Bracken
Bracken is probably the most widely distributed, recognisable and common fern in the northern hemisphere; it is found on every continent. This fern's vast geographic range contributes to its morphological variability, which has frequently been carved up into numerous separate species. The debate over the precise definition of Pteridium aquilinum continues.
Ferns have two distinct, free-living phases in their life cycles. In the dominant phase (the sporophyte), the plant most familiar to us, spores are released and carried by the wind to suitable, damp locations. The spores germinate producing tiny, green, membrane-like structures: prothalli (the gametophyte). Prothalli produce the sex cells: motile sperm and immobile eggs. The sperm swim to the eggs, through films of water, forming zygotes that germinate to produce 'fernlings' and eventually mature ferns.
When individual bracken fronds mature they are approximately triangular, highly divided and can reach three metres in height and at least half this size in breadth. The spores are produced along the margins of the frond undersurface from specialised structures (sporangia) protected by the down-rolled margin. Individual plants produce millions of tiny, brown spores annually, sometimes forming carpets of bracken prothalli in appropriate habitats. Most of prothalli will die but a few survive to produce ferns with tough, underground, perennial stems (rhizomes) that throw up new, deciduous leaves each year. The shape of the elaborate vascular system in the broken stalk of a bracken frond, with is likeness to a double-headed eagle, is one explanation for the species' scientific name. Another derivation of the name evokes the resemblance of a bracken frond to an eagle's wing.
In Britain, unrolling, pale green, bracken leaves (fronds) are familiar features of well-drained soils in woodland, moorlands and heaths in late spring. Historically, bracken was actively encouraged as it was used as a valuable fodder and bedding plants for livestock. The alkaline-rich ash from burnt bracken fronds was used in glassmaking and as a source of detergent for washing clothes. Dried bracken fronds are sometimes used in horticulture to protect tender plants from frosts. In East Asia, North America and Oceania people have used young bracken fronds as a vegetable or starch source. However, excessive bracken consumption has been linked with some human cancers and livestock poisoning. Today, bracken our view of bracken has changed; it is often considered an invasive species as it rapidly colonises areas exploited by people, to the exclusion of other plant species.
Further reading
Marrs RH, Watt AS 2006. Biological flora of the British Isles: Pteridium aquilinum (L.) Kuhn. Journal of Ecology 94: 1272-1321.
Stephen Harris