Psilotum nudum (L.) P.Beauv. (Psilotaceae)


Whisk fern

Psilotum nudum growing in Oxford Botanic Garden.  Sporangia and enations of Psilotum nudum (Oxford Botanic Garden).

Psilotum nudum, one of two species in the genus Psilotum, is widely distributed across tropical and sub-tropical areas of the Americas, Africa, Asia and Australasia, with a small, endangered population found in southern Spain. Psilotum was long considered a 'fern ally', a surviving remnant of an extinct Devonian flora (because of its apparent similarities to the fossil plant Rhynia). However, recent molecular evidence places it within the true ferns and suggests a close relationship to the Ophioglossaceae, a family of ferns that includes the curious ferns called moonworts (Botrychium).

Psilotum is the only living vascular plant to lack both leaves and roots, hence the generic name that derives from the Greek word for bare or smooth. The dominate phase of the whisk fern life cycle, the sporophyte, has distinctive dichotomously branching stems which are solely responsible for photosynthesis and gas exchange. In place of leaves, the stems bear minute scales (enations) that lack vasculature. Prominent reproductive bodies, comprising three fused sporangia, are borne in the axils of enations on the upper parts of stems. Moisture is absorbed through hair-like rhizoids into a basal rhizome. The other stage of the whisk fern life cycle, the gametophyte, is completely subterranean and resembles a piece of rhizome. However, the gametophyte is unable to photosynthesize so is nourished by endomycorrhizal fungi; this unusual arrangement is also found in the Ophioglossaceae.

Whisk ferns are evergreen plants that grow in a variety of habitats, such as lithophytes in cracks in rock faces or epiphytically where humus accumulates in the forks of tree branches or wide leaf bases of palms. Whisk ferns generally have an erect habit, reaching a height of 50 cm. The other species in the genus, Psilotum complanatum, readily hybridises with Psilotum nudum to form the hybrid Psilotum xintermedium.

Psilotum nudum became a popular pot plant in Japan during the Edo period leading to the selection of many different cultivated varieties with evocative names such as 'Flying Cloud Pavilion'. Dozens of these varieties are illustrated in the Matsubaran fu, a list of cultivars published in 1836. Psilotum is difficult to grow from spores and is usually propagated by divisions. Despite that, it often appears spontaneously in glasshouse collections.

The plants have a variety of historical uses in Hawaii where spores were collected and used in the same way as talcum powder, while the stems, known as 'chickens' feet' were used in a children's game.

Further reading

Christenhusz et al. (2011) A linear sequence of extant families and genera of lycophytes and ferns. Phytotaxa 19: 7-54.

Hoshizaki BJ & Moran RC (2001) Fern grower's manual. Timber Press.

James Penny