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Saxifraga rosacea Moench, Irish Saxifrage

Account Summary

Extinct. Sub-oceanic boreal-montane.

Interestingly, fossil material of S. rosacea has been found at Derryvree, near Maguiresbridge, Co Fermanagh, in a Middle Midlandian full-glacial deposit, but there are no modern records this far north in Ireland (Colhoun et al. 1972).

Growth form and preferred habitats

This fluffy, loose, evergreen, stoloniferous perennial is polymorphic, displaying considerable phenotypic (plastic) variation with changing natural environments, but it can be easily recognised at any time of year by its distinctive radiating, prostrate, leafy barren shoots producing a loose mossy cushion or weft growth habit. The prostrate sterile shoots often bear small, leafy bulbils in the leaf axils, called gemmae. The majority of leaves are three-lobed, but some are linear, unlobed. As is the case with many other saxifrage species, most of the foliage of plants growing in drier situations turns bright red in summer. When growing in normal light intensities, the barren shoots are negatively phototropic, the growing tips bending as if to seek out darker growing conditions (Webb 1950a).

Mossy Saxifrage is a characteristic plant of shady, wet to fairly damp ledges on north-facing limestone cliffs and on damp to dripping, mossy banks overhanging streams. On account of the damp oceanic climate of NW Ireland that Fermanagh enjoys, however, it also grows in more open, sunny, rather dry, rocky, usually upland, limestone grassland, in or on eroded limestone pavement and on slightly damper areas on stabilised calcareous scree slopes. Although in Fermanagh and elsewhere S. hypnoides is frequently closely associated with limestone or base-rich soils, Webb & Gornall (1989) point out that over its whole distribution range, which is very much centred on B & I, it can also grow freely on siliceous rocks, provided the soil receives some flushing by groundwater. Occasionally, elsewhere, S. hypnoides can even flourish on sand-dunes.

Genetic variation

The chromosomes of mossy saxifrage species are very small, crowded and numerous, making them difficult to count. The existence of multiple chromosome pairing at meiosis, in some forms at least, also makes accurate counts difficult, so that only vegetative mitoses give reliable counts. Some counts of Irish material communicated by Prof Webb must be considered only tentative and approximate estimates (Webb 1950a & b). However, despite these problems, S. hypnoides in B & I is known to possess two cytotypes with chromosome numbers 2n=26 and 2n=52, although other earlier Irish counts suggest 2n=48 and 2n=64 (Webb 1950a & b; Webb & Gornall 1989). Geographically, plants of S. hypnoides from W Ireland and Wales are diploid (2n=26), while those from N Ireland, N England and Scotland are tetraploid (2n=52) (R.J. Gornall, in: Preston et al. 2002). There are no consistent morphological differences between the diploid and tetraploid chromosome cytotypes (Parker 1979).

In their Hybrid Flora of the British Isles (Stace et al. 2015), the authors list four chromosome counts for S. hypnoides, 2n=26, 48, 52 & 64, and for the closely related S. rosacea (Irish Saxifrage) which also occurs in Ireland, 2n=48, 52 & 64. Both these species are self-fertile and field observation shows there has been introgression and transfer of characters between them in the past when their distributions overlapped (Stace et al. 2015).

A third closely related species restricted to southern continental Europe (SE France, N & C Spain and E Portugal) is S. continentalis (Engler & Irmscher) D.A. Webb (= S. fragosoi Sennen), which previously was considered a subspecies of S. hypnoides. It also has cytotypes 2n=26 & 52, and differs most from S. hypnoides in having more numerous and consistently present summer-dormant buds that are covered with almost entirely membranous, translucent outer leaves. It has basal leaves that are 1-7-lobed, often 5-lobed, rather than 3-lobed as in S. hypnoides (Webb & Gornall 1989).

The basic chromosome number in this group of Saxifrages was firstly thought to be x=8 (Webb 1950a), but has subsequently been revised to x=13, a degree of change that says much about the difficulties of chromosome counting in these species (Webb & Gornall 1989). In any event, at least one S. hypnoides cytotype is polyploid, and must have arisen following hybridisation. Hybridisation takes place readily between species in this section of the genus (Section Dactyloides Tausch, the 'mossy' saxifrages), there being no serious sterility barrier between the members due to the degree of polyploidy they share (Webb 1950a & b).

The range of phenotypic modification, produced by the plasticity that S. hypnoides displays, is closely paralleled and overlapped by the range of variation produced by its genetic diversity (Webb 1950b).

Flowering reproduction

From May or late April to July a small proportion of the leafy branches develop flowers. Shading has a marked inhibitory effect on the number of flowering stems produced (Webb 1950a). The flowering stems branch in their upper half and bear 1-5, but occasionally up to seven pure white, shining flowers in a loose panicle. Flower buds are pendulous (nodding), when first developed, clearly differentiating the plant from the closely related S. rosacea which has erect flower buds (Webb 1950a). Otherwise the best distinguishing feature of these two species is the aristate tip of S. hypnoides leaf lobes, ie each being drawn out into a fine, colourless hair-like point, while the leaf segment tips of S. rosacea are obtuse to mucronate (an abrupt point), but never aristate-apiculate (Parker 1979; Webb & Gornall 1989).

Flowers of S. hypnoides are regular, 10-15 mm in diameter, hermaphrodite, scentless and strongly protandrous (ie the anthers mature before the stigmas are ripe), meaning that cross-pollination is strongly favoured, although self-pollination is still possible (Webb 1950b). The open nature of the unfertilised flower, with its only slightly domed central ovary (3/4 inferior, embedded in the receptacle, ie nearly epigynous), means that the nectaries are fully exposed between the ovary and the insertion of the petals on the flat receptacle. This makes the plentiful nectar available even to short-tongued flies, sawflies and gall-wasps, so that a wide range of insect visitors can feed and cross pollinate them (Webb 1950a; Proctor & Yeo 1973; Fitter 1987). Individual flowers last from 8-12 days from opening to petal fall. The fruit is a dry capsule and the many seeds it contains are black and covered with numerous low, rounded papillae (Sell & Murrrell 2014).

There do not appear to be any estimates of the number of seed normally produced per capsule in wild sites. In late autumn, the dry fruit capsule partially splits to release the seeds, which are shaken out by swaying of the flowering branch in any breeze over a period of time. Seed dispersal is probably further assisted by flowing water and, over small distances, by wind (Webb 1950a).

Germination does not require a chilling period, but probably most seed does not germinate until the following spring (Webb 1950a). The current author (RSF) cannot find any published information on buried seed longevity for S. hypnoides, but results for other Saxifraga species indicate that in most of them seeds are transitory, persisting for only one year (Thompson et al. 1997).

Vegetative reproduction

Although S. hypnoides produces a spreading weft of prostrate shoots that root at intervals, its powers of vegetative reproduction and spread by this means are probably minor compared to seed. The shallow rocky soils, the cliff ledge and other more grassy habitats S. hypnoides usually occupies, tend to be ecologically discontinuous, restricting any form of gradually spreading increase by vegetative growth (Webb 1950a).

Fermanagh occurrence

In Fermanagh, S. hypnoides has been recorded in a total of 25 tetrads, 4.7% of the total in the VC. It is almost exclusively found around the Knockmore and Marlbank limestones in the hilly W & SW of the county, but it does extend SE to an outlier on the limestone knoll of Knockninny, overlooking Upper Lough Erne where Dr Wade first recorded it in 1804.

Irish occurrence

In NI, apart from Fermanagh, S. hypnoides grows along the base-rich basaltic scarps of Cos Antrim and Londonderry (H39, H40), and elsewhere in Ireland the species is only common, widespread and locally abundant in the Burren, Co Clare (H9) and on the Ben Bulbin mountain limestones in Cos Sligo and Leitrim (H28, H29).

British occurrence

The distribution of S. hypnoides in Britain has been reasonably stable over the last 50 years, although there may have been some decline in N Scotland (R.J. Gornall, in: Preston et al. 2002). In Britain, Mossy Saxifrage is essentially a plant of northern and western mountains, extending from the far north of Scotland to S Wales and N Somerset (Cheddar), but there also are scattered garden escapes further south in England, although most of these are probably of hybrid origin (Webb 1950a; Preston et al. 2002).

European occurrence

Beyond the shores of B & I, S. hypnoides is common and widespread in Iceland and the Faeroes, while on the continental mainland it is remarkably confined to just a few localities in W Norway, and two small remote outliers further south, in Belgium and in France in the Vosges, near Gérardmer. It has not been seen in the latter location for several decades (Hultén & Fries 1986, Map 1027; Webb & Gornall 1989, Map 55; Jalas et al. 1999, Map 3186).

Threats

Possibly sheep grazing and trampling.