Pinguicula lusitanica L., Pale Butterwort

Account Summary

Growth form and preferred habitats

P. lusitanica is a short-lived, wintergreen, rosette-forming, shallowly fibrous rooted perennial which like other members of the genus is stem-less and carnivorous. It usually overwinters as a leaf rosette, but occasionally behaves as an annual. It does not reproduce vegetatively as do the other species of the genus in B & I, which produce bulbils in the leaf axils towards the end of the growing season. An individual plant only persists for 2-3 years and it flowers annually, although in unfavourable conditions it may be delayed until the following year (Heslop-Harrison 2004).

The plant traps small insects and other organic matter, including pollen grains, on its basal rosette of sticky, pinkish or pale yellow-green leaves, sometimes tinged with purple and regularly patterned with thin, red veins. The leaves, which usually number 5-12, are ovate-oblong, 1.0-3.0 cm long × c 1.0 cm wide, rounded-obtuse at the apex, their margins entire and strongly inrolled or upcurved. The leaves are softly fleshy in texture and their upper surfaces are densely covered with minute, sticky glands. When the leaves trap an insect (chiefly small flies, aphids, spring-tails and small, creeping insects), they digest the animal's soft tissues with externally secreted exo-enzymes that are subsequently reabsorbed to supplement the meagre nutrient supply available to the roots from the wet, extremely acidic, boggy soils on which the species grows. The sticky leaves act like glue-laden fly-paper and will also capture pollen, seeds and small leaf fragments that will be externally digested and absorbed, adding to the Pale Butterwort's nutrient benefit (Lusby et al. 1996).

Pale Butterwort occurs on wet, flushed, open or bare peat patches on bogs, heaths and wet, short-turf, grazed grassy moor vegetation. More rarely, it grows on poorly drained mineral soil, again where it is constantly irrigated or flushed with slight to moderately base-rich ground water and the vegetation is grazed, removing tall, vigorous competing species (Garrard & Streeter 1983).

While P. lusitanica grows on wet, flushed ground, plants seldom survive repeated or prolonged immersion. Pale Butterwort tends to associate with other weakly competitive species such as Carex pulicaris (Flea Sedge), Narthecium ossifragum (Bog Asphodel) and several Drosera spp. (Sundews), all of which are low-growing and can tolerate the more or less nutrient-starved, cold, wet soil conditions. In taller vegetation, such as amid tussocks of grasses such as Molinia caerulea (Purple Moor-grass), or rushes (Juncus spp.) or sedges (Carex spp.), P. lusitanica is confined to gaps of bare ground between these vigorous plants, and a moderate level of trampling by livestock or deer helps to keep vegetation sufficiently open for germination and establishment to occur without destroying developing seedlings and young plants of the species. The size of local populations is often limited by the scale of these vegetation gaps, which tend to support numbers between ten and 200 individual Pale Butterwort plants (Lusby et al. 1996).

In common with other Pinguicula species, while P. lusitanica plants may be favoured by some level of grazing and trampling helping to keep the turf open, they do not withstand trampling because their leaves are brittle and very thin in nature, and their short, shallow fibrous root system only provides weak anchorage to the substratum. These same fragilities of leaf and roots also make the plants intolerant of damage inflicted by prolonged inundation and it avoids larger flushes and streams for this reason. Plants usually congregate in small populations of 10-20 individuals, varying in number in any habitat from year-to-year, depending upon the temperature, rainfall and humidity of the particular season (Heslop-Harrison 2004).

Fermanagh occurrence

In Fermanagh, P. lusitanica has been recorded in 47 tetrads (8.9% of those in the VC). Although the distribution map displays Pale Butterwort as widely scattered mainly in the western half of the county, there is strong evidence of a decline due to local loss of suitable habitats, as only 32 of the tetrads have post-1975 records.

Native status and phytogeographical links

P. lusitanica is one of four Fermanagh species belonging to the 15 strong Hiberno-Cantabrian (or less correctly 'Lusitanian') species element of the flora of B & I (Botanist in Ireland, paragraph 35). This plant grouping is not exclusive to Ireland occurring also in Britain, but to a much lesser extent. The alternative common name for Pale Butterwort is 'Western Butterwort', which is apt as it does have a decidedly western distribution throughout B & I (see below).

P. lusitanica is unusual among the Hiberno-Cantabrian species element in that it extends much further north than any other member of the group, reaching its limit in Orkney (Lusby et al. 1996). In Ireland, it is much more widely scattered than it is in Britain, but it is still more frequent in the western counties. Pale Butterwort is also much less disjunct than most other Hiberno-Cantabrian species, occurring on the continent from Brittany south along the western seaboard to Spain and Portugal (the old name of which was 'Lusitania'). The Lusitanian or Cantabrian species share a common distribution along the western seaboard of Europe from Portugal northwards to Ireland and Britain, and some also extend into the Mediterranean region. Webb (1983) regrouped these species, placing P. lusitanica with 29 other Irish species in a purely 'Atlantic' distribution group.

Forbes (1846) was the first to suggest that the Hiberno-Lusitanian species might be the oldest members of the flora of the British Isles, having reached Ireland before the advent of the Pleistocene Ice Ages, that they survived the glacial events in a SW refuge, thus making them Tertiary relics. Praeger (1934) supported this view and Webb (1983) did not greatly modify it. However, the Hiberno-Cantabrian species are almost all intolerant of hard frost and they also have a requirement for a more or less constant high relative humidity, a feature they share with members of the heath community in this same broad geographical region.

Partly because of its intolerance of frost, but also because the scarcely disjunct distribution of P. lusitanica is matched by that of three other Hiberno-Cantabrian species (Erica vagans (Cornish Heath), Euphorbia peplis (Purple Spurge)and Sibthorpia europaea (Cornish Moneywort)), Perring (1962) regarded these four species as Post-glacial migrants which travelled north along the old Atlantic coastline as the glaciers retreated. More recent paleoecological studies of glacial retreat, ancient shorelines and plant migration make this notion very difficult to maintain, however, and eventually they might have to be recognised as archaeophytes, ie plants that travelled with early man and that were either intentionally or accidentally introduced.

On the other hand, their peculiar ecological properties, their similar but very restricted local Irish distributions, often in absolutely remote bogland sites, all still lend weight to the argument that they really are native species. If so, somehow despite the inherent improbabilities, they must have overcome the odds and jumped across the marine barriers to achieve their known distribution at unknown dates in post-glacial times, entirely without mans' assistance. Having said this, it is very unlikely this species could have survived the last Midlandian glaciation in situ in Ireland as has been suggested. Evidence against post-glacial re-immigration is growing and increasingly convincing, leaving only the possibility of accidental human re-introduction for this and other temperature-sensitive species.

Perennation and mode of reproduction

P. lusitanica overwinters as a leaf rosette and unlike other temperate members of the genus, it does not form detachable, mobile gemmae or hibernacula (ie specialised winter resting buds) as a form of vegetative reproduction. Instead, Pale Butterwort is entirely dependent on seed production for its dispersal and establishment. This dependence on seed reproduction is probably the factor that limits P. lusitanica to the W & S of the British Isles, making it vulnerable to late spring and early autumn frosts (Lusby et al. 1996).

Flowering reproduction

P. lusitanica has a long flowering period, which many Floras suggest stretches from June to October (eg Butcher 1961; Clapham et al. 1962); RHN found it still in flower in mid-November 2001 after a very mild almost frost-free autumn. This probably represents a secondary period of flowering induced by shorter days or lower night temperatures during September, followed by mild October and November weather. Each flower survives only 3-4 days if unpollinated (Heslop-Harrison 2004).

The bisexual flowers are produced singly, in succession, on long, pedicel stalks (or scapes), arising from leaf axils. The number of flowers produced over the flowering period varies from 1-8, and the slender pedicels range in height from 6-15(-20) cm. The green calyx is 2-6 mm and slightly 2-lipped, the upper lip deeply 2-lobed, rounded at apex. The lower lip is 3-lobed and similar. The pale, pinkish-mauve, 2-lipped corolla is 7-11 mm long, and is often yellow in the tubular throat. The corolla has a blunt, down-curved, rear spur, 2-4 mm in length.

There are just two stamens, with short, 1.5 mm, stout filaments, the anthers pale yellow. The style is absent, the sub-rotund stigma sitting directly on top of the ovary. The flowers are small, pale in colour and decidedly inconspicuous, so it is not surprising that self-pollination appears the norm, although the flowers are obviously designed for insect-pollination (Proctor & Yeo 1973). They may still be visited and pollinated by bees and other hymenoptera to a limited extent (Fitter 1987). The insect species that pollinate the slightly larger and more colourful flowers of P. vulgaris (Common Butterwort) are very much larger than those that become trapped and digested by the leaves, so entomophily (insect-pollination) is not incompatible with the insectivorous habit.

The fruit is a small capsule, 4 mm long, almost globular. Seeds are 0.5-0.6 mm, oblong, brown or red-brown, their surfaces reticulate and rather deeply honeycombed (Butcher 1961; Sell & Murrell 2009). After pollination and fertilisation the flowering scape extends significantly (by 2-3 cm) to carry the developing fruit well above the level of the leaf rosette. Seed set is generally very good and large numbers (unspecified) of the small seeds are released from capsules and are carried on the wind. In wet weather, the capsule closes up to keep the contained seeds dry (Heslop-Harrison 2004).

In common with P. vulgaris, seedlings of P. lusitanica have not been observed in the wild in B & I, but they probably would not appear until the following spring. The first leaves (cotyledons) are very tiny, measuring less than 1 mm in length, a fact which goes a long way towards explaining the absence of seedling observations. However, P. lusitanica is short-lived and has no means of vegetative reproduction, and therefore is entirely dependent upon seed production and seedling establishment for its survival, both locally and in the longer term. Normally seed production is annual and appears prolific, but establishment in a suitable environment devoid of competition remains a critical factor determining survival (Heslop-Harrison 2004).

The current author (RSF) cannot locate any detailed information on seed production, field longevity, dispersal distance and establishment from seed, which suggests a series of interesting research projects awaits someone.

British and Irish occurrence

P. lusitanica belongs to the Oceanic temperate phytogeographical element (Preston & Hill 1997) and displays a strong western tendency in its distribution across the whole of B & I. It is frequent all along the western seaboard of Ireland, and more scattered elsewhere across the C & E of the island, but even within these restricted areas, it is local and sometimes inexplicably absent from conditions that look eminently suitable. In Britain, it stretches from Cornwall to West Sutherland and N & W Scotland (including the Hebrides but not Orkney and Shetland) via outliers on the Welsh coast of Pembrokeshire and the Isle of Man (BSBI Atlas 2; Lusby et al. 1996; F.J. Rumsey, in: Preston et al. 2002; Heslop-Harrison 2004). 

European occurrence

In Europe, the species distribution includes W France and the Cantabrian coast of Spain and Portugal. It also extends southwards to just reach NW Morocco and one site in Algeria (Clapham et al. 1962; Heslop-Harrison 2004; Sell & Murrell 2009).

Names

The name 'Pinguicula' derives from the female diminutive of the Latin, 'pinguis', meaning 'fat', possibly a reference to the greasy surface of the sticky leaves (Melderis & Bangerter 1955; Gilbert-Carter 1964). The Latinised specific epithet 'lusitanica' means 'Portuguese'. The common name 'Butterwort' first appeared in print in Gerard's (1597) Herball, referring to Pinguicula vulgaris, Common Butterwort, the plant supposedly being able to encourage or protect the milk-producing capacity of cows, thus ensuring a good supply of butter (Grigson 1974).

Threats

Loss of sites through drainage, peat-cutting, moor-burning and afforestation, plus eutrophication of ground water from agricultural practices. Because of its complete dependence on seed reproduction, it is less tolerant of trampling and disturbance than related species.

References

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