''Pilularia globulifera'', or pillwort, is an unusual species of

fern The ferns (Polypodiopsida or Polypodiophyta) are a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having specialized tissue ...

in the family Marsileaceae. It is native to western Europe, where it grows at the edges of lakes, ponds, ditches and marshes, on wet clay or clay-sand soil, sometimes in water up to deep.

Description

Pillwort has slender, cylindrical, rush-like fronds up to tall that are shaped like croziers as they unfurl. It has a pea-shaped, 4-chambered sporocarp about in diameter, each chamber formed from a modified leaf and containing several sori bearing both

megasporangia A sporangium (from Late Latin, ; : sporangia) is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. Virtually all plants, fungi, and many other groups form sporangia at some point in their life ...

and

microsporangia A microsporangium () is a sporangium that produces microspores that give rise to male gametophytes when they germinate. Microsporangia occur in all vascular plants that have heterosporic life cycles, such as seed plants, spike mosses and the a ...

. The species is thus

heterosporous Heterospory is the production of spores of two different sizes and sexes by the sporophytes of land plants. The smaller of these, the microspore, is male and the larger megaspore is female. Heterospory evolved during the Devonian period from is ...

Habitat

Pillwort grows on silt and mud at the margins of lakes, ponds and other watercourses that are submerged for at least part of the year. Some of the plants growing in association with this species in the UK include water celery (''

Apium inundatum ''Apium'' is a genus, as currently circumscribed by Plants of the World Online, of 12 species of flowering plants in the family Apiaceae, with an unusual highly disjunct distribution with one species in the temperate Northern Hemisphere in the We ...

''), marsh pennywort (''

Hydrocotyle vulgaris ''Hydrocotyle'', also called floating pennywort, water pennywort, Indian pennywort, dollar weed, marsh penny, thick-leaved pennywort and white rot, is a genus of prostrate, perennial aquatic or semi- aquatic plants formerly classified in the fam ...

'') and lesser spearwort (''

Ranunculus flammula ''Ranunculus flammula'', the lesser spearwort, greater creeping spearwort or banewort, is a species of perennial herbaceous plants in the genus ''Ranunculus'' (buttercup), growing in damp places throughout the Boreal Kingdom. It flowers June/Jul ...

''). In its habitat in shallow water on pond margins or in poached wet grassland, it seems to grow well in bare locations where it faces little competition. Populations vary greatly from year to year; it sometimes "disappears" from a site only to recur there many years later, and cleaning out a ditch may stimulate it to reappear.

Distribution

This is a rare species, declining as its wetland habitats are reduced by eutrophication and drainage, but is regarded as of least concern by the

IUCN Red List The International Union for Conservation of Nature (IUCN) Red List of Threatened Species, also known as the IUCN Red List or Red Data Book, founded in 1964, is an inventory of the global conservation status and extinction risk of biological ...

. It is listed on Schedule 8 of the Wildlife (Northern Ireland) Order 1985, but it has not been seen there since 1970 and may now be extinct in the province. It is protected under the

Wildlife and Countryside Act 1981 The Wildlife and Countryside Act 1981 (c. 69) is an act of Parliament in the United Kingdom implemented to comply with European Council Directive 79/409/EEC on the conservation of wild birds. In short, the act gives protection to native species ...

in the rest of the UK, where it is now classified as vulnerable. It is listed as threatened or endangered in nearly all the countries in which it grows.

Uses

Pillwort can be grown in a "bog garden" or as a marginal aquatic in a garden pond.

References

{{Authority control Salviniales Plants described in 1753 Taxa named by Carl Linnaeus Ferns of Europe Flora of England