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Mangroves in Australia: page 3
Plant Survival Strategies
Salt
Many mangroves stop much of the salt from entering their systems
by filtering it out through their roots. Some species can keep
out more than 90% of salt in sea water. Others quickly pass the
salt out of their systems once it has entered. Their leaves have
special salt glands which pass the salt out. You can see and taste
the salt coating the leaves. Still others collect the salt in
bark or in older leaves that are about to drop. Some mangroves
use more than one of these methods.
Mangroves also have features that conserve water: thick waxy leaves,
fleshy leaves, pores in the leaves that are sunken below the surface
so that wind doesn't dry them out.
Do mangroves need
salt?
Apparently not. Some species have been kept in pots where they
have grown healthily and flowered regularly when given only fresh
water. However, experiments have shown that the best growth occurs
where the plants live in sea water diluted by about 50 per cent
with fresh water.
Unstable Ground
Apart from the salt, mangroves also have to cope with being water-logged, and with unstable soils that may be lacking in oxygen. Mangrove plants have come up with quite similar ways of dealing with these difficulties:
Roots
Roots support
a plant and take in essential nutrients and oxygen. In unstable soil an extensive
root system is necessary in order for the trees to remain standing.
Most mangroves have more of the plant below the ground than above
it. The main mass of roots, however, is generally within the top
two metres of the soil, as the oxygen supply is in that layer.
Cable roots and anchor roots provide support. Small roots come
from these to collect nutients from the rich surface soil. Other
roots collect oxygen from the soil.
Because little oxygen is in the mud, many mangroves raise part
of their roots above the surface. These roots are covered in special
breathing cells to draw in air. To avoid getting buried in the
build up of soil sediment, the breathing roots can grow up vertically.
Pollution is a problem : oil blocks the breathing cells and the
plant can suffocate.
Red, stilt or spider, mangrove is subjected to high wave action and has stilt or prop roots. These spread far and wide, providing numerous anchors for the tree as well as a large surface area for the breathing cells. Extra stilts can grow from the branches or trunk, and develop many breathing cells as soon as they reach the mud.
Grey mangrove grows peg roots, which act like snorkels.
Orange mangrove develops cable roots which have grown above the surface of the mud and then down into it again.
Looking glass mangrove has buttress roots which are like flattened, blade-like stilt roots.
Spreading new
plants
The fruits and seedlings of all mangrove
plants can float, which is how the plant sends its seeds away
to grow in another area. Generally the seeds float away and lodge
in mud, where they begin to grow. Some kinds will only germinate
when temperatures or salt levels are satisfactory. Some species
do not drop their seeds, but begin to grow out of the base of
the fruits to form long spear-shaped stems and roots that grow,
attached to the parent tree, for one to three years. They reach
lengths of up to a metre before breaking off the parent plant
and falling into the sea. They float horizontally until they can
lodge in mud in a less salty place, where they turn vertically
, roots down and buds up. They then begin to grow rapidly.
The cannonball mangrove produces a large fruit, about 20cm in diameter containing up to 18 tightly packed seeds. When ripe, it explodes and scatters the seeds, which float away on the sea.
The seed of the
looking-glass mangrove has a prominent ridge on one side.
This can act as a sail when the seed is in the water.
Click on
this link to see some pictures of mangroves
http://www.forestlight.co.uk/cgi-bin/gallery.pl?fn=display&gallery=4
If you use any part of this in your own work, acknowledge this source in your bibliography like this:
Sydenham, S & Thomas, R. Mangroves in Australia. [Online]www.kidcyber.com.au (2003)
Updated ©kidcyber [2008]