Cerastium diffusum Pers., Sea Mouse-ear
Account Summary
Native, very rare, possibly a mis-identification. European temperate.
Growth form and preferred habitats
This little, hairy, winter- (or occasionally spring-) annual forms an overwintering basal leaf rosette, dull dark-green in colour (Dark-green Mouse-ear is an alternative English common name for the species, and it used to have the scientific binomial, C. atrovirens Bab.). A plant of light, dry sandy or gravel soils, C. diffusum is a common species of open coastal habitats that rarely, or very occasionally, strays inland where it occupies a range of open, disturbed habitats with suitable soils. According to Jonsell et al. (2001), in Fennoscandia it does not prefer saline conditions, but rather, it tolerates sea spray.
In Britain, the inland habitats are generally associated with dry grassland by paths, wall tops, along roadside verges where salt is regularly scattered for accident prevention in frosty weather, waste ground and rough grassland. Previously, during and in the years after the Second World War, C. diffusum also commonly occurred in Britain on the ballast between railway lines, but following economic contraction of the railway mileage, it has declined in this type of site since then.
Sexual reproduction
The plant flowers from May to July. Diffusely branched flowering shoots are decumbent or ascending, up to 30 cm tall and densely covered with short, sticky glandular hairs. The small flowers, 3 to 4 mm across have four (or less frequently five) notched petals, equal in length or shorter than the sepals. The flowers are rarely visited by insects and automatically self-pollinate (Clapham et al. 1987). The straight capsule is 6 mm long and typically contains 30 seeds, although there can be up to 45. The seeds are rounded, 0.7 mm across, pale- or reddish-brown in colour, bearing blunt tubercles (Salisbury 1964, p. 159; Jonsell et al. 2001, p. 151). Seed dispersal is presumably primarily by wind, assisted by human activity disturbing sites where it grows.
Fermanagh occurrence
There are two Fermanagh records from the dry limestone grassland at Rahallan Td, S of Belmore Mountain, an area now famous as one of the sites of Neotinea maculata (Dense-flowered Orchid). The plant was listed in two tetrads at Rahallan on 15 May 2005 by RHN accompanied by Dr D. Cotton, the BSBI recorder for Cos Sligo and Leitrim (H28 and H29) and Mr F. Carroll. Unfortunately, no voucher was collected, and these two interesting finds are therefore unacceptable as new county records.
The only other Cerastium listed in the Fermanagh Flora Database for this site is C. fontanum (Common Mouse-ear). We therefore regard the records of C. diffusum as possible errors, and the species remains on our list of desiderata.
Fossil history
No information has been obtained on this subject.
British and Irish occurrence
In N Ireland, C. diffusum was previously frequent on the shores of Lough Neagh, but it has markedly declined to rarity (Flora of Lough Neagh; FNEI 3; McNeill 2010). In Britain, Sea Mouse-ear is very common on coasts and increasingly scarce inland (New Atlas; Braithwaite et al. 2006).
European and World occurrence
C. diffusum is variable and polymorphic to the extent that in Flora Europaea three subspecies were recognised, of which only subsp. diffusum is definitely found in Britain & Ireland (Clapham et al. 1987; Tutin et al. 1993; Stace 2010). C. diffusum subsp. diffusum is almost entirely restricted to W, C & S Europe, extending northwards to S Sweden and eastwards to Ukraine. Again, the world map shows it largely confined to coastal sites (Hultén & Fries 1986, Map 753), the oceanic distribution of this European temperate species suggesting that it is more frost-sensitive than warmth demanding (Jonsell et al. 2001). Hultén & Fries (1986) recognise that the distribution is incompletely known, and there exists a small number of scattered introductions in N America. It is very puzzling to work out how and by what agency this annual species managed to cross the Atlantic Ocean, but it cannot be doubted that it succeeded in doing so on a number of occasions. The answer must somehow involve accidental or incidental human transport.
Names
The genus name 'Cerastium' is from the Greek 'keras' or 'kĕrastēs' meaning a horn or horned respectively, from the fact that some species have curved seed capsules resembling horns as they emerge from the calyx (Johnson & Smith 1946; Stearn 1992). The Latin specific epithet 'diffusum' means loosely spreading or diffuse (Gilbert-Carter 1964). The English common name 'Sea Mouse-ear' is a recent book name of no folklore significance.