Introduction

Ferns are the second largest vascular plant group on earth with more than 9000 living species currently placed in four classes: (1) whisk ferns – Psilotopsida, c. 92 species, (2) horsetails – Equisetopsida, c. 15 species, (3) marattioid ferns – Marattiopsida, c. 150 species, and (4) leptosporangiate ferns – Polypodiopsida, c. 9000 species (Smith et al., 2006). Their origin dates back to the Late Devonian or early Carboniferous more than 350 million years ago (Pryer et al., 2004). They reproduce by haploid spores, which grow into a free-living gametophyte, usually a photosynthetic prothallus with microscopic male and female organs. The male sexual organs, the anteridia, release mobile sperms that swim to the female sexual organs, the archegonia (often on the same prothallus), and fertilise an egg that remains attached to the prothallus. The resulting zygotes divide by mitoses and grow into the diploid sporophytes, usually with characteristic rhizome and fronds (Lloyd, 1974). Most of the ferns are perennial hemicryptophytes (less commonly tree ferns, rarely annuals) that produce up to millions of tiny, long-lived, mostly wind-dispersed spores every year (Smith et al., 2006). The notable exceptions are some genera of Polypodiaceae (e.g. Grammitis, Jungermannia), which produce relatively few, chlorophyllous spores per frond that live only days or weeks (Schaefer, 2001a) and some water- or bird-dispersed heterosporous ferns (Marsileaceae, Salviniaceae).