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Flowering Marine Plants
FLOWERING MARINE PLANTS
When we think of flowers, we usually think of plants that grow on the land. However, some marine plants also flower, and even though their flowers may seem insignificant in comparison the lilies, wildflowers and petunias that we love, they are worth investigating, and your investigation will take to some very interesting and sensitive environments.
The main groups of flowering marine plants are seagrasses (which are particularly under threat across the planet), mangroves, and salt marsh plants.
These marine plants are not true glasses. The roots and horizontal stems (rhizomes), often buried in sand or mud, anchor the grasses and absorb nutrients. Leaves, usually green, are produced on vertical branches and also absorb nutrients. The stems and leaves contain veins and air channels so they can carry fluid and absorb gases. Seagrasses flower but the flowers are simplified and the pollen is moved by the tide. They produce small seeds which can move long distances by currents or inside fish. Populations often extend by complex rhizome systems, ie. systems of interconnected stems that connect shoots below the sediment surface.
These plants are found in shallow temperate, subtropical and tropical coastal waters. They rely on light to convert carbon dioxide into oxygen (photosynthesis). The oxygen is then available for use by other living organisms. The number of species is greater in the tropics than in the temperate zones. Since their maximum depth is affected by light, turbidity greatly reduces the depth to which they grow.
Seagrass communities are one of the most productive and dynamic ecosystems. They provide habitats and nursery grounds for many marine animals, and act as substrate stabilisers. In some coastal areas, entire fisheries may depend on the productivity of these seagrass beds.
Because of their high cellulose content, these marine plants are very tough, and relatively few species can graze on them. However, seagrass meadows are a major food source for a number of grazing animals, particularly the dugong and the green turtle which mainly feed on seagrass. An adult green turtle eats about two kilograms of these plants a day while an adult dugong eats about twenty eight kilograms a day. According to the World Conservation Union (the IUCN), dugongs are listed as vulnerable. Globally, green turtle are listed as Endangered by the IUCN.
The richest mangrove communities occur in tropical and sub-tropical areas where the water temperature is greater than 24ºC in the warmest month. Mangroves are true plants with roots, stems and leaves that have become specially adapted for life in seawater. Mangroves are among those very few land plants that can live in inter-tidal areas and can tolerate a constantly changing mixture of salt and fresh water. They have prop roots which extend downwards from the trunks or snorkel roots that extend upwards from the roots into the air to enable oxygen absorption. They exude excess salt in old sacrificial leaves.
Mangroves are also valuable in nutrient recycling, improving water quality, recharging ground water, flood control, erosion control and stabilisation of river banks. Mangrove colonies usually support rich communities. Populations of bats, birds, insects, and even monkeys live in the branches, while fish, crabs, shrimps, oysters and many seaweeds can be found in the tidal mud zone.
The main challenge for mangroves is coping with salt. For many mangroves, the first line of defence is to prevent much of the salt from entering by filtering it out at root level. Some species can exclude more than ninety percent of salt in seawaters. Mangroves have also adapted to salt water by quickly excreting salt which has entered the system. The leaves of many mangroves have special salt glands which are among the most active salt-secreting systems known. It is quite possible to see and/or taste the salt on the leaf surfaces of species which choose this method. Some mangrove species concentrate salts in bark or in older leaves which carry it with them when they drop.
Mangroves reproductive strategies are unique in the plant world. They disperse propagules (parts of plants that aid in dispersing seeds and other reproductive material) via water; these may have reached varying degrees of embryonic development while the propagule is attached to the parent tree. Most of the mangrove propagule are long (cigar-shaped), and may drop into the mud of sand directly below the parents, and if conditions are right, they will grow. The propagule of some species spend a period in the water during which their embryonic development continues.
Salt Marsh Plants
Salt marshes are coastal wetlands rich in marine life. They are sometimes called tidal marshes, because they occur in the zone between low and high tides. Salt marsh plants (e.g. Cod grasses and Halophytes) are a true grass found in mud flats that are submerged only at high tide. These plants have adapted to thrive in harsh, semi-aquatic environment and saline soils.
Salt marshes contain of a low diversity of plants - rushes, grasses, and sedges – that possess the adaptations needed for salt excretion and gas exchange: salt glands through which salt is exuded, small leaves and stout stems. The tangled roots and stems of these plants stabilise the marsh bottoms and trap dissolved nutrients and materials with each cycle of the tide, and these are converted by bacteria into food for microscopic and larger animals within those ecosystems. In fact, salt marshes produce energy through photosynthesis at a much higher rate than some crop plants. They also provide safe habitats for many animals.
Coastal environments provide a fascinating diversity of plant and animal forms, including these flowering plants. Because they are sensitive and specialised ecosystems, they are very susceptible to changes in climate, water levels and other environmental changes, and they are therefore at risk. This alone makes them worth visiting and exploring.
The authors of this article are staff of ACS Distance Education. This school operates from both Australia and the UK, offering over 350 courses (Hobby and Vocational). See www.acs.edu.au, www.acsedu.co.uk
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