Isoetes lacustris L., Quillwort

Account Summary

Growth form and preferred habitats

In Fermanagh, this small submerged perennial can sometimes be clearly seen growing in sheltered shallow water on the gravelly bottoms of acidic, nutrient-starved, unproductive lakes, as at Lough Nabrickboy in Big Dog Forest. More often it occurs in deeper, medium-sized, brown-water lakes on the Western Plateau, or in more mesotrophic, ie moderately productive, lowland water bodies. In both these situations, it can form a lawn-like turf and become the dominant bottom-growing plant. The presence of the species is often only betrayed when its stiff, evergreen 'quills' are washed up in the plant debris along the shoreline after stormy weather, sometimes in considerable quantity.

Like the vegetatively similar Lobelia dortmanna (Water Lobelia), I. lacustris is not a competitive species and in nutrient-rich waters it is easily overgrown by algae or by faster-growing aquatic macrophytes or both. Probably for this reason, Quillwort tends to occupy rather deeper water, typically from 0.5 to 2 m, but it can survive down to 6 m deep in order to avoid competition from more light-demanding vascular plants (Page 1997; Jonsell et al. 2000). I. lacustris appears to avoid small lakes and silt- or peat-bottomed mountain tarns, habitats more ecologically suited to I. echinospora (Spring Quillwort).

The four air chambers which traverse the length of each dark green tubular leaf are easily seen when it is sectioned at right angles, and they serve to distinguish the plant from either Littorella uniflora (Shoreweed) or Lobelia dortmanna, two other species with similar stiff 'isoetid' style of leaves, with which it frequently occurs in stony or sandy lake shallows. When Lobelia dortmanna flowers, it becomes a much more conspicuous plant than either of the other two species mentioned, and since it is so similar to I. lacustris in its ecological requirements and tolerances, it is a very good indicator of the likely presence of Quillwort at a site.

Another way in which I. lacustris is very readily distinguished from Littorella uniflora is by its brown (not white) roots (Jermy & Camus 1991). The four elongate air chambers in Isoetes leaves are supported and divided by numerous crosswalls (ie septae). The function of the air chambers within the photosynthetic green leaves is associated with the unusual method of carbon metabolism of these aquatic species, which is called 'Crassulacean Acid Metabolism (CAM)'. This physiology is most frequently found in desert succulent plants such as Yucca brevifolia (Joshua tree) (Crawford 1989, pp. 140-2). The extensive roots of I. lacustris absorb carbon dioxide gas from the mud sediment and store it along with carbon dioxide from night-time respiration as malic acid. The plant can thus recycle and store inorganic carbon, a scarce essential element in this habitat (and see also the species account of Lobelia dortmanna) (Farmer & Spence 1985; Boston 1986; Preston & Croft 1997).

Fermanagh occurrence

Considering the large number of lakes in Fermanagh, many of them upland, it is not surprising that this VC is the N Ireland headquarters of this aquatic species in terms of frequency, having been recorded 35 of tetrads, 6.6% of those in the VC. The two lakes in Fermanagh where it has not been recorded since the 1940s are Lough Scolban in the west of the VC and Lough Skale further east.

Irish occurrence

Compared with Co Fermanagh, the low frequency of Quillwort in Cos Down and Antrim (H38 and H39), and its very slight representation in both Cos Armagh and Londonderry (H37 and H40) is rather unexpected. In Ireland overall, I. lacustris has been recorded at least once in the past in 19 of the 40 VCs (Scannell & Synnott 1987).

The New Atlas hectad map shows that Quillwort is predominantly distributed in the N and W of both Britain and Ireland, although in the latter it is also present in eastern sites, for instance in both the Mourne mountains, Co Down (H38) and the Wicklow mountains south of Dublin (H20). There are also a few isolated sites close to the SE coast of Ireland in Co Waterford and South Tipperary (H6 and H7).

British occurrence

The distribution in Britain is very decidedly Scottish, Cumbrian and Cambrian (ie Welsh), but it does have a very few southern outliers in S Devon (VC 3). The solitary early 19th century E England occurrence at Prestwick Carr in S Northumberland (VC 67) is long extinct (Swan 1993; Preston & Croft 1997).

European and world occurrence

Quillwort has essentially a sub-Atlantic distribution in NW Europe, centred on Scandinavia and the British Isles. However, there are isolated, widely scattered outliers as far afield as Iceland, the Pyrenees and the Urals (Jalas & Suominen 1972, Map 18). Beyond Europe, I. lacustris is also found in S Greenland and in NE North America (Hultén 1958, Map 247; Hultén & Fries 1986, Map 9; Jonsell et al. 2000).

Variation

Hultén (1958) and Hultén & Fries (1986) both map the North American form of this plant as 'I. lacustris var. macrospora' Dur., and it has also at times been elevated to species rank and gone through several synonyms which these authors list. However, many botanists reckon this variety or species is best submerged back into I. lacustris (Preston & Croft 1997). A number of other varieties with one or more unusual characters have been described, including an Irish form, var. morei Syme, from a lake near Bray, Co Wicklow, which has very long leaves (Brunker 1950). An interesting viviparous form, with vegetative buds instead of sporangia, has been recorded from Lake Windermere (Page 1997). These variants, however, are now simply regarded as the evolving products of long geographic isolation and the consequent inbreeding of an ancient species.

Herbivory and dispersal

Upland game birds such as grouse are reported to feed on Isoetes species in North America (Fassett 1957) and their relatives may also do so in Britain and Ireland, although the current author (RSF) cannot locate any mention of this herbivory in the literature. It is considered feasible that the distribution of the species may reflect the North Atlantic migration pattern of wildfowl, most probably that of geese, which might carrying the spores, or much less likely, transport vegetative parts of the plant (Page 1997). The possibility of avian transfer is supported by the existence of the endemic form I. azorica, since it is hardly possible to imagine any mechanism of transport apart from water birds visiting the remote island group of the Azores on their regular migration route (Ridley 1930).

Names

The genus name 'Isoetes' appears to have been coined by the ancient Roman scientist Pliny, combining two Greek words 'isos', meaning 'equal' and 'etos', meaning 'a year', ie 'equalling one year'. This refers to the idea that the plant did not change with the seasons, meaning that it was evergreen. However, it is also thought that Pliny originally applied the name not to this species, but rather to a member of the Crassulaceae (Gilbert-Carter 1964; Johnson & Smith 1946). The specific epithet 'lacustris' is Latin meaning 'associated with lakes' (Gilbert-Carter 1964). The English common name, 'Quill-wort' or 'Quillwort' is an 18th century name, given to this inconspicuous and not well recognised plant from its supposed resemblance to a bunch of quills (Prior 1879; Grigson 1974).

An alternative name, 'Merlin's Grass', is a translation of a Welsh name 'Gwair Merllyns', where 'gwair' means 'hay' and Merllyn was the name of a Welsh prophet. The Welsh name of the plant appears in a manuscript account of Samuel Brewer's botanical journey through Wales in 1726, which is preserved in the British Museum, and it is quoted as follows in Britten & Holland (1886), "At Llyn Ogwen (Carnarvonshire) I saw the horses very greedily eating of that which was cast upon the shore and that on the water; and the people tel [original spelling] me that they wait there every day for it, and leave good grass growing near it; and that it improves cattle better than any grass; and that the fish like it as well. The fish are larger there than any of the other lakes, which they attribute to the eating of [this plant], which they call Gwair Merllyns.'

Threats

Eutrophication (ie cultural nutrient enrichment) of lowland sites and silting of upland ones, the latter at least generally attributed to forestry operations.

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