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.

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.

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.

• Colonizing and Populating Habitats : Introduction
• Liverworts
• Mosses
• Fungi
• Chapter Summary (useful for revison)
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