Pilularia globulifera

Taxonomy 

Name: Pilularia globulifera L.

Family: Marsiliaceae

Common name: Pillwort.

It has no synonyms.

Chromosome No.: 2n = 26 (Stace 2010).

Pilularia globulifera

Identification 

This is a tiny fern, about 8cm tall, which is easily recognised by the characteristically unfurling leaves and the large (3 mm), round sporocarps, if present (Page 1997; Stace 1997). The only similar fern in Europe is P. minuta, which occurs in the western Mediterranean but not in Britain (Tutin et al. 1993). It is perhaps most easily confused with seedlings of Juncus, or submerged Juncus bulbosus plants; specimens should always be collected or photographs taken if a new site is found.

 

Distribution 

It is endemic to Europe, occurring in most Western European countries and extending eastward as far as Poland and Italy (Fl. Europaea). It is widespread throughout Britain and Ireland, excepting only the Northern Isles, but restricted to suitable habitats, which are limited to about ten scattered clusters.

BSBI Hectad Map 

Click on the map to view full-size on the BSBI Maps Scheme website.

Status 
  • Rarity: Nationally Scarce (Stewart et al., 1994) in Britain. Rare in Ireland. 
  • Origin: native throughout.
  • Threat: in the New Atlas (Preston, pearman & Dines 2002) it is given a Change Index of -0.03, representing a small decline; but it is one of those species that is always well recorded, and the Change Index over-compensates for recent recording levels. The map shows it has been lost from nearly half of all its known sites. Cheffings & Farrell (2005) list it as Near Threatened.
  • Conservation: Pillwort is one of the few species that is widely considered a universal axiophyte - i.e. it is always important for conservation even in areas where it is relatively common.
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Ecology 

Jermy, in Stewart, Pearman & Preston (1994) gives a detailed account of its habitat in shallow water on pond margins, in poached wet grassland and even, occasionally, in mires. Populations can vary greatly from year to year, often responding quickly to low water levels leaving exposed bare substrate. Jermy (ibid.) reports that spores can develop through the gametophyte stage to produce new sporophytes in just 17 days, which makes it an opportunistic coloniser. He questions whether the sporocarps remain dormant for long periods, or whether new habitat is colonised by fresh spores from nearby populations. In the north of Scotland it seems to occupy quite a different niche. Most of the records are for the sides of rivers and lakes where, perhaps, the scouring effect of running water creates the open conditions it requires.

Further Work 

We have no real idea of whether this species is in decline or the change index is not reliable for an historically well-recorded species, or for a plant that is mobile. A detailed study of its historical range and populations is needed. A controversial aspect of the conservation of Pillwort has been the readiness of organisations to plant it. It has been introduced or re-introduced to numerous sites in Scotland including a place on the Island of Rum where it is may not have occurred naturally. At Dowrog Common in Pembrokeshire there was a reintroduction attempt that appears to have failed, but the plant re-appeared years later when appropriate management was introduced (S.B. Evans, pers. comm.). this leaves a nagging doubt about whether it really is a native population or not, and the reintroduction attempt is now seen as unfortunate. A study of the genetics of this population might usefully resolve the question.

References 
  • Page, C.N. 1997. The Ferns of Britain and Ireland, 2nd edition. Cambridge University Press, Cambridge.
Citation 

Lockton, A.J. (date accessed). Species account: Pilularia globulifera. Botanical Society of the British Isles, www.bsbi.org.uk.

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