The fern life cycle is different from the life cycle of flowering plants. Ferns use alternation of generations, with a sporophyte diploid (2n) generation and a gametophyte (n) generation. The reason is that ferns evolved for life in a damp environment, using water for sexual reproduction. The plant with fronds we recognize as a “fern” is actually just the sporophyte phase of the plant’s life.
Parts of a Fern
Understanding the life cycle is easier if you know the parts of a fern:
A mature fern consists of three primary components: fronds, roots, and rhizomes. Fronds are the leafy parts of the fern that give them their characteristic feathery appearance. The root system anchors the fern to its substrate and helps absorb water and nutrients. The rhizome, which may be underground or partially above ground, serves as the plant’s stem. Roots and fronts emerge from the rhizome.
Fern fronds house a fascinating feature: sporangia. Sporangia are tiny cases called sori found on the underside of fronds that produce and contain spores. These spores are the means by which ferns propagate.
Understanding Alternation of Generations
The fern life cycle involves alternation of generations. This term refers to the two distinct multicellular life stages that occur in the life cycle of algae and some kinds of plants: the sporophyte and the gametophyte stages.
The sporophyte stage is diploid, meaning it contains two sets of chromosomes (2n). This is the stage we recognize as the mature plant. The sporophyte produces spores via a process called meiosis, which results in cells with half the number of chromosomes, or haploid cells (n).
These haploid spores then grow into the gametophyte stage, which is small and often not readily recognizable. The gametophyte produces gametes – eggs and sperm. These gametes fuse during fertilization and form a new diploid cell, or zygote, that grows into the next sporophyte, thereby completing the cycle.
Steps of the Fern Life Cycle
It doesn’t matter whether you start the life cycle with the sporophyte or gametophyte, as long as you complete it.
- The diploid mature sporophyte performs meiosis and produces haploid spores in the sporangia on some fern fronds. Meiosis is the process that makes eggs and sperm in flowering plants and animals.
- The sporangia release the spores. Once these spores find a suitable location with enough moisture, they germinate, growing into tiny heart-shaped gametophytes. Another name for the gametophyte is the prothallus. The prothallus has small leaflets for photosynthesis and rhizoids for obtaining nutrients and water from the surface. It is much smaller than a sporophyte fern, so this generation is not very noticeable unless you look for it.
- The gametophyte (prothallus) contains two types of reproductive organs. Antheridia produce sperm, while archegonia house the eggs.
- Under wet conditions, sperm from the antheridia swim towards the archegonia and fertilize the eggs.
- The fertilized egg, or zygote, is now diploid. It remains attached to the gametophyte while it grows into a young sporophyte. As the sporophyte matures, it produces fronds, roots, and eventually develops sporangia on its fronds, each filled with spores. When the sporangia burst open, they release their spores, and the cycle begins anew.
Other Forms of Fern Reproduction
While ferns predominantly reproduce via spores, there are other methods of propagation. Some ferns reproduce vegetatively, a form of asexual reproduction. This often occurs through the growth of new plants from rhizomes spreading into new soil and spouting fronds, or from bulbils, small bulb-like structures found on the fronds of some fern species (proliferous frond tips). As the fern grows, the weight of the frond drops them toward the ground, where they “root” and later separate from the parent plant. This strategy finds use in propagating ferns, as all of the offspring are genetically identical to the parent.
Certain ferns also engage in apogamy and apospory, types of asexual reproduction that bypass typical sexual reproduction stages. In apogamy, a sporophyte develops from the gametophyte without fertilization. Ferns use apogamy when conditions are too dry for fertilization. In apospory, a gametophyte develops directly from the sporophyte tissue, without the production of spores.
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- Pryer, Kathleen M.; et al. (2001). “Horsetails and ferns are a monophyletic group and the closest living relatives to seed plants”. Nature. 409 (6820): 618–622. doi:10.1038/35054555
- Ranker, Tom A.; Haufler, Christopher H. (2008). Biology and Evolution of Ferns and Lycophytes. Cambridge University Press. ISBN 978-0-521-87411-3.