English: Pillwort.
Synonym: Calamistrum globuliferum.
Clades: Marsileaceae.
Genus: 3 to 6 species.
Region: western Europe; temperate northern and southern hemispheres, Ethiopian mountains; Australia, New Zealand, western South America.
Habitat: on silt and mud at the margins of lakes, ponds and other watercourses; in poached wet grassland; at the edges of lakes, ponds, ditches, marshes, on wet clay or clay-sand soil, sometimes in water up to 30 cm dee; in bare locations with little competition.
Ecology: growing in association with Apium inundatum, Hydrocotyle vulgaris, Ranunculus flammula; rare due to declining wetland habitats; protected, classified as vulnerable.
Use: ornamental in bog gardens, marginal aquatic in a garden pond.

BotanyFern; small; aquatic; submerged for at least part of the year; grow well in bare locations where it faces little competition.
Root: rhizomes, creeping.
Leaves: slender; leaves thread-like; cylindrical; rush-like; up to 8 cm tall; shaped like croziers as they unfurl; not subdivided; tapered to a point.
Sporocarp: pea-shaped, 4-chambered; about 3 mm in diameter; each chamber formed from a modified leaf and containing several sori bearing both megasporangia and microsporangia; heterosporous.