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Pinguicula vulgaris L., Common Butterwort

Account Summary

Native, frequent and widespread. Circumpolar boreal-montane, but distribution rather disjunct in Asia.

1881; Stewart, S.A.; Lough Navar area.

Mainly April to October.

Growth form and preferred habitats

This rosette-forming, insectivorous, short-lived perennial overwinters as a mobile, rootless bud, which together with bulbils enables vegetative spread in addition to seed production. The plant in summer bears 4-7(-11) yellow-green, fleshy, rosette leaves 2.5-5.0 × 1.0-2.0 cm, which lie flat to the ground, shallowly anchored by a tuft of fine, fibrous roots. The upper leaf surfaces are covered with stalked glands holding secretion droplets of fluid that give the leaves their characteristic greasy look and sticky feel.

The essential growth requirements of P. vulgaris appear to be associated with constant high humidity during the growing season, usually requiring frequent precipitation and continuously seeping water through the soil under the plant, and bare ground or relatively open, less-than-ideal growing conditions, offering a low level of competition from neighbours. Suitable conditions occur in sloping sites throughout Fermanagh on heavily leached or naturally nutrient-poor, flushed peaty or gravelly soils. While Common Butterwort is locally frequent and widespread and occurs on both peaty acid and calcareous soils, it is much more prevalent on bare, wet acid peat on bogs, and in flushes in heaths and on upland moors, or on wet lakeshores where there is some degree of soil water movement. Less commonly, it grows on permanently wet rock ledges and in crevices on damp, dripping cliffs and occasionally also in limestone flushes. The constant availability of seepage water appears much more significant than the type of substratum on which it grows and the pH range varies very greatly, dependent upon habitat, but can be as low as 3.1 and as high as 8.0 (Heslop-Harrison 2004). Elsewhere in Europe, P. vulgaris is regarded as indifferent to soil type (Blanca et al. 1999).

P. vulgaris is completely absent from taller vegetation and most observed populations consist of between 1-50 plants growing with associates which include another carnivorous species, ie Drosera rotundifolia (Round-leaved Sundew), the semi-parasitic Pedicularis sylvatica (Lousewort), plus a range of sedges common and less so, including Carex dioica (Dioecious Sedge), C. echinata (Star Sedge), C. flacca (Glaucous Sedge), C. lepidocarpa (Long-stalked Yellow-sedge), C. panicea (Carnation Sedge), C. pulicaris (Flea Sedge), Carex viridula subsp. oedocarpa (= C. demissa) (Common Yellow-sedge), Eriophorum angustifolium (Common Cottongrass), E. latifolium (Broad-leaved Cottongrass) and Schoenus nigricans (Black Bog-rush).

The rosette leaves are delicate and brittle and cannot survive any grazing or trampling by large herbivores including sheep, goats and deer during the growing season. However, the ground on which it grows is usually unenclosed and the trampling effect of grazing stock may help keep the vegetation rich and open and at the same time limit the dominance of taller, more vigorous species to the benefit of P. vulgaris. Heavily trampled, poached ground may be beneficial in the longer term, providing bare soil niches suitable for seed germination and possible establishment (Heslop-Harrison 2004).

The established strategy of P. vulgaris is categorised as 'SR' meaning it is considered a Stress-tolerant Ruderal (Grime et al. 1988, 2007).

Benefit of carnivory

The yellow-green, rather greasy to the touch, fleshy rosette leaves are efficient fly-traps, equipped with stalked, sticky, glandular hairs which regularly ensnare considerable numbers of very small, midge-sized flies, winged aphids, springtails and other tiny creeping insects. It is not obvious exactly how the leaves attract their prey, although they do seem to give off a slightly fungoid odour. There is no nectar or other food reward, apparently, but once the victims are trapped they are soon overwhelmed and suffocated by the release of further mucilage from the plant. The presence of new material on the leaf surface stimulates digestive glands to secrete five or more exo-enzymes and acids which breakdown the soft tissues of the trapped organic matter. In addition to the unfortunate insects, this may include pollen and other plant and animal detritus. The resulting digested soup-like fluid is reabsorbed by the leaf surface and helps supplement the meagre mineral nutrient supply the roots of the plant receive from either peaty or lime-rich soils (Slack 1988, p. 107).

Karlsson & Carlsson (1984) showed experimentally that P. vulgaris plants supplied through their leaves either with insects, or with selected nutrients in appropriate concentrations in agar blocks, gained significant growth and increased biomass only when additional phosphate was made available. Nitrogen on its own, however, did stimulate root growth and produced an increase in the root/shoot ratio. Earlier work found a definite interaction between insect trapping and nutrient uptake by the roots of P. vulgaris (Aldenius et al. 1983) such that, compared to controls, plants supplied with insects showed a larger increase in N content than could have been due to withdrawal from the insects alone. Subsequent work, however, showed that this interaction occurred only at very high soil nutrient concentrations and at more ecologically relevant nutrient levels it may not occur at all.

The conclusion was that the role of carnivory in P. vulgaris is not simply to provide the plant with macro-nutrients, such as phosphate (PO4) or nitrogen (N) on their own, but rather that additional micro-nutrients obtained from the digestion of insect or other organic tissues play an important interactive role in the complete nutrient relations of the plant when they are combined with both N and PO4 (Karlsson & Carlsson 1984).

Overwintering and reproduction

While Common Butterwort is a perennial with a characteristic basal rosette of yellowish leaves, these completely die off in the autumn, so that the plant overwinters as a totally dormant bud, or 'hibernaculum'. This bud does not even retain roots, so it becomes mobile to a certain extent. In addition, depending upon the vigour of the original plant, a number of much smaller buds called 'brood buds' (gemmae or bulbils), may develop by vegetative means towards the end of the growing season. They are formed in the axils of the last foliage leaves of the year and may assist in clonal local dispersal. Although the number of bulbils produced by P. vulgaris in temperate regions is small, they probably provide an effective means of reproduction since opening bulbils are supplied with relatively large starch reserves stored in the bud scales. They usually establish daughter individuals around or near the parent plant, but they can be transported by water currents after autumn or winter rains, snow melt, or spring flooding (Heslop-Harrison 2004).

Between May and July mature Butterwort rosettes may develop one to three or more, leafless (scapose) flowering stems, each bearing a solitary, irregular, two lipped, 'gullet-type', dark purple, blossom. The flowers secrete nectar in a spur and give off scent that attracts small bees as pollinators. Despite this, the flower is reported in most situations to be neglected by insects and it habitually self-pollinates (Raven & Walters 1956, p. 47).

After fertilisation, by whatever means, the resultant fruit capsule, 0.5-1.0 cm, is ripe 2-3 weeks after the flowers have been pollinated. The developing fruit capsules are held horizontal at first, but become erect so that the seeds are not released unless the capsule is shaken. The scape also elongates as the capsule ripens, carrying the seed higher above ground. The capsule swells and splits open apically along the margins of the placenta into 2 to 4 valves. In dry weather, the valves open to release the seeds, which are numerous, tiny, oblong, lightweight and powdery and are easily dispersed by wind and water (Hutchinson 1972; Heslop-Harrison 2004). In subarctic regions, 110-140 seeds are produced per capsule (Karlsson 1986), but the current author (RSF) has not been able to discover comparable figures for seed production in B & I.

Seeds appear in the soil seed bank, but their properties of longevity, or otherwise, are unknown (Thompson et al. 1997). Grime et al. (1981) found 50% germination occurred after six months under experimental conditions, and Heslop-Harrison (2004) states that seeds do not normally germinate in B & I until the spring following their production, germination capacity being around 100% when the seeds are overwintered at normal outdoor temperatures.

Seedling establishment in the wild is precarious since the tiny seed size means the embryo is provided with negligible food reserves and suitable wet sites, free from competition by other species, are rare. For these reasons, seedlings have not been observed anywhere in B & I (Heslop-Harrison 2004).

Fermanagh occurrence

It is very frequent in Fermanagh on the peatlands of the Western Plateau, but is only thinly scattered elsewhere. It has been recorded in 108 tetrads, 20.5% of those in the VC. Unlike P. lusitanica (Pale Butterwort), it has not suffered really significant loss of suitable habitat locally, since 97 of the Fermanagh tetrads have post-1975 records.

While the plant is associated mainly with wet peat on bogs and flushes on upland moors, it only ascends to around 450 m, and it has not yet been recorded from Cuilcagh summit ridge at around 600-660 m. Perhaps the most unexpected site in the VC is on limestone pavement on the summit of Knockmore, but it also appears on the limestone scree above Doagh Lough in the same area. Normally one would imagine these habitats would be too well drained for P. vulgaris to occupy, but in Fermanagh, with its high level of regularly spaced precipitation, clearly these relatively dry habitats are sufficiently wet to allow it to occur and persist.

British and Irish occurrence and changes

P. vulgaris is much more frequent in the N and W of both B & I and it has been recorded in every Irish VC except East Cork (H5) (Perring & Walters 1976; Scannell & Synnott 1987). The New Atlas more clearly shows that in Ireland, apart from East Cork which is completely blank, there are four additional VCs where there are no post-1970 records, namely Mid-Cork (H4), Kilkenny (H11), Carlow (H13) and Monaghan (H32), while in Wexford (H12), Louth (H31) and Armagh (H37) the species in each case is recorded in only one or two hectads. The decline in the presence of P. vulgaris in these parts of Ireland reflects a continuing gradual encroachment or destruction of suitable habitat for the species in the S & E, which further emphasises the essential N & W character of the distribution of P. vulgaris on the island.

Very much the same picture applies in Britain, where the even greater pressures from drainage, agricultural intensification and other industrial and urban development have engendered losses of Common Butterwort in S & E England from more than half the hectads that existed in 1930. Elsewhere in Britain the species appears relatively stable in comparison with these areas (F.J. Rumsey, in: Preston et al. 2002).

European and world occurrence

The flower of P. vulgaris is rather variable in size, form and colour, so that in the past several varieties were described among the European forms (Hultén 1958, p. 230), although these are not mentioned in Flora Europaea 3 (S.J. Casper in: Tutin et al. 1972, p. 296). P. vulgaris is widespread but very unevenly distributed throughout N, W & C Europe, just extending south into N Africa and eastwards to the W Ukraine. In Germany and in Scandinavia, for instance, the plant is absent from many areas and in S Europe it is confined to the mountains. Beyond this, it is almost circumpolar, although again there are major gaps in the distribution, eg in Siberia, and it is again very uneven in its occurrence in N America (Hultén & Fries 1986, Map 1717).

Names and Uses

The genus name 'Pinguicula' is derived from the Latin 'pinguis' meaning 'fat' or 'greasy', a reference to the appearance of the sticky leaves (Gilbert-Carter 1964). The Latin specific epithet 'vulgaris' means 'common'. The name 'Butterwort' refers to the old notion that the plant was a magic charm protective of cow's udders, thus ensuring a supply of butter (Grigson 1974). In past times the plant was also widely used in cheese making as a substitute for rennet, having the ability to curdle milk (Vickery 1995) and this probably is the origin of an Ayrshire common name, 'Thickening-grass' (Grigson 1955, 1987).

Threats

Various forms of development involving drainage, fertiliser application and general change of land-use.

References

Perring, F.H. and Walters, S.M.(eds.) (1976); Scannell, M.J.P. and Synnott,D.M. (1987); Karlsson, P.S. and Carlsson, B. (1984); Grigson, G. (1974); Grigson, G. (1955, 1987); Vickery, R. (1995); Gilbert-Carter, H. (1964); Hulten, E. (1958); Hulten, E. and Fries, M. (1986); Raven, J. and Walters, M. (1956); Thompson, K., Bakker, J.P. and Bekker, R.M. (1997); Grime, J.P., Hodgson, J.G. and Hunt, R. (1988, 2007); Tutin, T.G. et al.(Eds.) (1972); Hutchinson, J. (1972); Slack, A. (1988); Aldenius, J., Carlsson, B. and Karlsson, S. (1983); Blanca et al. 1999; Heslop-Harrison 2004; Karlsson 1986; Grime et al. 1981; Preston et al. 2002;