Fern life cycle and diversity

Blechnum attenuatum and other ferns growing on Montagu Pass in the Western Cape. This is the sporophyte phase of the fern life cycle. Photo: Diane Turner, iSpot.

The factsheet in the June 2016 issue of Veld & Flora follows on from the factsheet in the March 2016 issue of Veld & Flora, vol. 102(1), and the Veld & Flora Factsheets on the Classification of Life in vol. 98(3) September 2012 and Plant Classification in vol. 101(4) December 2015.


Fern leaves with sori. Photo: Clare Archer, iSpot.

 A DIVERSITY OF FERNS

Ferns or Pteridophytes are a group of primitive plants. It includes the familiar fern with its graceful fronds, but also encompasses a great diversity of fern-like plants that grow in a variety of habitats from shady, damp forests to deserts. Like Mosses and Liverworts (Bryophytes) they have no seeds, flowers or fruit, and reproduce by means of spores. They mostly grow on dry land but they still need water in order to reproduce. Unlike Bryophytes they have a vascular system (specialized tissue for transporting water and nutrients – xylem and phloem).

Ferns are usually referred to as Pteridophytes, which includes all spore-bearing, vascular plants. In older classifications these plants were informally divided into ‘true ferns’and ‘fern allies’. The ‘fern allies’ comprised the Clubmosses, Spikemosses, Quillworts, Whisk Ferns and Horsetails. New research in molecular biology has necessitated a few changes in fern classification. To reflect evolutionary relationships more accurately, scientists now propose to divide spore-bearing, vascular plants into two formal groups: Lycophytes and Monilophytes.

Lycophytes include the Clubmosses, of which there are three genera in southern Africa: Huperzia, Lycopodiella and Lycopodium, Spikemosses which contain a single genus Selaginella with ten species in southern Africa and the Quillworts, which also contain a single genus, Isoetes, with at least 14 species in southern Africa. This ancient group of plants was once prolific with extinct giant species of Clubmosses growing up to 40 m high.

Monilophytes include the Snaketongue Ferns of which one genus, Ophioglossum, is indigenous to southern Africa, Whisk Ferns of which one species, Psilotum nudum, is indigenous to southern Africa, the Horsetails of which one species, the African Horsetail (Equisetum ramosissimum), is indigenous to southern Africa, the Potato Ferns of which only one species, Ptisana fraxinea, is indigenous to southern Africa, and the Leptosporangiate Ferns, which is the largest and most diverse group that contains around 270 species indigenous to southern Africa.

The African Horsetail (Equisetum ramosissimum subsp. ramosissimum var. altissimum) is the only species of horsetail in South Africa. The spores are borne under sporangiophores in the cone-like structures at the tips of some of the stems. Photo: Clare Archer, iSpot.

Almost all land plants reproduce by means of two distinct, alternating life forms: a sexual phase that produces and releases gametes or sex cells and allows fertilisation, and a dispersal phase. This is an adaption which frees plants from a life under water and allows them to live and reproduce on dry land. The sexual phase is known as the GAMETOPHYTE or haploid (n) generation and the dispersal phase is the SPOROPHYTE or diploid (2n) generation.

In ferns, the dispersal phase culminates in the production of haploid (n) spores (i.e. each spore has a single set of chromosomes) which are released from capsules (sporangia) borne on the underside of fern leaves. These spores germinate and grow into tiny, free-living plants –the prothallus. This is the sexual (or gametophyte) generation of the fern plant. Haploid (n) sexual cells or gametes are formed in the sex organs (the antheridia and archegonia) on the underside of the prothallus. The male gametes (sperm) then swim towards the female gametes (eggs) in the archegonia of the prothallus. Thus, even though ferns mostly grow on land, they still need the presence of water to facilitate fertilisation. Once fusion of the egg and sperm – or fertilisation – takes place, a diploid (2n) zygote forms with two sets of chromosomes. The zygote grows by cell division, in a process called mitosis, into the dispersal (or sporophyte) generation of the fern plant which is the familiar fern that we see growing all about.

The diploid(2n) sporophyte fern plants produce sporangia or capsules on the undersides of their leaves in which haploid (n) spores are produced. The black dots we normally see on the leaf are sori (singular sorus) which are clusters of sporangia. The spores inside the capsules have undergone a process of cell division called meiosis that results in four daughter cells each with half the number of chromosomes of the parent cell. The spores are dispersed and germinate and grow into haploid gametophyte plants – and so the cycle continues.

So although the gametophyte (n) and sporophyte (2n) are two different plants, in ferns the sporophyte is what we see and know as a fern. This is different to the life cycle of mosses, conifers and flowering plants, which are covered in previous and subsequent factsheets.

Some ferns dispense with the sexual phase of the cycle and reproduce vegetatively. See ‘The ferns of the Ntendeka Wilderness’ in Veld & Flora 67(4), 118-120, December 1981.

READ MORE
In the 1981 issue of Veld & Flora, read about ‘The ferns of the Ntendeka Wilderness’ Veld & Flora 67(4), 118–120. In the 1998 issue, read about ‘The ferns of Mariepskop’ Veld & Flora 84(4) 116-117. In the 1994 issue, read ‘The weedy ferns of Ferncliffe: Unusual invaders threaten Natal’s flora’ Veld & Flora 80(3), 88-90.
'To be or not to be a fern ally' by Ronell and Arrie Klopper, Pteridoforum 80: January 2007.

Discover more about South African ferns by visiting iSpot –  and searching for ‘ferns’ or Wikipedia.

LINKS TO THE CURRICULUM

GRADE 11 Life Sciences, Strand 1: Diversity, Change and Continuity. Topic: Biodiversity of Plants. Content: Grouping of bryophytes, pteridophytes, gymnosperms and angiosperms.

GRADE 12 Life Sciences Strand 1: Life at Molecular, cellular and tissue level. Topic: Meiosis: the process of reduction division purposes of reduction division (gametogenesis and exceptions: mosses, ferns), Importance of meiosis: diploid to haploid: production of gametes.

TEXT by Caroline Voget with assistance from Dr Ronell R Klopper, South African National Plant Checklist Co-ordinator, SANBI. The following books and websites were consulted: The Story of Life and the Environment: An African perspective by Jo van As, Johann du Preez, Leslie Brown and Nico Smit, published by Struik Nature and also from online sites including: Iziko Museum’s website, the Online Textbook, Prentice Hall, Wikipedia and The Encyclopaedia of Life.