Potentilla anglica Laichard., Trailing Tormentil

Account Summary

Growth form and preferred habitats

As its English common name indicates, P. anglica is a trailing, procumbent, mat-forming perennial, possessing a thick, branched rootstock bearing a varying mixture of 3-, 4- and 5-nate leaves and with leaf stipules entire or 3-lobed, not deeply divided or lobed as in P. erecta (Tormentil). Small, simple, undivided leaves also occur. The spreading aerial stems root at their tips later in the growing season and the four or five petalled flowers regularly contain up to 20 carpels and set good seed, in comparison with the 4-12 carpels that are usual in P. erecta (Webb et al. 1996).

Although the two closely related taxa overlap in their ecology, P. anglica tends to be a more lowland species than P. erecta and it also frequents less acid, better drained, light or sandy soils than the latter: it especially avoids damp, cold, iron podsols with their impeded drainage. The habitats where these two species overlap include low-growing vegetation on heathy pastures (including those near the coast), inland heaths, dry roadside-, riverside- and railway-banks, tracksides, hummocks in damp fields, woodland rides, paths and margins, field edges, quarries and shingly waste ground (Stace et al. 2015).

Hybrid origin of the species

P. anglica is an example of an allopolyploid species of sudden origin that arose by hybridization between two tetraploids (2n=28), P. erecta and P. reptans (Creeping Cinquefoil). Both these species are highly fertile and self-incompatible, and there are incompatibility barriers between the two which make hybrids between them very difficult but not impossible to obtain (Matfield & Ellis 1972). At some stage, a hybrid involving both species was formed, possibly involving an unreduced gamete from P. reptans, creating a hexaploid hybrid (2n=42). This hexaploid must then have back-crossed with P. erecta in a second stage of the process, again without reduction of its gamete, combining to form a new, fully fertile octoploid hybrid (2n=56) that is P. anglica Laichard. (Matfield & Ellis 1972; Harold 1994).

As one would expect of a species of hybrid origin, its morphology is intermediate between its two parents, but P. anglica also has some features that are directly attributable to its higher chromosome number. Each stem node bears one to several leaves with petioles of variable length. The leaflet number, leaf size and petiole length all decrease through the growing season so that a plant may look more like P. reptans in early summer, but becomes more like P. erecta later on. P. anglia roots at its stem nodes, but does so less readily than P. reptans. Directly due to the higher chromosome count, the leaflets tend to have a lower length:breadth ratio than those of the parent species and the pollen grains are larger. Most significantly, the additional sets of chromosomes in P. anglica lead to a breakdown in the self-incompatibility mechanism that is so characteristic of the two parent species. This means a single, isolated specimen of P. anglica is able to self-pollinate and set good seed, making seed fertility a useful diagnostic character for this species (Harold 1994).

However, matters are complicated by the relatively frequent occurrence of another hybrid, P. × suberecta, a cross between P. erecta and P. anglica. P. × suberecta is morphologically very similar to P. anglica and always occurs in the presence of both its parents. Although P. × suberecta is not completely sterile and may set a few seed, it does not reproduce vegetatively and, therefore, it tends to die out. Nevertheless, its existence confuses the identification of P. anglica to a very considerable extent (see my account of this hybrid for more details).

Fermanagh occurrence

Trailing Tormentil is widespread and locally frequent in Fermanagh in short grass on well drained banks, slopes and roadsides. Although recorded from 117 Fermanagh tetrads, representing 22.2% of the total number, P. anglica remains almost certainly under-recorded in the current Flora survey. It is probably quite often confused with the more common and widespread P. erecta (Tormentil) with which it ecologically overlaps and forms the partially fertile hybrid, P. × suberecta. RHN and the current author (RSF) are sorry to report we have never found this hybrid in Fermanagh.

There are nine tetrads in Fermanagh with pre-1975 records only for P. anglica, but this is too few to suggest there has been any real decline in presence.

British and Irish occurrence

The hectad map in the New Atlas (accompanied with a warning regarding confusion with the hybrid) displays P. anglica as widespread throughout Ireland, but more local and patchy in its occurrence and with many old records in C Ireland.

The same map indicates a widespread presence in W England to the south of Lancaster and also in Wales. The distribution becomes more scattered and local further N & E and also in the E & SE of England, where evidence suggests that it is declining. In Scotland, P. anglica is frequent only in Dumfries and Galloway and in the Clyde and Forth conurbations, although isolated outliers exists in the western isles and in NE Aberdeen (D.J. McCosh in: Preston et al. 2002).

European and world occurrence

Beyond these isles, the native range of P. anglica is restricted to a small region of cool temperate W & C Europe, with a few introductions beyond. The SW limit is in the Pyrenees and it stretches north to reach the S Baltic region of Sweden and Finland. It is absent from the Iberian and Italian peninsulas but stretches east to the western border of the former USSR (Sell & Murell 2014). P. anglica is probably endemic to Europe. The plant mapped on Corsica (Kurtto et al. 2004, Map 3522) is tetraploid and should be recognised as a separate species. A claim by Fernald (1933) that the population in Newfoundland is native appears unsupportable, particularly in view of other widely scattered outliers of this species in N America that are acknowledged naturalised introductions (Hultén & Fries 1986, Map 1121). The same applies to outliers in Madeira, the Azores and Iran, where the species is almost certainly introduced (Kurtto et al. 2004). See also P. sterilis below.

Threats

None.

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