Lythrum portula (L.) D.A. Webb (= Peplis
portula L.), Water-purslane
Account Summary
Native, very rare, but very possibly under-recorded. European temperate.
1902; Abraham, J.T. & McCullagh, F.R.; Castle Caldwell estate.
June to August.
Growth form and preferred habitats
While it is never found in Fermanagh in quantity, this dwarf, more-or-less prostrate, creeping, branched, reddish, quadrangular-stemmed annual, with opposite, fleshy, simple leaves and very inconspicuous, 2 mm diameter purplish or pink flowers, solitary in the leaf angles, occurs on various types of bare muddy, gravelly or stony ground that floods in winter, but which dries out in summer. Alternatively it features in very shallow water on the margins of lakes, pools, streams, ditches and puddles. While it normally grows in shallow water or on damp ground, L. portula can occasionally be found in water up to 1 m deep (Preston & Croft 1997).
Due to its fairly small size and very inconspicuous flowers, this distinctly calcifuge, lime-avoiding annual, which is often very local in its occurrence, can be, as here, either difficult to detect, easily overlooked, or mistaken for something else equally diminutive.
Apart from the receding margins of all kinds of water bodies, L. portula should be looked for in damp depressions in sand and gravel pits and in temporarily wet ground near flushes, and in rutted tracks and rides. While it is restricted to acidic, base-poor, mineral soils, Water-purslane avoids strongly acid, nutrient-impoverished conditions and it only rarely grows in open areas on organic peat (Preston & Croft 1997). The occurrence of a large but local population of the species on limestone at Coole Lough in the Burren, Co Clare (H9) is very exceptional, but this is just another example of anomalous behaviour by normally calcifuge species that takes place in this ecologically and biogeographically highly unusual coastal region of W Ireland (Webb & Scannell 1983).
Variation
Two subspecies have been distinguished, somewhat arbitrarily on the basis of their mature fruits and a ratio cline in the length of their outer calyx teeth. They are also partly distinguished by their separate geographical distributions, subsp. portula being widespread in Europe and extending southwards to S Italy, Sardinia and Corsica, while subsp. longidentata, which has epicalyx segments 1.5-2.0 mm, four times larger than those of subsp. portula, is more western in its distribution, being recorded from France, Portugal, Spain, Algeria, the Azores, and is the predominant form in at least western parts of Britain and Ireland (Allen 1954b). However, intermediates occur in the area between the two subspecies where they overlap (eg on the Isle of Man) and they do not appear to differ in their ecology, rendering them in the opinion of the current author (RSF) of only doubtful utility and certainly better considered as varieties (Allen 1984; Preston & Croft 1997; Sell & Murrell 2009).
Flowering reproduction
The tiny pinkish flowers are produced from June to October. They have an epicalyx of six segments, 0.5-2.0 mm, linear-lanceolate and with a long acute apex. The calyx has six lobes, each with a small appendage. The six pink petals, when present, are very thin (they are described as 'fugacious' or 'caducous', meaning they fall off early). The stamens are either six or twelve and are inserted beneath the petals. The single style and stigma are very short and solitary and the superior ovary sits in a shallow, bell- or cup-shaped hypanthium.
The flowers are usually self-pollinated and produce a small, sub-globose, flattened seed capsule, about 1.5 mm in diameter containing many seeds. Submerged flowers do not open and are automatically pollinated in the bud (ie they are cleistogamous) (Melderis & Bangerter 1955). Plants growing in standing water fruit less readily than those in terrestrial conditions (Allen 1954b).
The seeds are small (0.7 × 0.5 mm), ovoid and grey and, despite the apparent lack of a dispersal mechanism, they somehow, mysteriously, manage to reach suitable isolated habitats, such as sand quarries and gravel pits (Preston & Croft 1997). Buried dormant seed survives for at least five and, perhaps, many years (Croft 1994; Thompson et al. 1997).
The established ecological strategy as determined by Grime et al. (1988, 2007) is given as R/SR, meaning it is intermediate between Ruderal and Stress-tolerant Ruderal. This designation, together with the fact that seed germination requires light, suggests that some degree of soil or substrate disturbance is essential to bring dormant seeds to the surface and stimulate their germination (Preston & Croft 1997).
Vegetative reproduction
The usually creeping stems root at their nodes and, since they are rather brittle, fragments may disperse in water or mud and help propagate the plant (Preston & Croft 1997).
Fermanagh occurrence
In Fermanagh, Water-purslane has been recorded just 16 times in a total of 13 tetrads, 2.5% of those in the VC. Eleven of the tetrads have post-1975 records and, as the tetrad map indicates, they are located mainly in the NW of the county.
British and Irish occurrence
While L. portula may very well be under-recorded in Fermanagh, the New Atlas hectad map indicates that in B & I its distribution is very much better known now than was the case in the earlier BSBI Atlas (Walters & Perring 1962) and it is more frequent and wider in its occurrence than previously known. This is especially the case in all western parts of Britain and in the Scottish midlands, between Glasgow and Edinburgh, where the species is now seen to be most prevalent. In Ireland, L. portula has a strong representation in the SW, but is widely scattered throughout the island.
Having said this, while the hectad map of B & I shows L. portula is frequent and widely scattered, there is clear evidence of a species decline since the 1950s, especially in SE England and the English Midlands. The losses are most readily explained by the typical loss of habitats associated with seasonally flooded ground, such as infilling, overgrowing, or draining of ponds, and the repair of wet ruts in resurfaced tracks (Preston & Croft 1997; R. Wilson, in: Preston et al. 2002).
European and world occurrence
L. portula is essentially a lowland, European temperate species extending from S Scandinavia to S Europe and to scattered sites in N Africa and W Asia. It has occasionally spread within and somewhat beyond its original range, for instance into isolated localities in more northern areas of Fennoscandia (Hultén & Fries 1986). It has also been introduced to California, C & S America, New Zealand and, possibly, also to N China (Hultén & Fries 1986, Map 1349; Sell & Murrell 2009).
Threats
None.