| Fern any pteridophyte
plant having few leaves, large in proportion to the stems, and bearing
sporangia on the undersurface or margin. It is a primitive group which
is distinctive in having a characteristic arrangement of leaf in the bud.
It is coiled in a manner known as 'circinate vernation'. During onward
development the coiled bud elongates and becomes uncoiled assuming a fiddlehead
shape or the familiar Crozier. There are about 12,000 species of fern
now living in the humid tropics; common representatives are sword ferns,
lady ferns, tree ferns, etc.
The young coiled leaves
of some ferns are eaten as vegetables in many parts of the world
including Bangladesh. In the Indian subcontinent about 900 species
have been recorded. Bangladesh has about 250 species, found mostly
in the northeastern hilly forests. Many epiphytes and some climbers
are reported. |
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A common fern, Nephrodium
sp |
Aquatic ferns like Azolla, Salvinia, Ceratopteris
are common in all waterbodies during monsoon. The tiger fern (Achrosticum
aureum) is an integral part of vegetation in the sundarbans
and other coastal regions. Due to habitat changes many ferns such as,
Cyathea, Ctenitis, Lygodium circinatum, and many filmy and
epiphytic ferns, are now facing different categories of threats.
Tree fern A good number of ferns
are shrub-like or giant tree-like. Such large ferns are popularly known
as tree ferns. About 300 species of tree ferns have their habitats in
the tropical and sub-tropical parts of the world. They generally grow
in fairly cool and high humid localities, usually at fairly high elevations
where they are bathed by mists, fogs and rains. They may attain a height
of about a metre to 20 metres. The trunk of the tree fern is quite different
from the common tree trunk; there is no secondary growth and only apical
growth. A crown of leaves are present at the tip of the trunk like in
the common palm. But the young leaves are commonly found coiled at the
centre of the crown in characteristic fern fashion. The matured leaves
act as a reproductive organ by producing a spore on the undersurface.
About 200 million years ago tree ferns flourished and constituted the
dominant vegetation of the world. The famous coal beds of Carboniferous
age have mostly evolved from these group of plants.
The present-day tree ferns belong to genera Cibotium,
Dicsonia, Cyathea and Angiopteris. Of these only three species
under Cyathea, (C. gygantea, C. glauca and C. spinosa),
and one species under Angiopteris (A. evecta) grow in the
eastern hilly regions of Bangladesh. Of these C. spinosa and C.
gygantea are considered as threatened species. [Mostafa Kamal Pasha]
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