Taxonomy

Accepted name:

Luronium natans (L.) Raf., Floating Water-plantain (Stace 1997).

Synonyms:

2n = 38.

Identification

This is one of the most difficult plants in Britain for identification. It is extremely variable in shape, with submerged leaves up to half a metre long (pers. obs.) and ribbon-shaped, or just a centimeter across and almost round (Stace 1997). The key feature for identification is the presence of stolons which, in the absence of flowers, is the only reliable diagnostic feature that separates it from Baldellia ranunculoides and Alisma plantago-aquatica.

All records have to be supported by evidence of at least a photograph and preferably a voucher specimen (but only if collected under licence).

Status

Luronium natans is protected in UK law under the Wildlife & Countryside Act, which makes it illegal to uproot or damage plants, or to collect them without a licence (which is issued by Natural England, the Countryside Council for Wales or Scottish Natural Heritage).

It is also a priority species in the European Habitats Directive, legislation, which means that all native sites for it are designated SSSI or cSAC which, under the CROW Act, the landowner is obliged maintain in favourable condition. It is one of the most highly protected plant species in Britain.

Distribution

According to Cook et al. (1975) and Preston & Croft (1997), L. natans is restricted to Europe and is declining throughout its range. Cook (ibid.), however, observed that it appeared to be expanding its range in Britain, and the New Atlas (Preston, Pearman & Dines 2002) seems to confirm that with a slight recorded increase overall, and recently confirmed records for Ireland and Scotland.

Given the problems of identification, little is certain about this species. It was first recorded in Scotland, for instance, by the normally reliable William MacGillivray (Withering’s Arrangement 1841), but that record is often discounted on the grounds that it was not found there again for a long time.

The core populations in Britain are in the lakes of Mid-Wales and the Shropshire Meres, with outlying populations in the canal network and in heathland pools as far away as Dowrog Common in Pembrokeshire and South Weirs in the New Forest. There are always suggestions that it is introduced when it crops up in new sites - for example, at Calthorpe Broad and Potter Heigham in Norfolk (Preston & Croft 1997) - but there is little direct evidence of that. It is a difficult plant to grow, and canal ecologists have tried to introduce it to new sites with little success.

Ecology

The importance of L. natans to conservation is matched by its highly specialised ecological requirements which could be summarised as ‘clean water.’ It grows in depths of up to 2m in oligotrophic lakes in Wales (Trueman, Morton & Wainwright 1995) or in shallower mesotrophic water bodies. It requires high levels of light (Hill et al. 1999) so algal growth, shade and turbidity can be fatal to it.

The sites where it occurs include oligotrophic to mesotrophic lakes and pools, often those with fluctuating water levels, as it tends to flower and fruit prolifically when exposed on bare mud.

It is mostly a lowland plant, reaching its altitudinal limit at Llyn Bugeilyn in Montgomeryshire (v.c. 47) (Pearman 2004).

It also still occurs in several canals, but it disappears when the canals are restored to navigation and boat traffic increases above a very low level. Such low usage sometimes occurs on the most uninspiring stretches of canal in urban areas.

There are several old records for it in rivers, although it is rarely found in that habitat now.

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Further work

The most important thing is to continue to collect good quality, validated records of this species. Its distribution in Scotland and Cumbria is still uncertain, and the origin and persistence of outlying populations is a matter of much interest and speculation.

With a BAP species such as this, a lot of conservation work is happening, such as introductions and translocations. These should be documented on Conservation Evidence, if not written up elsewhere.

References

Cook, C.D.K., Gut, B.J., Rix, E.M., Schneller, J. & Seitz, M. 1974. Water Plants of the World. W. Junk, The Hague.

Hill, M.O., Mountford, J.O., Roy, D.B., & Bunce, R.G.H. 1999. Ellenberg’s indicator values for British plants. HMSO.

Lockton, A.J. & Whild, S.J. 2005. Rare Plants of Shropshire, 3rd edition. Shropshire Botanical Society, Shrewsbury.

Pearman, D.A. 2004. Altitudinal Limits of British Plants Database.

Preston, C.D. & Croft, J.M. 1997. Aquatic Plants in Britain and Ireland. Harley Books, Colchester.

Trueman, I C, Morton, A & Wainwright, M. 1995. The Flora of Montgomeryshire. Montgomery Field Society & Montgomeryshire Wildlife Trust, Welshpool.

Withering, W. 1841. A systematic arrangement of British plants, 5th edition, corrected and condensed by William MacGillivray. Scott, Webster & Geary, London.