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How mangroves tolerate their environment
Mangrove plants have special adaptations that allow them
to grow in soft, salty, oxygen-deficient soils - something most other plants cannot do.
A
supply of oxygen to the roots is vital for plant growth and nutrient uptake. Because mangrove
soils are often anaerobic some mangrove plants have developed specialised structures called
pneumatophores. These above-ground roots are filled with spongy tissue and have numerous small
holes in the bark which allow oxygen to be transferred to the below-ground root system. They
can also provide structural support for trees in the soft, muddy mangrove soils. There are
four types of pneumatophore - stilt or prop type, snorkel or peg type, knee type and ribbon or
plank type (see How To Use This Guide). Knee and
ribbon types may be combined with buttress roots at the base of the tree.
The water supply available to plants in mangrove forests is often saline.
This water needs to be desalinated before it can be used. Membranes in cells at the root
surface exclude most of the salt. Some salt, however, still enters the root system. This
passes through the plant and is stored within the plant in its leaves, stems and
roots. Consequently, there
are often very high concentrations of salt in mangrove plant leaves. In some species salt is
secreted from special leaf glands (e.g. Grey Mangrove), while in others the salt is
stored until the leaf dies and is shed.
Because of the limited availability of freshwater in
the salty soils of the intertidal zone, mangrove plants have developed ways of limiting the
amount of water that they lose through their leaves. They can restrict the opening of their
stomata (small pores on their leaf surfaces which exchange carbon dioxide gas and water vapour
during photosynthesis) and also have the ability to vary the orientation of their leaves. By
orientating their leaves to avoid the harsh midday sun, mangrove plants can reduce evaporation from
their leaf surfaces.
Mangroves also have buoyant seeds that are suited to dispersal in water.
Unlike most plants, whose seeds germinate in the soil, many mangrove plants are viviparous i.e.
their seeds germinate while still attached to the parent tree. Once germinated the seedling
grows either within the fruit (eg. Aegialitis, Acanthus, Avicennia and
Aegiceras),
or out through the fruit (eg. Rhizophora, Ceriops,
Bruguiera and Nypa) to form what is called a propagule (a seedling ready to
go). When the propagule
is mature it drops into the water and remains dormant while it disperses until it lodges
safely in the soil,
sprouts roots and begins to grow. The development of a propagule of the Red Mangrove is
illustrated in Propagule Formation. Other species within the same
group of plants, called the Rhizophoraceae family, reproduce in a similar way.
Plants that belong to the Rhizophoraceae family are
Ceriops species,
Bruguiera species, and
Rhizophora species.
However, many of the mangrove species growing in the intertidal zone eg.
Xylocarpus species,
Heritiera littoralis and Lumnitzera
species reproduce in the conventional manner.
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