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Fern life cycle

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The life cycle of ferns is different from other land plants as both the gametophyte and the sporophyte phases are free living. This interactive illustrates the alternation of generations in ferns.

Transcript

Sporangia (Clustered in sori)

The sporangia is the structure which produces spores. In ferns the sporangia are usually aggregated into particular bigger structures, if you turn over a fern frond and you might see lines and those are aggregations of the sporangia.

Meiosis

The typical big fern plant, what it does is by meiosis produces spores and the spores have half the number of chromosomes of the big parent plant.

Spores

So a spore is the product of meiosis. In ferns its tiny, if you think of a ruler and think how big a centimetre is and divide that by ten then you've got a millimetre and then divide that by ten again and then halve it that is about the size of the spore.

Spore germination

Spores are released into the wind. If those spores happen to land somewhere suitable they will grow into what is called a gametophyte and that is a whole separate individual plant. Its very tiny, maybe the size of your fingernail, and it’s just like a little, thin, small green plate. What that does is it will produce the sex cells, the eggs and the sperm.

Gametophyte (prothallus)

The prothallus is the fern gametophyte. It is a green, photosynthetic structure that is one cell thick, usually heart or kidney shaped, 3–10 mm long and 2–8 mm broad. It is very difficult to find in the bush as it is so tiny. It doesn’t have roots, stems or leaves but it does have rhizoids that anchor it to the soil and help with absorption. The underside of the prothallus is where the gametes are produced from the male and female sex organs. The prothallus has both male and female sex organs.
Acknowledgement: Public domain.

Antheridium

The antheridium is the male sex organ. These are small spherical structures that produce flagellate sperm.
Acknowledgement: Ohio State University.

Archegonium

The archegonium is the female sex organ. These are flask-shaped structures that produce an egg which is reached by the sperm swimming down the neck.
Acknowledgement: Ohio State University.

Fertilisation

The sperm need to swim through water in order to get to the eggs, the eggs are housed or maintained in the gametophyte, and that dependence on water is why ferns are so often linked to wet habitats. If the sperm do manage to get to an egg, fertilisation occurs and that is where the two, the sperm and egg come together. That doubles the number of chromosomes and that gives rise to a whole new typical fern plant again and the cycle repeats.

Developing sporophyte

The young fern plant develops from the prothallus.
Acknowledgement: George Shepherd.

Mature sporophyte

The mature fern plant consists of 3 major parts – the rhizome, the fronds and the sporangia. The mature fern plant is the sporophyte that produces spores which are released from sporangia
Acknowledgement: Steve Attwood.

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