Hippuris vulgaris L., Mare's-tail

Account Summary

Growth form and preferred habitats

The genus Hippuris is often (but not always) considered to contain just a single species that is widespread over the whole world. Other botanists recognise two or more closely related species and several varieties have also been described (Preston & Croft 1997). At the same time, H. vulgaris is regarded as one of the most highly evolved species in the flora of B & I, its extremely minute flowers displaying a remarkable degree of biological reduction and its adaptations to the aquatic environment being so extremely successful, they have been repeated in several totally unrelated flowering plant families, plus in ancient cryptogamic genera such as Equisetum and Chara. H. vulgaris is such a successful species in reproductive and ecological terms, it has achieved circumpolar distribution in the northern hemisphere, plus a limited presence in the southern. H. vulgaris is a rhizomatous, glabrous, heteromorphic perennial, having two growth forms, submerged aquatic and aerial emergent. The latter is unbranched, jointed and has many crowded whorls of leaves giving it a very similar appearance to a developing Horsetail (Equisetum) and thus, by comparison, it has been given the English common name, 'Mare's-tail'.

The emergent plant is most often observed and is very easily recognised by its erect, fleshy but rigid, hollow stems, rising 15-30 cm above the water surface, bearing the minute flowers and leaves that are 10-20 mm long, in symmetrical whorls of six to twelve. The submerged or floating form of the plant produces flaccid, translucent stems up to a metre long, bearing very much longer whorled leaves, 30-40 mm, than does the emergent form (Sell & Murrell 2009). The submerged leaves have wavy margins and a soft tip, while the aerial emergent ones are entire-margined and are hard tipped. The hollow stems have numerous well-developed air spaces that transport oxygen down to the submerged parts of the plant (Melderis & Bangerter 1955).

In B & I, H. vulgaris is occasional to locally frequent or abundant in shallow, sheltered, still or slow-moving mesotrophic to eutrophic, nutrient-rich, waters of lowland lakes, ponds, slow-flowing streams and ditches. It is most frequent in lime-rich waters and is usually rooted in fine mineral or muddy organic substrates (Garrard & Streeter 1983; Preston & Croft 1997).

Fermanagh occurrence

H. vulgaris has been recorded in 54 Fermanagh tetrads (10.2% of the total), 48 of them containing post-1975 finds. As the tetrad map indicates, the species is mainly confined to southern parts of Lower Lough Erne, plus Upper Lough Erne and the River Finn catchment. Elsewhere in the VC, it is much more scattered in isolated, shallow water, lowland situations.

Mare's-tail is abundant in some lowland lakes, particularly in and around marl or limestone shores, including the Green Lough turloughs (ie vanishing lakes in limestone country linked to subterranean caves), and along the marly soils of the River Finn. It is also occasional to rare on more acidic terrain, as in flushes on Sphagnum bog, and in recently created temporary pools in disused quarries and sand pits.

Flowering reproduction

The flowers are borne singly in the leaf axils of the emergent portion of stem in June and July. As Hutchinson (1948) commented, "… the calyx of the bisexual flowers is very minute around the top of the inferior ovary, there are no petals, there is only one stamen, one subulate style (ie with a fine, abrupt, sharp point), one pendulous ovule, and of course only one seed in the small, nut-like fruit. Reduction could hardly go any farther in a bisexual flower!" The extremely reduced, wind-pollinated flowers can be bisexual or unisexual on the same plant. The tiny flowers go through two distinct stages: firstly the female stage in which the tapering, pointed style is prominent and the red anther is sessile and still unopened, and secondly the male phase, with the style withered and the anther now elevated on a slender filament, open, exposing the pollen to the wind. The flowers set seed efficiently and disperse them very effectively (Proctor et al., 1996; Preston & Croft 1997).

After fertilisation, the inferior ovary and its pendulous ovule develop into a very small, ellipsoid nut containing a single seed. In terms of seed dispersal, Ridley (1930) quotes Praeger (1913) who found the seeds were capable of floating for over 15 months and remained viable throughout. Ridley (1930) also lists various ducks which eat and transport the (presumably undigested) nuts internally, thus enabling them to reach and colonise isolated so-called temporary 'dew ponds' in quarries and in sand pits remote from connected water systems. H. vulgaris is also tolerant of saline conditions in damp to wet coastal habitats. The established strategy of H. vulgaris is categorised by Grime et al. (1988, 2007) as CR, meaning Competitive Ruderal, which the current author (RSF) believes, describes the ecological behaviour of this often extensive colony-forming aquatic species very well.

Fossil record

While fossil pollen of H. vulgaris is very rare, its mericarp fruits are amongst the most characteristic and, in older deposits, the commonest of Pleistocene plant remains recorded in B & I (Godwin 1975). The fossil record shows H. vulgaris was present in these islands in all of the last five interglacial periods and it has definitely been present throughout all zones of the current Flandrian warm period (in Ireland, called the Littletonian). Its fruits have also been recognised from all the last four glacial stages and, in the latest, the Weichselian, the records are extremely abundant, occurring in association with a rich periglacial (ie near-to-glacier) flora and fauna. From this evidence, it seems highly likely that H. vulgaris has persisted in Britain throughout the whole Pleistocene era (Godwin 1975).

Irish occurrence

While H. vulgaris has been recorded at least once in every Irish VC (Cen Cat Fl Ir 2), the impression this creates is quite misleading. The New Atlas hectad map shows that the species is very much more consistently found in lowland VCs with permeable aquifers associated with fissured rocks. Generally speaking, these conditions apply across the Central Plain of Ireland and in much of Ulster (Haughton et al. 1979) and they literally underlie the main rivers and lakes from Coleraine on the NE coast to Limerick in the SW, where the great, long River Shannon enters the Atlantic.

Elsewhere in Ireland, Mare's-tail is only very thinly scattered across the province of Munster and the southern half of Leinster, where the rocks are more impervious and the rivers are much less impressive.

British occurrence

H. vulgaris is widely distributed and sometimes abundant in lowland parts of England and Wales, especially in limestone regions. It is represented across all latitudes from the Channel Isles to the northern tip of Shetland (VC 112). It is, however, less frequent in W Scotland, Wales and the SW of England. It ascends to around 900 m in Scotland, but is rare above 400 m (C.D. Preston, in: Preston et al. 2002). It is reported to be declining in parts of Essex and Sussex (VCs 13, 14 and 18, 19), but increasing in other areas such as Gloucestershire (VCs 33, 34). As it is frequently used to decorate and oxygenate garden lakes and ponds, and is readily available in the horticultural trade, deliberately planted material may escape into the wild, or perhaps H. vulgaris is sometimes discarded when it becomes too vigorous and abundant for the particular garden setting (Preston & Croft 1997).

European and world occurrence

H. vulgaris is described as being ± ubiquitous in Scandinavia and Hultén & Fries 1986 (Map 1376) show it as very frequent and circumpolar in the northern hemisphere in arctic, boreal and temperate zones, although it becomes scarce to rare and widely scattered towards the Mediterranean basin. It is also present as a native species in Chile from 50° S, and southwards to Tierra del Fuego. The Chilean plant appears identical with that of the northern hemisphere (Hultén 1974). While H. vulgaris is present throughout the subarctic and lower arctic regions, and reaches a northernmost locality at 76° 49' N in E Greenland, in the Canadian E Arctic, however, it is rare north of the Arctic Circle (Godwin 1975).

Names

The genus name 'Hippuris' is from the Greek 'hippos', 'horse' and 'oura', 'a tail', the stems with their crowded hair-like leaves supposed to resemble a horse's tail and hence 'Horse-tail'. This applied equally to Equisetum species and botanists call them by that name. Hippuris vulgaris being a flowering plant, rather than a cryptogam, botanists distinguish it as it were a female form of 'Horse-tail', ie 'Mare's-tail', following the ancient classical example, where Equisetum species were called by the medieval Latin name 'cauda equina' (also translating as 'Horse-tail') and H. vulgaris in relation to this was 'cauda equina femina' (Turner 1538; Gerard 1597; Grigson 1974). The Latin specific epithet 'vulgaris' simply translates as 'common' (Johnson & Smith 1946).

The name 'Horse-tail' or 'Horsetail' has never been in common English folk useage for Equisetum spp., or for Hippuris, according to Grigson (1955, 1987). Thus 'Horse-tail' and 'Mare's-tail' are two perfect examples of English 'book names', invented by a botanical writer and subsequently taken up, repeated and promoted by others, but not used elsewhere. Traditional local English folk names listed for H. vulgaris include 'Cat's-tail' (the commonest name), 'Fox-tail', 'Bottle-brush', 'Joint-grass', 'Paddock's Pipes' (ie 'Toad's Pipes') and 'Witch's Milk'. Similar names listed for Equisetum species include 'Fairies' Spindles', 'Trowie Spindles' ('trow' is a reference to a goblin or devil), 'Snake-pipes', 'Toad's Pipes' and 'Paddock's Pipes'. Both Equisetum and Hippuris have far and wide longstanding folk associations with fairies, goblins, toads, snakes, witches and the Devil (Old Man's Beard) (Grigson 1955, 1987).

Threats

None.

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