Buckler ferns, with their shuttlecock-like arrangement of fronds, are one of the commonest and most complex genera of British ferns. Once, the buckler ferns were thought to comprise a single species, the male fern or 'Filix mas'. Today, about 20 species and interspecific hybrids are recognised in Britain. Outside of Britain, Dryopteris is found on most continents, and comprises more than 150 species.
Ferns disperse themselves over long distances using spores. A spore is a single cell with a cell wall rich in sporopollenin, one of the most chemically-inert biological substances on the planet. Each spore is produced by cell division (meiosis) in special structures called sporangia, and has one complete set of chromosomes in its nucleus. The sporangia are crowded in pustules (sori), on the lower surface of the Dryopteris frond, and protected by a kidney-shaped flap of tissue (indusium).
A sporangium looks like a bladder on a stick; the explosive release of its contents into an air stream is crucial for successful fern dispersal. Biologists have known for decades about the basic release process; sporangia split open when dehydrated, whilst energy stored in cells walls ejects the spores. However, recent biomechanical investigations show the mechanism is analogous to that used by medieval catapults, especially the mangonel. The crucial structure is the annulus, a strip of about a dozen cells with unevenly-thickened walls, which divides one side of the sporangium.
Siege engine designers discovered four stages were essential if projectiles were to be hurled long distances from a catapult. The catapult needed to be charged with energy, which had to be stored until a trigger mechanism released the missile cradle. Furthermore, a mechanism was needed to arrest the movement of the missile cradle, so the missile was released. The annulus fulfils all these functions.
The annulus is charged as water is lost from the cells by evaporation; increasing water tension forces the thickened walls together. Energy is stored as the annulus begins to curve and the opposite side of the sporangium splits apart, revealing the spores. Water tension increases inside the annulus cells until cavitation (air bubble formation) suddenly occurs. The catapult is triggered and the sporangium partially returns to its former shape. Annulus wall properties mean the shape change occurs in two phases of different periods, approximately 10 milliseconds and several hundred milliseconds, respectively. The phases are the necessary arrest to ensure spores are flung from the sporangium.
Ingold, CT 1965. Spore liberation. Clarendon.
Noblin X et al. 2012. The fern sporangium: a unique catapult. Science 335: 1322. See also
Rünk, K et al. 2012. Biological Flora of the British Isles: Dryopteris carthusiana, D. dilatata and D. expansa. Journal of Ecology 100: 1039-1063.