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Part XIX : Colonizing and Populating Habitats : Seeds and Spores Index

Seeds and Spores : Introduction
Liverworts
Mosses
Fungi
Chapter Summary (useful for revision)
Questions relating to this chapter

Topic Chapters Index

 

Fern prothallus © Paul Billiet 

A fern prothallus

 

Ferns and coal

300 million years ago many parts of the Earth were covered in dense fern forests. It was a group of ferns called horsetails which were particularly important in those ancient forests. The forests grew very well in swamps which covered part of the regions which we call Europe and North America today.

This age of the Earth, 300 million years ago, is called the Carboniferous Period by geologists. In the rocks which belong to this period we find large amounts of coal.

Coal begins to form when vegetation dies in wet swampy areas. The water does not allow the vegetation to decompose properly so it turns into a material called peat. Peat is still being formed today and in certain regions, such as Scotland and Ireland, it is dug up, dried and burned as fuel.

If the peat is covered in sediment and if it is compressed under pressure the peat will turn into coal.

COLONIZING AND POPULATING HABITATS

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Seeds and Spores

The Pteridophytes: Ferns and Horsetails

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.

 

Life cycle of fern © Paul Billiet 

 

On the underside of the frond you may 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.

As the spore case ripens it dries out. One side of the spore case is made of thicker cells than the other. This makes the spore case burst open and throw the spores into the air.

 

Sporangia on the underside of fern pinnules  © Paul Billiet 

Sporangia on the underside of fern pinnules

 

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.

It is on the prothallus that fertilization takes place 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. The life cycle for the fern called Polypodium is shown in the diagram above.

 

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