About Pilularia americana A.Braun
Pilularia americana A.Braun, commonly called American pillwort, is an aquatic fern in the order Salviniales. Like all other members of this order, it is heterosporous and produces hard, seed-like sporocarps. Its globose sporocarps clearly distinguish it from the closely related genus Marsilea, which has flattened sporocarps. Ferns in the genus Pilularia have lost their leaf blades, leaving only grass-like stipes, but this species still retains the circinate vernation that is characteristic of most ferns.
The American pillwort has a well-established range across most of California and south-central Oregon. Outside of this area, it is only known from scattered local occurrences in eastern North America. It is unclear if this pattern reflects a truly sporadic distribution, or if the species is simply extremely easy to overlook because it is small and grass-like. Close examination is often required to correctly identify it as a Pilularia. The species is also reported to occur across Central and South America, though further scientific study may confirm these populations are actually the separate species Pilularia mandoni A. Braun. Recent research has found that the North American plants traditionally classified as P. americana actually consist of two distinct cryptic species: populations across most of the United States are true P. americana, while populations in southernmost California and Mexico are genetically distinct and very closely related to the Australian and New Zealand species Pilularia novae-hollandiae. These southern populations may eventually be reclassified as part of P. novae-hollandiae.
This species grows easily in cultivation when provided with a suitable, uncrowded habitat. It may die back during drier or colder parts of the year, and regenerate the following growing season from its dormant sporocarps. Although it is classified as an aquatic fern, it prefers to grow either emergent (in shallow water, with its fronds extending above the water surface into air) or completely emersed (with all fronds growing out of water), and it roots best in wet mud.
