True ferns are a group of seedless vascular plants that make up the class pteropsida. They are closely allied to horsetails and whisk ferns, that on the basis of DNA analysis are also considered to be ferns. The ferns evolved in the Devonian period and nowadays comprise about 12,000 species. Ferns reproduce via spores that are formed in socalled sporangia, also known as sori (singular, sorus) in ferns. Sori are formed in distinctive patterns on the undersides of green leaves or on special, non-green, leaves, called sporophylls.
The dominant life stage of ferns is diploid (like in most plants, but not mosses) and is called the sporophyte. Upon reaching maturity the sporophyte starts producing spores that are haploid. Spores are the major unit of dispersal in ferns and a single sorus can produces millions of spores. The small size of the spores enables them to spread in excess of a thousand kilometers, although the majority falls within a few meters from the mother plant. After dispersal the spores germinate and form into tiny, often hearth shaped, leaf-like structures that are called gametophytes or prothallia. The tiny prothallium is where fertilization occurs and from which a new individual emerges. Ferns can be either homosporous or heterosporous. Homosporous ferns have only one type of spores that can produce male, female and often bisexual gametophytes. Heterosporous ferns have two types of spores: the female type is called megaspore and only produces eggs, while the male microspore only produces sperm. The fact that homosporous ferns can produce bisexual gametophytes enables an unusual form of self fertilization, namely intra-gametophytic selfing (sensu Haufler 2002). This creates the opportunity to produce new populations from a single spore. This theoretical advantage has made sure that these species are always marked as good colonizers and indeed homosporous ferns make up an unusually large proportion of the floras of the Pacific isles. On the other hand intra-gametophytic selfing can only lead to a completely homozygous offspring and this lack of genetic variablility is likely to increase the risk of extinction for these populations. After successful fertilization of an egg of a female gametophyte a new sporophyte will be formed.
- Smith, A. R., K. M. Pryer, E. Schuettpelz, P. Korall, H. Schneider & P. G. Wolf. 2006. A classification for extant ferns. Taxon 55(3):705–731 available here
- Campbell, N.A, Reece, J.B. 2002. Biology 6th edition. Pearson Eduction, Inc., publishing as Benjamin Cummings. San Francisco, USA
- Baker, H. G. 1955. Self-compatibility and establishment after 'long-distance' dispersal Evolution 9(3): 347-349
- Haufler, C. H. 2002. Homospory 2002: an odyssey of progress in Pteridophyte genetics and evolutionary biology BioScience 52(12): 1081-1093