Cystopteris fragilis (L.) Bernh., Brittle Bladder-fern

Account Summary

Growth form and preferred habitats

Brittle Bladder-fern is a widespread, very variable, small deciduous fern in our survey area. Strongly calcicole, it is frequent in permanently shaded, rocky ground, Ash woodland, or walls and quarries where plant competition is limited. It frequently associates with Asplenium trichomanes (Maidenhair Spleenwort), Phyllitis scolopendrium (Hart's-tongue) and Ceterach officinarum (Rustyback). Its thin, delicately cut, deciduous fronds sometimes reach 25 cm or more in length, although usually they are much shorter, around 10 cm long. Separate but similar fertile fronds bear numerous sori. Each young sorus is protected by a thin, membranous indusium which is slightly inflated and is pear- or bladder-shaped. The sori turn dark brown or black in colour as the sporangia ripen, and by this stage the indusium has shrunk and become inconspicuous, facilitating the release of spores on the slightest breeze.

Fermanagh occurrence

This little fern is a typical plant of the Fermanagh limestones, base-enhanced dolomitized sandstone scarps or other calcium-bearing rock habitats of the west of the county. In Fermanagh, it has been recorded in 64 tetrads, 12.1% of those in the VC. Seven tetrads contain pre-1976 records only. Typical local habitats include ± permanently shaded, damp crevices on N-facing, sometimes wooded cliffs and ravines, rocky slopes, screes, swallow-holes, caves and narrow fissures (ie grykes) in limestone pavement. As the tetrad distribution map indicates, apart from the calcareous or base-rich natural rock outcrops of W Fermanagh, Brittle Bladder-fern is only occasional elsewhere in the county and here it is confined to scattered man-made habitats, such as quarries and the weathered lime-mortar of shaded old walls and bridges.

British and Irish occurrence

In Britain & Ireland, the distribution of C. fragilis is markedly northern and western, being strongly associated with the wetter and higher rocky ground in these regions. In other parts of these isles, C. fragilis ascends well over 915 m in sheltered, moist cliff crevices, but in the very oceanic climate and with the relatively low relief base-rich rocks in Fermanagh, the highest it reaches is the very modest 400-420 m of Trien Mountain. The complete inability of the species to tolerate summer drought means that it is more or less absent or very rare in the S & E of both Britain and Ireland (Page 1997).

In Ireland, the distribution and frequency of Brittle Bladder-fern most closely matches that of Hymenophyllum wilsonii (Wilson's Filmy-fern), another very delicate, even thinner-textured fern species, the distribution of which is also very much governed by constantly high atmospheric humidity levels (Jermy et al. 1978; New Atlas).

Variation

C. fragilis is a very variable polymorphic fern with a number of named varieties, some of which are very probably linked to the different chromosome numbers recorded for the species which form a polyploid series. Tetraploid and hexaploid plants, plus the pentaploid hybrid between them, have been found in the British Isles. The degree of frond dissection appears to be correlated with chromosome number: the more dissected it is, the more likely the plant is hexaploid. An octoploid form has been found in Europe, which might also crop up if searched for (Jermy & Camus 1991; Page 1997).

European and world occurrence

In Europe, the species is very widely distributed throughout moister northern and middle latitudes, becoming somewhat less evident in the Mediterranean basin, yet reaching the Azores (Jalas & Suominen 1972, Map 110; Page 1997). A closely related species, previously regarded as C. fragilis subsp. diaphana (Bory) Litard., occurs throughout Madeira (Press et al. 1994), and another related form also occurs in the Canary Islands (Page 1997).

Taken in the broadest sense, C. fragilis s. lat. is an extremely widespread circumpolar species, its natural range stretching right across N America as well as throughout Europe and Asia. Taxonomic varieties of it also occur in large parts of Africa down to the Cape, in Australia, Tasmania, New Zealand, in Kerguelen and South Georgia, as well as in S America and the Falkland Isles (Hultén 1962, Map 55; Hultén & Fries 1986, Map 52).

Names

The genus name 'Cystopteris' is Greek, combining 'kustis', 'kystis' or 'cystis', meaning "a bladder", and 'ptěris', "fern", referring to the bladder- or pear-shaped outline of the indusium (Gilbert-Carter 1964). The shape of the latter always reminds me of the large flasks of coloured water that in my Londonderry childhood of the 1950s often graced pharmacists' premises, and which still act as icons of their profession. The Latin specific epithet, 'fragilis' usually translates as 'brittle', 'fragile' or 'easily broken', which in this case applies to the brittle stipe or stalk of the frond. Translating the Scientific name thus, we derive the English common name 'Brittle Bladder-fern'. However, an alternative translation of 'fragilis' is 'wilting quickly' (Stearn 1992), which is also appropriate for this fern and can readily be applied to its delicate fronds.

Threats

Upgrading and 'tidying' of old walls and bridges.

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