Fern life cycle
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
Meiosis
Meiosis is a type of cell division that produces gametes – cells that contain half the number of chromosomes than the parent cell. In ferns, these cells are the spores.
Transcript
Dr Leon Perrie
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.
Video: © University of Waikato
Spores
Spores are small reproductive structures that are released from the sporangium.
Transcript
Dr Leon Perrie
So a spore is the product of meiosis. In ferns, it’s tiny. If you think of a ruler and think how big a centimetre is and divide that by 10, then you've got a millimetre, and then divide that by 10 again and then halve it. That is about the size of the spore.
Video: © University of Waikato Micrograph of spore, Australasian Pollen & Spore Atlas, Creative Commons 3.0
Spore germination
Released spores grow into a gametophytes – very small heart-shaped structures.
Transcript
Dr Leon Perrie
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. It’s 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.
Video: © University of Waikato Micrograph of spore, Australasian Pollen & Spore Atlas, Creative Commons 3.0
Gametophyte (prothallus)
Gametophytes contain both female (archegonium) and male (antheridium) sex organs. They mature at different times in order to increase the chances of cross-fertilisation and genetic variation.
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
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
Antheridium
The antheridium is the male sex organ. These are small spherical structures that produce flagellate sperm.
Acknowledgement: Ohio State University
Fertilisation
Fertilisation occurs when the fern’s egg and sperm combine to form a zygote. Ferns require water to enable the movement of the sperm to reach the egg. A zygote is a combination of genetic material from both the egg and sperm and contains a complete set of DNA to form a new fern plant.
Transcript
Dr Leon Perrie
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.
Video: © University of Waikato Archegonium image courtesy of Ohio State University
Developing sporophyte
The zygote develops from the prothallus (fern gametophyte). It grows using mitosis and develops into a young fern plant.
Acknowledgement: George Shepherd
Mature sporophyte
The mature fern plant consists of three major parts – the rhizome, the fronds and the sporangia. The mature fern plant is the sporophyte structure that produces spores, which are released from sporangia.
Acknowledgement: Steve Attwood
Sporangia (Clustered in sori)
Sori are clumps of sporangia that hold the reproductive spores. They are found on the underside of fern fronds.
Transcript
Dr Leon Perrie
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.
Video: © University of Waikato
