Hydrocotyle vulgaris L., Marsh Pennywort

Account Summary

Growth form and preferred habitats

One or more orbicular, crenate, peltate leaves, 8-35 mm in diameter, borne on sparsely hairy petioles up to 25 cm long, arising from slender, pale green stems, long-creeping in mud or floating in water and rooting at the nodes, make Marsh Pennywort a readily recognised aquatic perennial. H. vulgaris grows in usually sunny, open, moist or wetland situations and is extremely common and patch-forming in a wide range of moderately acid, infertile muddy ground, or in short-sedge sward of Sphagnum or other species of moss-lawn, and in shallow, still aquatic habitats including ditches. It is only occasionally found in half-shady situations, but it can form an understorey to taller species such as Juncus effusus (Soft-rush) and in Phragmites reed beds (Grime et al. 1988, 2007; Jonsell & Karlsson 2010).

Taxonomy

The genus Hydrocotyle has been moved about from family to family on numerous occasions. It has been included in the Apiaceae (= Umbelliferae) (eg Tutin 1980; Stace 1997; Sell & Murrell 2009), or the Araliaceae (Ivy family) (eg Jonsell & Karlsson 2010), or sometimes it is given its own family, the Hydrocotylaceae (Pennywort family). A morphological study of a few representatives of the two closely related families Apiaceae and Araliaceae indicated that neither is monophyletic (Judd et al. 1994, quoted in Jonsell & Karlsson 2010). Hydrocotyle was found to be in the same clade as Araliaceae based on molecular studies (Chandler & Plunkett 2004, quoted in Jonsell & Karlsson 2010). A morphological character uniting Hydrocotyle with Araliaceae is the hard endocarp on the seed or mericarp (Jonsell & Karlsson 2010). However, most members of the Araliaceae are woody, have either one or five styles, and the fruit is a berry (Sell & Murrell 2009). None of this fits Hydrocotyle which lies rather better in a subfamily within the Apiaceae, the Hydrocotyloideae, although it differs from the rest of the family in possessing leaf stipules and the fruit does not have oil bodies. In truth, Hydrocotyle appears anomalous in either of the two closely related families and while Sell & Murrell (2009) preferred to keep the genus within the Apiaceae, Stace (2010, 2019) decided to recognise the Hydrocotylaceae.

Fermanagh occurrence

Patches of H. vulgaris are a very familiar sight around Fermanagh since this truly amphibious perennial has been recorded in 234 tetrads, 44.3% of those in the VC. It is particularly frequent around the sheltered, grazed lowland shores of Upper Lough Erne, where the muddy-organic substrates, seasonally flooded water-meadows, marshy pastures with temporary wet depressions, swampy fen and fen-carr, and shallow, eutrophic, peat-brown lakeshore waters must provide near-ideal growing conditions for the species.

In both lowland and more upland bog sites, such as for instance on the Western Plateau, it is commonly scattered on the margins of pools and peaty ditches, as well as in more moderately nutrient-rich, flushed organic muds on grassy moorlands, blanket-bogs and heaths. Here it grows where taller, shading and more vigorous species are limited or excluded by grazing, trampling, periodic flooding, or other forms of disturbance or exposure, and also by the restricted availability of plant nutrients (Grime et al. 1988, 2007).

Despite its leaning towards wet to moist, moderate to mildly acid, often infertile conditions, H. vulgaris does occur in some more calcareous situations too, eg around the shores of Lower Lough Erne, in pools on the distinctly marly River Finn and around Rooskey turlough (ie a 'vanishing lake' in limestone terrain, where drainage is vertical into a subterranean cave system). In Scandinavia, Jonsell & Karlsson (2010) consider H. vulgaris is indifferent to lime.

In the frequently wet, mild, oceanic climate of Fermanagh, where prolonged drought is very rare, Marsh Pennywort can also occur in a purely terrestrial mode on much drier, but still moist or constantly damp peaty banks, for instance in open areas in wet deciduous woods, eg along paths in the Correl Glen NR and the Cladagh River Glen NR (also known as the Marble Arch NR).

Vegetative reproduction

The slender creeping horizontal shoots of H. vulgaris root at the nodes and form more or less extensive clonal patches, allowing the plant to quickly colonise adjacent ground bared by any form of disturbance (Grime et al. 1988, 2007). Fragmentation of the stem as a result of grazing, trampling or flooding, also enables detached material to disperse by water and re-establish the plant vegetatively in fresh sites.

Flowering reproduction

Although the casual observer will always be unaware of it, the plant may produce its minute, extremely reduced, insignificant, well-hidden flowers from June to August in very small inflorescences, on peduncle stalks only about half the length of the leaf stalks (ie 3-18 cm high). The little axillary umbel clusters are borne on these short leafless stalks springing from nodes on the horizontal stems. There are three to six almost sessile flowers in each very imperfect umbel, which is sometimes elongated to form a secondary cluster. The 3 mm diameter flowers have very small or no calices, and pink or greenish-white, entire petals. The tiny schizocarp fruit measures 1.5 × 2.5 × 0.7 mm and has two much flattened, keeled, nearly circular carpels and four lateral ridges on the twin mericarps (Melderis & Bangerter 1955; Butcher 1961; Sell & Murrell 2009). In these very inconspicuous flowers, self-pollination is possible and probably obligatory. Although the anthers mature in succession, the stigmas mature before the last of the five anthers bursts to release its pollen, allowing selfing to occur.

Irrespecitive of how pollination and fertilisation is achieved, seed regeneration in H. vulgaris may be less important than vegetative reproduction in many situations, the plant often being flowerless due to excessive competition (Grime et al. 1988, 2007). Seed is still significant in the longer term, however, both to maintain the species genetic variation and for long distance dispersal between already colonised and fresh or newly created habitats. No persistent soil seed bank has been detected.

Mericarp fruits float and, in addition, adhesion in mud on the feet or feathers of ducks or other waterfowl has been proposed as the external transport mechanism for short distances; seed recovered from the gut of wide-ranging, migrating ducks such as Gadwall, suggests that they may be the vector to much more isolated stations (Ridley 1930; Grime et al. 1988, 2007).

British and Irish occurrence

The subfamily to which H. vulgaris belongs, the Hydrocotyloideae (previously treated by taxonomists as a separate family, the Hydrocotylaceae) is mainly distributed in the S hemisphere and H. vulgaris is the only native member in B & I. H. vulgaris is very common and widespread in most of Ireland, but becomes significantly less frequent in the drier and more fertile SE part of the country. It is also very widespread at all latitudes in Britain, but again there is a marked species predominance in the wetter and more acidic environs of the west and it is absent or rare in large areas of the English Midlands and N Scotland (Tutin 1980). Drainage and development since about the 1960s has wiped out many former sites of the species, especially in SE England (M. Southam, in: Preston et al. 2002).

European and world occurrence

Beyond the shores of B & I, H. vulgaris is mainly confined to W, C & S Europe, the greatest frequency being in W & C regions. Rather unusually for a plant quite widely distributed in Europe, the species does not extend appreciably into Asia (Sculthorpe 1967). It does occur, however, in W Iceland, S Scandinavia to around 60°N, in the remote Azores and previously in N Africa, although it has not been seen there recently (Tutin 1980; Hultén & Fries 1986, Map 1382; Sell & Murrell 2009). Previous reports of the species in New Zealand are rejected and discounted by Webb et al. (1988).

Names and folklore

The English common name 'Marsh Pennywort' is the most frequently one applied to H. vulgaris (an obvious reflection of its orbicular leaf shape), but there are at least nine alternative common names applied in different regions around B & I (Grigson 1955, 1987). Other understandable common names include 'Fairy Tables', and several that refer to a once widespread erroneous belief that sheep grazing the plant were liable to contract flukeworms causing liver rot, hence names such as 'Farthing Rot', 'Flowkwort', 'Sheep Rot', 'Shilling Rot' (another coin reference), 'Water Rot' and 'White Rot' (Grieve 1931; Grigson 1955, 1987). Another plant that has been similarly accused is Ranunculus flammula (Lesser Spearwort), which does contain acrid toxic protoanemonin, although it really does not affect stock to any observable extent, as they avoid consuming it (Cooper & Johnson 1998).

There is a suggestion that H. vulgaris might be the plant used by herbalists in Co Limerick for dressing burns, although, again, that might be another mis-identification for the more likely Potamogeton natans (Bog Pondweed) (Allen & Hatfield 2004)

Threats

Drainage and development, including the intensification of agriculture. Not greatly threatened at present due to the upland nature of the majority of its sites, but lowland habitats are particularly affected by continuing changes in land management.

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