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The Spore-producing Plants
The Pteridophytes: Ferns and Horsetails
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Pteridophyte comes from the Greek words "pterida" meaning fern and "phyta"
meaning plant.
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The fern plants have large complex bodies
with extensive roots underground. Some of them can grow as
large as trees. Millions of years ago forests of tree ferns
covered the Earth but today these forests are mainly
restricted to parts of New Zealand. The fern leaf or frond is
supported by a long stem called a rachis from which grow small
leaves called pinnules.
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The
underside of a fern leaf showing groups of spore cases. |
On the underside of the frond, it is
possible to find small groups of spore cases. The spore cases
are usually found under the pinnules at the ends of the frond.
The shape of the groups of spore cases are useful in
identifying the species of fern plant.
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The fern leaf is divided into sections called pinnules.
Each pinnule has spore cases on its underside. |
As the spore case ripens it dries out. One
side of the case is made of thicker cells than the other. This
makes the spore case burst open and throw the spores into the
air.
The spores are microscopic and so they are
easily carried by the wind. If the spore lands in a suitable
habitat, it will germinate and grow into a small heart shaped
plant called a prothallus. This prothallus is not yet a fern
plant.
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The spore case loses water and splits open, releasing its spores.
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It is on the prothallus that fertilization
takes places in the life cycle of the fern. The sperms are
produced on one part of the prothallus and they swim in a film
of water towards the eggs. The fertilized egg cell will grow
into a young fern. Once the fern plant has established itself
the prothallus dies.
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