (family Plumbaginaceae), Aegiceras, and Acanthus ilicifolius (family Acanthaceae). Another mechanism to avoid toxic levels of ions is to produce succulence, i.e., dilution of salts via having watery tissues. Salt dilution is said to be the reason for having large hypodermal cells on the upper (adaxial) side of leaves, covering the photosynthetic tissues, e.g., in Avicennia and Rhizophora, or large, vacuolate cells in the middle tissue in Sonneratia and Laguncularia.

Germination of seeds while still attached to the mother plant is called vivipary. Vivipary and cryptovivipary (not visually obvious) are exceedingly rare among plants, but these are found in many early colonizing, pioneer species on mangal, including Avicennia, Rhizophora and all other Rhizophoraceae, Aegiceras, Pelliciera, Aegialitis, and the aggressive estuarine species Nypa fruticans. The classic example of vivipary is Rhizophora mangle which is able to traverse broad ocean regions by producing large seedlings that float horizontally, undamaged by salinity. These seedlings can be washed up on sand or mud flats, where they settle to establish new populations. In this case, the hypocotyl develops as a long, stiff axis, sometimes exceeding a foot in length. Vivipary is alleged to be a strategy not only for seawater flotation, a dispersal mechanism, but to avoid the toxic effect that chlorides have on germination. By germinating while on the mother plant, and thereby drawing nutrients under lower salt stress, the young plant can increase its salt resistance before falling into the seawater environment.




	Even within a dense mangrove swamps, seedlings from the mangrove family (Rhizophoraceae) occasionally take root.




		*One of the widespread Old World species of the mangrove family (Rhizophoraceae) is Ceriops tagal, here shown in fruit before vivipary has occurred.*





		A classic example of vivipary in plants has been the red mangrove, Rhizophora mangal (Family Rhizophoraceae). While the fruit persists on the mother tree, the embryo forms an extremely long hypocotyl. The young seedling will eventually drops from the plant, then either root in surrounding mangrove swamp mud or drift on water potentially to colonize a different site.




	Seedlings of Rhizophora mangal (Family Rhizophoraceae), the common New World red mangrove, have taken hold in mud and lava of an Hawaiian mangrove swamp near Hilo. Mangroves are not native to the Hawaiian Islands but were introduced to the archipelago.


	A plant of of the red mangrove, Rhizophora mangal (Family Rhizophoraceae) has secured itself by forming aerial stilt roots on this Hawaiian mud flat covered with lava rock. Younger seedlings are attempting to become established in this population.


	Mangrove trees appear to have special mechanisms to permit them to take up water from the very saline muddy soil without making their water conduits salty.


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