biological flora of british islands. hymenophyllum. (richards &evans 1972)

Hymenophyllum Tunbrigense (L.) Sm.Author(s): P. W. Richards and G. B. EvansSource: Journal of Ecology, Vol. 60, No. 1 (Mar., 1972), pp. 245-268Published by: British Ecological SocietyStable URL: http://www.jstor.org/stable/2258054Accessed: 06/07/2010 16:43

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245

BIOLOGICAL FLORA OF THE BRITISH ISLES Hymenophyllum

BY P. W. RICHARDS AND THE LATE G. B. EVANS

School of Plant Biology, University College of North Wales, Bangor

List Br. Vasc. P1. (1958) No. 7.1

Hymenophyllum tunbrigense (L.) Sm.

A mat-forming bryophyte-like perennial, growing flattened against vertical or inclined rocks or tree trunks, with numerous overlapping, usually pendulous, deeply pinnatisect leaves arising from very slender, wiry, much-branched interlaced rhizomes bearing tufts of adventitious roots at the nodes. Leaves bright pale green when young, often becoming bluish or brown (not blackish) when older, translucent, with mat surface, 30-6-0 (rarely 0-5-15-0) x 2 cm. Lamina one cell thick (except at the veins), more or less oblong to ovate-oblong in outline, pinnate with winged rhachis, the pinnae divided more or less dichotomously but irregularly, with 5-11 oblong predominantly acroscopic segments, each with a single vein almost reaching the apex: margin finely toothed. Cells of lamina in surface view 60?1 x 39?1 ,um, with c. 30-40 rather large chloroplasts lining the lateral walls. Petiole very slender, wiry, j-3- the length of the lamina, in exposed habitats tending to become bent at right angles to it.

Sori usually 5-10 per leaf and one per pinna, borne terminally on the segment nearest the rhachis; indusia green when young, brown to black when ripe, purse-like and flattened in the plane of the lamina, formed by two flattened broadly triangular to suborbicular valves which are free for about three-quarters of their length and irregularly toothed on their distal margins. Sporangia with oblique annulus, gradate in development. Spores 40-48 gm in diameter.

Variation in Britain, as far as investigated, seems to be entirely phenotypic (Evans 1964). No subspecies or varieties have been described from Europe. Populations appearing more or less similar to those of Europe are found in the Atlantic Islands, North America, various parts of the tropics and in the Southern Hemisphere; with the exception of those of the Atlantic Islands, and perhaps of eastern North America and Jamaica, these are probably not conspecific with European H. tunbrigense and require critical re-examination.

Native and locally abundant on rocks, more rarely on trees and on peaty soil among bryophytes, in moist, well-sheltered situations in the west and north of Great Britain, the Sussex Weald and in Ireland; less widespread and abundant than H. wilsonji and with a narrower ecological range.

I. Geographical and altitudinal distribution. Reported from thirty-seven vice-counties in Great Britain and twenty-five in Ireland. The present distribution in Great Britain extends from Cornwall and Devon in the south to Skye in the north, with an outlying group of stations on the sandstone rocks of East Sussex: extinct since at least 1875 in its locus

246 Hymenophyllum tunbrigense (L.) Sm.

classicus at the High Rocks, Tunbridge Wells (Wolley-Dod 1937). Occurs in south, west and north Ireland. Areas of highest frequency are North Wales, western Scotland and west and south-west Ireland. During the past hundred years this species has suffered from the destruction of its habitats and from unscrupulous collectors: extinction in some localities has increased the discontinuity of its distribution. The large number of localities in which it has not been seen since 1930 (Fig. 1) suggests that it is still diminishing in frequency over much of its area. It has often been confused with H. wilsonii and sometimes

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groups of stations ('nids atlantiques' of Christ 1910); as in Britain it is decreasing and is now extinct in many former stations. At present H. tunbrigense is certainly known only from France, Luxembourg, north Spain, Italy and the eastern coast of the Black Sea. In Luxembourg it is found in numerous sites in the 'Suisse Luxembourgeoise',

P. W. RiCHARDS AND G. B. EVANS 247

a district of sandstone rocks in the basin of the Ernz Noire (Reichling 1954). In West Germany it occurred at Bollendorf, in the Valley of the Sure, close to the Luxembourg frontier (Hess, Landolt & Hirzel 1967). In France there are many localities in the north- west (Manche, Cotes-du-Nord, Finistere) and the western Pyrenees (Basses Pyreenes): it is also known from a few localities in the north-east (Vosges) and has been reported from Seine-et-Marne, Orne and Eure-et-Loire. In Spain it is recorded from Galicia and from one locality in the Spanish Pyrenees (Lacoizqueta 1885). In Italy it occurs in several localities in the Alpe Apuani, Toscania (Gams 1935; Pichi-Sermolli 1936); also in Marche

6 30 24 70' 18 12- 18iE 12- 0 2 30 36 42 48 54 < 0.?- 6 72

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60*

FIG. 2. The distribution of Hymenophyllum tunbrigense (L.) Sm. in Europe. o, Native occurrence; @, occurrence of 'uncertain status'; +, extinct; x, probably extinct. The occurrences on the eastern coast of the Black Sea are outside the Flora Europaea

area and are not shown. Reproduced by permission of the Committee for Mapping the Flora of Europe.

(Pichi-Sermolli 1936). In the U.S.S.R. it is found on the eastern coast of the Black Sea at three stations near Batumi, Adzharya (Burchak-Abramovick 1962).

Formerly found in the Belgian Ardennes (Christ 1910; Wilpert 1937) and in Germany in the Saichsische Schweiz near Dresden, where it is now extinct at all recorded sites (Wilpert 1937). Formerly occurred in the island of Elba, and probably Corsica (Jovet 1933). Reported from Czechoslovakia, but not confirmed for many years (Dostal 1958). In Yugoslavia it is said to have been found near Zagreb in 1900 and has been doubtfully recorded from three other localities (Pevalek).


Outside Europe, H. tunbrigense is reliably recorded from the Azores, Madeira and the Canaries. Populations probably conspecific with it have been found in South Carolina (twenty-one stations-Taylor 1938; Anderson & Bannister 1952) and in Jamaica (G. R. Proctor, personal communication).

In Britain its altitudinal range is from sea-level (e.g. near Llanfrothen, Merioneth; Rhum, W. Inverness; Carlingford Lough, N. Ireland) to 1250 ft (380 m) in an isolated station on Tryfan, Caernarvonshire, at 1350 ft (412 m) at Wistman's Wood, Devon, and 1500 ft (460 m) on Priestleap Mountain, Kerry (Carroll, in Scully 1916). Hart's (1881) record from 2500 ft (762 m) in Tipperary is probably due to confusion with H. wilsonii which still grows in the locality though H. tunbrigense does not (D. A. Ratcliffe, personal communication). Mainly a plant of low altitudes and usually replaced by H. wilsonii above 1000 ft (305 m). Most Caernarvonshire localities are between 0 and 500 ft (152 m). The upper limit decreases northwards: in the north of England it is below 500 ft (152 m), in Scotland below 300 ft (90 m).

Found at 1750 m in the Pyrenees (Turmel 1952) and to 700 m near Batumi, U.S.S.R. (Burchak-Abramovick 1962), but most of its present or former localities in continental Europe are below 300 m.

II. Habitat. (a) Climatic and topographical limitations. The distribution of Hymeno- phyllum tunbrigense in Britain, though no doubt influenced by historical factors such as the destruction of former woodlands, seems to be mainly determined by the occurrence of suitable microclimates and substrata. In the moist, sheltered habitats in which it grows, the range of humidity and temperature are widely different from the ambient conditions and in most of Britain the macroclimate appears to be well within the limits of tolerance of the species. The microclimatic requirements of H. tunbrigense differ from those of H. wilsonji as pointed out under the latter.

H. tunbrigense is most often found on or among rocky outcrops, e.g. cliffs, block screes, especially when they are weathered so as to provide deep gullies, crevices or fissures and are to some extent shaded by trees or overhanging rocks. Though often found near streams and waterfalls, it is intolerant of flowing water and is absent from both the spray and drip zones of waterfalls and the flood zone of streams. The amount of shade or shelter demanded seems to vary with the humidity of the macroclimate. For example, in the very moist west of Scotland it sometimes grows on open hillsides; in North Wales the majority of its habitats are in crevices or on rocks with some shelter from trees, but it also grows in deep hollows in block screes where there are no trees. In the high level oakwoods of Dartmoor (Wistman's Wood, Black Tor Copse) it grows mainly in dark holes in the 'clitter' (boulders), but in Higher Piles Copse, a wood at an intermediate elevation, and in the valley woods of the same area, it grows in quite open positions on the boulders of the wood floor (Proctor 1962).

In the Sussex Weald H. tunbrigense colonies are found on the vertical or slightly oblique walls of deep fissures in outcrops of Tunbridge Wells (Neocomian; Wealden) sandstone; the colonies are usually restricted to one of the two walls. These Sussex habitats (and the former site near Tunbridge Wells) are in many respects very like those in Luxem- bourg where the species has a similar pattern of distribution (Assa 1924; Klein 1916, 1926; Heuertz 1933). Assa states that in Luxembourg and in Saxony the colonies do not extend up rock faces for more than one-third of their height; in Britain colonies are not thus restricted, except in some sites which are liable to long periods of desiccation. There is some discrepancy between Assa's and Klein's account of the Luxembourg sites, but it seems that most of the sites have a northerly aspect and none faces south. The study by

P. W. RICHARDS AND G. B. EVANS 249

Evans (1964) of the aspect of twenty-three Caernarvonshire localities, each with several H. tunbrigense sites, showed that seventeen localities had a N, NE or NW aspect; the remaining six mostly faced SE, but in all of these the effect of aspect was modified by local features, e.g. three were in deep gorges near waterfalls.

In localities where both H. tunbrigense and H. wilsonii occur as epiphytes, as in south- west Ireland, H. tunbrigense usually grows on the lower part of the tree trunks and H. wilsonii at higher levels, often extending on to the branches; the zones of the two species meet but scarcely overlap (see under H. wilsonii, pp. 261-2).

Temperature. Throughout the British Isles mean and maximum temperatures in both summer and winter are probably well within the limits of survival of the species, though it is possible that at its northern and upper altitudinal limits it is in some way restricted by temperature, perhaps by an insufficient average number of day-degrees above some threshold value.

No British localities have a mean July temperature of more than 170 C or a mean maximum of more than 220 C. Localities in Italy and elsewhere in continental Europe doubtless have much higher summer temperatures. Doignon (1954) found that in 1947 plants transplanted from Finistere to Fontainebleau near Paris survived respectively a maximum of 40 and 44 successive days in which 30? C was exceeded. In cultivation in the laboratory, growth appears to be normal at an average temperature of c. 17? C, provided that a high humidity is maintained, though sori are rarely produced under these conditions.

The mean January temperature at all British localities is probably above 3 90 C and the mean minimum above about 10 C. The lower limit of temperature tolerance of the species is much lower than this. Johnson (Sowerby & Johnson 1859), who cultivated H. tunbrigense, noted that it was 'little if at all susceptible to cold'. Very low temperatures were tolerated in the localities in Saxony and by Doignon's transplants at Fontainebleau which withstood a minimum of -20? C in 1954; between 1946 and 1954 they survived 600 days of frost and long periods without thaw, while in three of these years there were 5 consecutive weeks with daily frosts and in two of them the plants were enveloped in a thick covering of ice for periods of 15 days. It is not clear from Doignon's paper whether his temperatures were measured in situ or taken from meteorological records. In North Wales snow cover for periods of up to a month has produced no ill effects (see Vc).

Rainfall and potential evapo-transpiration. With the exception of the Sussex localities, which have an average rainfall of about 30 in. (75 cm) and at least 175 rainy days, all British localities have 40 in. (100 cm) or more and the majority have over 60 in. (150 cm) and at least 225 rainy days. These figures are however of little importance, as in the drier areas of Britain habitats otherwise suitable for H. tunbrigense are lacking.

No detailed information is available about the rainfall in the continental localities, but in all of them it is probably 75 cm or more.

Information about potential evapo-transpiration in the British Isles is insufficiently detailed for a correlation with the distribution of H. tunbrigense but an examination of Green's (1964) map of the annual average potential water deficit (difference between the potential evapo-transpiration and rainfall) shows that most of the British localities (but again with the exception of the Sussex localities) lie in the areas with less than 3 in. (7 6 cm) water deficit.

Wind. Although it grows in coastal and mountainous areas subject to high winds, the sites actually occupied by H. tunbrigense are usually sheltered by trees, rock outcrops and other topographical features.


Microclimate. See H. wilsonii, 11 (a). Assa (1924) gives data on microclimatic conditions in Luxembourg.

(b) Subgtratum. H. tunbrigense commonly grows as a carpet over otherwise more or less bare rock surfaces, but in very humid situations it occurs on the ground among sphagnum, Leucobryum glaucum and other bryophytes covering peat or uncompacted humus, e.g. in west Scotland and in several woods near Maentwrog, Merioneth, or as an epiphyte on trees and shrubs (Quercus petraea, Ilex aquifolium, Calluna vulgaris, Vaccinium myrtillus), often associated with Igothecium myosuroides, e.g. near Betws-y-Coed, Caernarvonshire (Griffith 1895), in Coed Cymerau and other woods in Merioneth, and, often in great abundance, in Derrycunihy Wood, Killarney and elsewhere in south-west Ireland (see under H. wilsonii, Ila). Near Batumi, U.S.S.R., it also grows as an epiphyte as well as on rocks and is found on trunks of Alnus barbata and Prunus laurocerasus up to 2 m above the ground (Burchak-Abramovick 1962).

In its British localities H. tunbrigense grows on sandstone, granite, quartzite and many other types of rock of varied physical and chemical characteristics. The nature of the rock seems to be of no great importance, provided it is fairly hard and its texture, surface and position allow it to remain moist for long periods. Certain types of sandstone seem to be particularly favourable, probably because of their porosity and water- holding properties which provide a store of water in drought periods. Perhaps for this reason sandstone rocks are the usual substratum in areas such as the Sussex Weald, Luxembourg and Saxony which appear to be only just within the plant's limits of climatic tolerance.

H. tunbrigense grows most often on acidic rocks and Taylor (1960) states explicitly that it is not found on limestone: though this seems to be true, there is no evidence as to whether this is because the plant is a calcifuge or because the physical properties of limestones are unsuitable. Near Derryclare Lodge, Connemara (west Ireland) it grows on basic igneous or metamorphic rock (see III).

Under old, established colonies on rocks, raw humus or peat consisting of partly decayed leaves and rhizomes may accumulate to a depth of up to 3 cm: the whole mass acts as a ranker turf and becomes easily detachable, especially when dry (see Vc).

III. Communities. In the British Isles H. tunbrigense grows on rock faces and trees in various types of deciduous woodland (mostly Quercetum petraeae, but occasionally Quercetum roboris or Fraxinetum. Since its immediate associates are chiefly bryophytes and other ferns and the dominant tree appears to affect it mainly through its influence on the microclimate, it can perhaps best be regarded as belonging to one or more 'associules' or synusiae consisting mainly of bryophytes which form part of various woodland associations in certain topographical situations. In their account of the vegetation of Ireland, Braun-Blanquet & Tiuxen (1950) include both H. tunbrigense and H. wilsonii as differential species of the Isothecium myosuroides sub-association of the Blechno- Quercetum and of the Hymenophylletum.

In the very humid areas in west Ireland and west Scotland where Hymenophyllum tunbrigense occurs on hillsides without tree cover it forms part of the Festuca-Agrostis (and probably other) moorland associations.

Table 1 gives lists of species for a few selected stands including Hymenophyllum tunbrigense in North Wales and elsewhere. Higher plants other than trees commonly associated with it include mainly acidophilous species such as Anthoxanthum odoratum, Blechnum spicant, Calluna vulgaris, Deschampsiaflexuosa, Digitalis purpurea, Dryopteris pseudo-mas (D. borreri), Erica cinerea, Lonicera periclymenum and Vaccinium myrtillus.


Associated bryophytes include Sphagnum spp. (especially S. quinquefarium), Leucobryum glaucum, Dicranum majus, Thuidium tamariscinum and a very large number of hepaticae and other mosses, many of them Atlantic in distribution. Two of the most constant and abundant associates (very seldom absent) are Diplophyllum albicans and Isothecium myosuroides. In Luxembourg Lefort & Lawalree (1951) and Gams (1938, p. 404) include it in the Diplophylletum albicantis. Lists for the bryophyte 'associules' in which Hymeno- phyllum tunbrigense occurs in Derrycunihy Wood, Killarney, are given by Richards (1938). Paton (1954, 1956) gives some information on the associated bryophytes and their succession on the sandstone rocks of Sussex.

At Derryclare Lodge, Connemara, where H. tunbrigense occurs on boulders of basic rock in a wood of Quercus petraea and Corylus avellana (with some Fraxinus excelsior and Taxus baccata) it is associated with Ctenidium molluscum, Marchesinia mackaii and Plagiochila asplenioides, bryophytes usually regarded as more or less calcicole.

Hymenophyllum tunbrigense rarely appears to be in direct competition with the higher plants associated with it. It is able to maintain itself when Vaccinium myrtillus is rooting through it and various grasses are overshadowing it. Bryophytes seem to compete with it with varying success: it seems to coexist with Leucobryum glaucum more easily than Hymenophyllum wilsonii (Evans 1964) but on the steep slopes of wet, wooded ravines in North Wales it is sometimes overgrown by Sphagnum quinquefarium and dead or dying remains of Hymenophyllum tunbrigense can be found beneath the moss blanket. Competi- tion with H. wilsonii is dealt with under the latter species (III, p. 263).

Jovet (1933) and Turmel (1952) give some account of its associates in the West Pyrenees. Reichling (1954) gives lists for Luxembourg, and Gaume (1944) for Finistere.

IV. Response to biotic factors. Most of the effects observed are due to human activities. Tree felling is usually harmful through its effects on the microclimate, and has been regarded as the chief cause of extinction of Hymenophyllum in the localities in Saxony (Wilpert 1937) and in some Sussex localities (Rose 1957). No doubt many other sites in Britain have been lost for the same reason in past centuries.

Burning, if light, is not always fatal: recovery from rhizomes has been observed when all the leaves have been killed by fire (Evans).

V. (a) Gregariousness. Highly gregarious, typical stands consisting of numerous colonies or small patches. Each of these is presumably a single individual or a clone arising from one spore (but see Vlllc). The patches are often very irregular in shape and frequently form long horizontal strips at the base of cliffs, e.g. 3 x 075 m near Llanberis, Caerns., 22 x 1 ft (7 x 0 3 m) in South Carolina (Taylor 1938): they may be as much as 150 ml in area, e.g. at Betws-y-Coed, Caerns., Coed Crafnant, Merioneth. Praeger (1934) records patches of 24 ft2 (2-2 m2) in Donegal and 50 ft2 (4-6 m2) at Killarney in Ireland. The leaves occasionally grow scattered through grass and bryophytes, e.g. in North Wales and in the Western Isles, Scotland.

(b) Performance in various habitats. Performance varies with atmospheric humidity (liability to desiccation) and light. Under average conditions the mass of foliage (described by Smith & Sowerby (1841) as like 'half-ruffled plumage') is about 4 cm deep. In exposed sites this may be reduced to about 1 cm, but in environments combining constant high humidity and good illumination it may be 6 cm, some leaves reaching 15 cm. The maximum density of sori is found under conditions of moderate exposure, i.e. severe enough to reduce leaf expansion. In very moist shaded situations sori are not usually developed. Sporangia and spores seem more sensitive to desiccation than the leaves themselves and frequently show signs of damage, though the slow basipetal


Table 1. Species lists for British Hymenophyllum tunbrigense and H. wilsonii localities

Site 1 2 3 4 5 6 7 8 9 Hymenophyllum tunbrigense la - - 30 - - ? - -

H. wilsonji la o la - ? ? - ? Anthoxanthum odoratum If - sd - Calluna vulgaris r f Deschampsia flexuosa If r Galium saxatile o f Oxalis acetosella o 0 - + - - -

Potentilla erecta r - r Saxifraga spathularis - r 20 - ? + Sedum anglicum r o o Blechnum spicant va If o Dryopteris dilatata a o Lycopodium selago y r yr Polypodium vulgare agg. o r Dicranum majus o - - ? D. scoparium o o + - - - -

Hookeria lucens r? ? ? ? ? ? + - -

Isopterygium elegans la r Isothecium myosuroides va - - ? 60 -

Plagiothecium undulatum If - la - Pleurozium schreberi y r o - - - -

RhJacomitrium fasciculare a? ? ? ? ? ??+ - R. lanuginosum d? ? ? ? ? ??+ - Rhytidiadelphus loreus va r a ? Sphagnum palustre o o o S. plumulosum o - la Thamnium arbusculum (T. alopecurum) - - ? ? + ? + Thuidium tamariscinum va - o Diplophyllum albicans la If - ? -

Peia epiphylla if? ? ? ? ? ?? - -

Plagiochila spinulosa la - - ? + - + Scapania gracilis i a r

Details of sites and of species occurring in only one list. Site 1: Cerrig-y-rhiwdwr, Tremadoc, Caernarvonshire. Ground and rocks under 100 m.

Hymenophyllum tunbrigense in lower parts of stream valley in wood and H. wilsonii in upper parts. (Quercus petraea cd, Q. robur cd, Betula pubescens r, Corylus avellana Isd, Fraxinus excelsior f, Ilex aquifolium o, Sorbus aucuparia a, Hedera helix f, Lonicera periclymenum f, Conopodium majus r, Dactylis glomerata vr, Deschampsia cespitosa 0, Digitalis purpurea vr, Epilobium montanum vr, Geranium robertianum vr, Hieracium umbellatum agg. vr, Hypericum pulchrum vr, Luzula sylvatica la, Lysimachia nemorum vr, Melampyrum pratense la, Rubus fruticosus agg. f, Silene dioica vr, Solidago virgaurea, r, Umbilicus rupestris o, Vaccinium myrtillus r, Athyrium filix-femina f, Dryopteris pseudo-mas va, Pteridium aquilinum r, Thelypteris limbosperma o, Cynodontium bruntonii + Diphyscium foliosum +, Hedwigia ciliata +, Heterocladium heteropterum + Hylocomium armoricum la, Hypnum cupressiforme f, Mnium hornum If, Plagio- thecium succulentum +, Polytrichum formosum If, Rhabdoweisia denticulata ?, Sphagnum recurvum o, Thuidium delicatulum +, Calypogeia trichomanis o, Plagio- chila punctata +).

Site 2: Cam Madryn, Caernarvonshire. Boulder scree, north-facing at c.320 m. Hymeno- phyllum wilsonil among boulders. (Andreaea rupestris o, Hypnum cupressiforme var. ericetorum if, Polytrichum piliferum vr, Rhacomitrium heterostichum f, Lophozia ventricosa r).

Site 3: Connor Pass, Dingle, Co. Kerry. Grazed grassland on north-facing slope at c.410 m. Hymenophyllum wilsonii in scattered patches, mostly on rock and in more heavily grazed parts. (Achillea millefolium f, Agrostis tenuis cd, Carex caryophyllea vr, Epilobium nerterioides If, Erica cinerea o, Euphrasia frigida


Table 1 (cont.) (det. P. F. Yeo) o, Festuca ovina o, F. vivipara cd, Jasione montana o, Juncus squarrosus la, Luzula campestris f, Polygala serpyllifolia r, Ranunculus acris vr, Rumex acetosa r, Sagina ciliata lf, Trifolium repens la, Veronica officinalis vr, Viola riviniana r, Breutelia chrysocoma o, Polytrichum commune la).

Site 4: Derrycunihy Wood, Killarney, Co. Kerry. Sloping rock face. July 1952 (C. D. Pigott). (Bazzania trilobata +, Lejeunea flava +, Lepidozia pinnata +).

Site 5: Derrycunihy Wood, Killarney, Co. Kerry. Branches of Quercus petraea near to site 4. July 1952 (C. D. Pigott). (Dicranodontium denudatum +, Zygodon viridissimus +, Frullania tamarisci +, Lobaria laetevirens +, L. pulmonaria +, Stictina silvatica +).

Site 6: Lough Dreenaun, Waterville, Co. Kerry. Boulders beside the waterfall. May 1950 (C. D. Pigott).

Site 7: Lough Dreenaun, Waterville, Co. Kerry, wetter bases of boulders at site 6. May 1950 (C. D. Pigott).

Site 8: above Coomcallee, above Lough Currane, Waterville, Co. Kerry. Boulders at the summit of the ridge. May 1950 (C. D. Pigott). (Andreaea rothii +, Hylocomium splendens J, Herberta hutchinsiae +, Pleurozia purpurea +, Sphaerophorus globosus +).

Site 9: between Kidstones and Beech Hill, Long Sleddale, Westmorland. Wood of Quercus petraea; Hymenophyllum wilsonii in cushions of Diplophyllum albicans with other associated species. April 1955 (C. D. Pigott). (Amphidium mougeotil +, Fissidens osmundoides +, Blepharostoma trichophylla +, Coenogonium ebeneum +).

Lists 4-9 were made in areas of 1 m2 or less.

sequence of ripening of the sporangia often ensures that at least some sporangia in each sorus reach maturity.

The number of sori per leaf in one growth flush rarely exceeds ten, but several groups of sori per leaf are not unusual in constantly moist habitats. The number of sporangia per sorus and of spores per sporangium are relatively constant (Bower 1926).

(c) Effect offrost, drought, etc. Damage by low temperatures has not been reported and is unlikely in view of the observations from France and Germany quoted in 11(a). In laboratory experiments plants exposed to subzero temperatures for several days showed no damage. During long frosty periods with little snow cover extensive damage may occur, especially if there is bright sunshine from cloudless skies, but this is probably due to desiccation rather than to freezing. In the exceptional frost in Snowdonia in the early months of 1963 which lasted for some weeks perhaps one-third to half of the colonies lost most of their leaves. Snow-covered colonies survived better than those which were more or less without protection from snow, but, where colonies were subjected to alter- nate freezing and thawing, damage was increased. In dry frosty weather colonies may be s riously damaged by the peeling off of large portions from the rock surface.

VI. (a) Morphology of sporophyte. The leaves are borne on a system of extensively creeping, much branched and extremely slender rhizomes, which bear rather sparse adventitious roots. The vascular system of the rhizome is protostelic and contains only a very small number of water-conducting elements.

The most important morphological features of the leaves from an ecological point of view are their capacity for indeterminate apical growth (see VIc) and the structure of the lamina, which, as in most Hymenophyllaceae (cf. Bower 1926), is only one cell thick, except at the veins, and entirely lacking in stomata. A cuticle is present but it is very thin and not easy to demonstrate. There is no morphological difference between sterile and fertile leaves.

I J.E.


(b) Mycorrhiza. Not reported. (c) Perennation and vegetative reproduction. Herbaceous chamaephyte, perennating by

means of its rhizome system. Reproduction is probably by spores (see Vllla) and rarely by the breaking away of parts of the rhizome with attached leaves. Clowes (quoted in Moore & Lindley 1855) claimed that in cultivation the leaves were annual and many floras have repeated this statement, but our observations show that in nature and in laboratory cultivation they often persist for more than 1 year and are capable of continued growth. In many leaves there are flushes of growth in moist warm periods in both summer and winter. Not infrequently leaves can be found with spaced clusters of indusia and sometimes these can be dated back for as much as five seasons. Though the fronds may be potentially of unlimited growth, the great majority probably seldom persist for more than 2 years owing to droughts which kill the growing apex and often the whole leaf.

Vegetative spread is very slow; the maximum rate observed in Snowdonia is 6 cm in 3 years, with an average of 2 5 cm (10 rhizomes). One set of marked rhizomes was killed by drought after 12 months, but not one had extended more than 2 cm during this period. Colonies in Sussex marked by Mrs J. Paton had nearly all completely died when exam- ined by Evans 10 years later (1963). Longevity of colonies or clones is unknown, but is undoubtedly longer than a decade and may even attain several centuries.

(d) Chromosomes. 2n = 24 (Litardiere 1921) (material from France); n = 13 (Manton 1950) (from Loch Lomond, Scotland). The chromosomes, according to Manton, show a wider range of size than those of H. wilsonii.

(e) Physiological data. The following observations are summarized from Evans (1964). Water relations. In detached leaves placed in a series of controlled humidities (76,

43, 32, 15, 10 and 0 % R.H.) at 200 + 10 C a steady state was reached in c. 2 h at about 20-50% of the initial fresh weight (cf. 65-75 % in a similar experiment on H. wilsonii): the final weight fell progressively with the relative humidity (cf. H. wilsonii in which approximately the same final weight was reached at all humidities). When the leaves were returned to a high humidity (simulating night conditions after a dry day) water was absorbed from the atmosphere and the original fresh weight was regained in 10 h (cf. H. wilsonii which was still 20-60 % below the original weight after 10 h and apparently in a steady state). Mean rates of loss varied between 23-36% fresh weight per hour (cf. 40-57 % in H. wilsonii) and rates of uptake from saturated air varied from 2 7-2 9 % (cf. 1 9-2 0% in H. wilsonii).

The differences between the two species both when losing and gaining water suggest that the greater drought resistance of H. wilsonii depends not only on its protoplasm surviving desiccation better, but on the cells avoiding severe diurnal mechanical stress during drought periods. H. tunbrigense fails to avoid these stresses and appears to have even less control over water loss during changing ambient conditions.

The sensitivity of H. tunbrigense to desiccation over periods up to 11 days was mani- fested in progressive damage to the cells both with length of exposure at a given humidity and with saturation deficit at a given length of exposure. After 5 days at 50 % R.H. at 200 C about half the cells were killed. In H. wilsonji under similar conditions less than 10% of the cells were killed after 11 days: even after 5 days at 0% R.H. damage was slight.

Exceptional drought resistance, approaching that of H. wilsonii, was found in an isolated colony at 1000 ft (305 m) in a site in Snowdonia (Caernarvonshire) unusually exposed to drought and direct sunlight. Leaves kept at continuously high humidity in the labora-


tory developed an increased sensitivity to drought. Drought resistance in H. tunbrigense appears to be a plastic response at cellular level, which when combined with the plant's great morphological plasticity may confer unusual drought resistance on colonies in exceptional habitats.

In normal growth forms it was found that when plants are exposed to alternating periods of low humidity and saturated atmosphere there was little difference in the amount of damage compared with that of plants exposed to continuous periods at the same low humidity. On the other hand cell mortality was greatly increased if the plant was moistened by direct contact with liquid water. These experiments suggest that damage to the plant depends less on the total length of time that it is desiccated than on the duration of the periods of increasing or decreasing hydration, a rapid change of water content being more harmful than a gradual one. The cell damage which occurs when the plant is moistened by liquid water when desiccated may be a reason why H. tunbrigense does not occur near the spray zone of waterfalls and in the close proximity of running water.

The drought resistance of the gametophyte is very low, contrasting with that of H. wilsonii. In one experiment a large proportion of H. tunbrigense gametophytes died under conditions which caused less than 15 % mortality among gametophytes of H. wilsonii.

Light and carbon assimilation. Compensation points were investigated by the method of Lieth (Lieth & Ashton 1961). Samples of H. tunbrigense from Caernarvonshire localities were found without exception to reach compensation at lower light intensities than samples of H. wilsonii from nearly adjoining sites. In field samples the compensation point of H. tunbrigense was quite variable and lower in plants from more shady situations than in those from better-illuminated places. In experiments in which the compensation point at successively lower temperatures from 200 to 500 C was measured in plants of the two species which had been cultivated for some time under closely similar conditions it was found that in H. tunbrigense the compensation point remained constant while in H. wilsonji it gradually fell until at 500 C it almost equalled that of H. tunbrigense.

The consistently lower compensation point of H. tunbrigense as compared with H. wilsonii appears to explain the greater tolerance of the former for very poorly illuminated sites (see IIa above). The differential effect of temperature on the compensation point in the two species may throw light on the tendency of H. wilsonii to replace H. tunbrigense at higher altitudes.

At high light intensities the photosynthetic pigments of Hymenophyllaceae, especially xanthophyll, are damaged (Gessner 1940).

In an experiment (Richards & Jones, unpublished) in which leaves of the two species were kept in the dark until starch-free and then exposed to daylight in cages giving different degrees of shading, starch reappeared in the leaves of H. tunbrigense at lower daylight values than in those of H. wilsonii.

VII. Phenology. The growth flushes which occur under favourable weather conditions have already been mentioned (VIc). These may occur at any time of year, though a main flush usually develops in spring and a minor one in late summer or autumn. This is similar to the behaviour of many European mosses (Hagerup 1935). In Doignon's transplants at Fontainebleau (see II) the leaves were completely renewed four times during the 12-year period of observation. Indusia begin to appear soon after the expansion of the first few pinnae of a new flush. As growth of the leaf apex may continue for several seasons, successive groups of indusia may be found, each corresponding to one flush. Plants with ripe sporangia can be found over a long period of the year, partly owing to the gradate development of the sorus and partly owing to the sensitivity of individual


colonies to conditions in their immediate environment. Ripe spores have been found in every month of the year at one site or another in North Wales, but in most colonies the peak of spore production is in autumn and winter.

No information is available on the phenology of the gametophyte. VIII. (a) Reproduction of the sporophyte. Sporangia about thirty per sorus, with a

'typical number' of 258-512 spores per sporangium (Bower 1926). The potential output of spores per sorus may thus reach 7500-15000 (cf. about 3000 in H. wilsonii). Though spores are produced in abundance, young, newly-established sporophytes have not been found in spite of careful search, and effective reproduction, at least under present conditions in Great Britain, seems to occur only vegetatively. Germinating spores and very young gametophytes are often to be found inside the indusia, especially in very moist conditions or wet seasons, but no older gametophytes have been seen in nature. In the woods of south-east Ireland, where H. tunbrigense can sometimes be found as an epiphyte on quite young trees, establishment from spores must be presumed to take place. Apospory has not been reported.

(b) Discharge and dispersal of spores. The sporangia ripen in basipetal succession. During dry weather the indusial valves gape open exposing the sporangia: in droughts immature sporangia may be killed in this way. The sporangia have an oblique incomplete annulus and dehisce laterally but attempts to catch airborne spores near colonies with ripe sporangia have failed. Ripe spores are usually 3-celled and contain numerous chloroplasts. As young gametophytes are often found inside the indusia, spore discharge often takes place inside the closed indusium by breakdown of the sporangial wall, perhaps brought about by the internal pressure of the dividing spores. Dispersal may therefore be effected chiefly by rain-wash.

(c) Viability and germination of spores. Spores remain viable for long periods if kept at very high humidities under laboratory conditions (moderate temperature and illumination). Germination and subsequent growth of the prothallus is extremely slow. In laboratory culture not one gametophyte reached a 40-cell stage in 12 months.

(d), (e) Morphology and reproduction of gametophyte. In cultures the spores were usually, but not invariably, found to grow out in the 'tripolar' form which Goebel (1930) regards as characteristic of the genus. A 2-month-old gametophyte is shown in Fig. 3. By 18 months none had produced sex organs. The cells of the gametophyte are similar in size to those of the leaf lamina and are smaller than those of the gametophyte of H. wilsonii. Mettenius (1864) described a leaf-like prothallus: after 11 years it was sterile and only 1 mm long. Janckewski & Rostafinski (1875) give an account of gametophytes grown from spores collected in France near Cherbourg. These were ribbon-like when mature and produced sex organs: on two gametophytes attached embryos were seen. The morphology of the gametophytes and its reproductive organs in other species of Hymeno- phyllum have been described by Stokey (1940, 1948) who observed archegonia on the dorsal surface. Apogamy has not been reported.

Reproduction by gemmae is frequent in the Hymenophyllaceae, but does not seem to have been observed in this species.

(f) Hybrids. None known. IX. (a) Animal feeders or parasites. Nibbled leaves have been seen in North Wales,

suggesting feeding by small invertebrates. Slugs and woodlice have been observed feeding by day on the leaves, indusia and sporangia.

(b) Plantparasites. None observed. In North Wales and in Ireland epiphyllous hepaticae (mostly Lejeuneaceae) are occasionally found on the leaves. Duvigneaud (1945-1962)


lists epiphyllous hepaticae and lichens found on herbarium specimens of H. tunbrigense and H. wilsonii from Belgium and Luxembourg.

(c) Diseases. No fungal or other diseases have been reported. X. History. First reported as a British plant by Ray (1686). Found some years pre-

viously by Daire at the High Rocks, Tunbridge Wells, Sussex, and later shown to Ray by Doody (see Evans & Jermy 1962).

(b) (a)

0000

00 0~~~~~~~~~

FIG. 3. Germinating spores and young gametophyte of Hymenophyllum tunbrigense (L.) Sm. (a) Germinating spores in sorus, x 400, from Pistyll Cain, Merioneth. (b) Young

gametophyte in sorus, x 180.

Spores of Hymenophyllaceae belonging either to H. tunbrigense or to Trichomanes speciosum, associated with larger spores attributed to Hymenophyllum wilsonii, were found by Jessen, Andersen & Farrington (1959) in a deposit at Gort, Co. Galway, Ireland, regarded as dating from the Hoxnian Interglacial. In a deposit of the Hoxnian Inter- glacial at Kilbeg, Co. Waterford, Watts (1959) discovered numerous spores of H. tunbrigense and two sporangia with spores inside. The accompanying tree flora at Kilbeg includes evergreen trees associated with Alnus and Betula and resembles the present-day forest flora of the western Caucasus (where Hymenophyllum tunbrigense is still found): the herbaceous species belong to the Western Oceanic element. Spores, probably of H. tunbrigense, are frequent in Hoxnian Interglacial deposits at Nechells, Birmingham, during a stage when Picea and Abies were dominant: they are absent or rare in the succeeding Pinus phase (M. R. Kelly, personal communication). Remains of Hymeno- phyllaceae, dating from the same Interglacial, have been found in the Biarritz district in the western Pyrenees, an area where Hymenophyllum tunbrigense occurs today (Oldfield, in West 1961).

H. tunbrigense is undoubtedly a relict species in Europe and its present area, which is still decreasing and becoming increasingly discontinuous, must be the remains of what was once a larger and much more continuous range. Its concentration in the British Isles and the extreme west of continental Europe with widely scattered outlying stations further east is similar to the distribution of many Oceanic bryophytes. As has been suggested for the latter (Richards 1957), the most recent period in which H. tunbrigense occupied


a wide and more or less continuous range may have been the post-glacial climatic optimum (though it may well have occupied a still larger area in the warm Interglacials and even as far back as the Tertiary). Since this optimum period its area has become restricted and fragmented by climatic deterioration, and in recent centuries by the destruction of its habitats by man, through forest clearance, etc. (cf. Rose 1957).

The distribution of H. tunbrigense in Europe differs from that of H. wilsonii in being more southern and in having outlying relict stations (Sussex, Luxembourg, Saxony, Adzharia, etc.) far from the main area. The ability of H. tunbrigense to persist in these outlying stations probably depends on its tolerance of extremely poorly-illuminated habitats. This has enabled it to survive in deep crevices in sandstone rocks, etc., where the humidity and temperature regimes are suitable for it even in regions like Saxony where the macroclimate is far outside its limits of tolerance. H. wilsonii may also have once occupied a much larger area than it does now, but it has not survived in widely dispersed relict stations because its relatively high light requirement excludes it from the shady crevices which provide refuges for H. tunbrigense. Though more drought-resistant than the latter, H. wilsonii is not sufficiently tolerant of desiccation to be able to grow in the relatively well-illuminated situations which its light requirements demand except in regions of high atmospheric humidity (western Britain, north-west France, west Norway).

(For References see under Hymenophyllum wilsonii).

List Br. Vasc. P1. (1958) No. 7.2

Hymenophyllum wilsonii Hooker (H. peltatum auct. vix (Poir.) Desv., H. unilaterale auct. vix Bory ex Willd.)

A mat-forming bryophyte-like perennial, resembling H. tunbrigense in general appearance and size, but differing as follows. Leaves more erect and less closely appressed to the substratum, dark olive green, becoming blackish, never bluish-green when old, thicker in texture, less translucent and more rigid, with the pinnae curved back (especially when dry) and overlapping so that the lamina is not flat and appears narrower relative to its length: surface slightly shiny. Pinna segments fewer (3-5 instead of 5-11), the vein reaching the apex on mature segments. Leaf cells larger (94+2 x 39+1 ,um), more regular in shape and with more chloroplasts (c. 60-80 in surface view, cf. 30-40 in H. tunbrigense). Sori up to 10-15 per leaf, appearing stalked; indusia pear-shaped with strongly convex valves, the margins slightly overlapping, quite entire. Spores 62-78 ,um in diameter, more finely ornamented than those of H. tunbrigense.

Varies little, except in size; no subspecies or varieties have been described. Though H. wilsonii is very distinct and never intergrades with H. tunbrigense, the

specific status of H. wilsonii was disputed for many years after plants which would now be assigned to it were first described by Ray (1686). Ray subsequently saw specimens of H. tunbrigense from Kent, but regarded them as belonging to the same species until Plukenet later persuaded him that they were distinct (Evans & Jermy 1962). Linnaeus and others afterwards again combined them and it was not until 1830 when the bryolo- gist W. Wilson (who knew both species well in the field) had drawn Hooker's attention to them that their distinctness was clearly established (though Hooker subsequently united the two species once more). Taxa closely resembling the European H. wilsonji are found


in the Southern Hemisphere and on mountains in the tropics: some have regarded H. wilsonii as a synonym of the earlier described H. peltatum (Poir.) Desv. or of H. unilaterale Bory ex Willd. from Reunion (Bourbon). As with H. tunbrigense, the relationship of the European and extra-European populations requires further investigation: meanwhile the name H. wilsonii can be legitimately used for this plant if it is regarded as a species confined to north-west Europe and the Atlantic islands.

0 1 2 3 4

HYMENO- ll PHYLLUM 9 9 2

WILSONII Hook.

Filmy Fern -___

* 1930 onwards t 0 0 02 o Before 1930 S Ld w

7 0~~~ 0

8 9 5 006

0 0 * * *0O 0~~~~~~~~

00 Soo

I0 00 9

* ~~~~~~~~~~~~~~0 0 000 _

3 0 I~~~~~~~~~~~~~~~~

3 4~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Native. Widely distributed and locally abundant in the west and north of Great Britain and in Ireland, growing on rocks, in turf among bryophytes on mountains and in some districts on trees, more tolerant of exposure and desiccation than H. tunbrigense.

I. Geographical and altitudinal distribution. Its area in the British Isles (Fig. 1) overlaps that of H. tunbrigense, but it is more widely distributed (recorded in more than twice as many 10-km grid squares and from fifty-six vice-counties in Great Britain and twenty- nine in Ireland compared with thirty-seven and twenty-five respectively). In southern England it lacks outlying stations in the south-east, all its English stations lying north of a line running from Start Point (Devon) to the mouth of the Tees, but in Wales, north

260 Hymenophyllum wilsonii Hooker

England and Scotland it extends further east: it also extends further north (to Shetland). Like H. tunbrigense it has sometimes been recorded in error owing to confusion with liverworts.

In Europe outside Britain it is known only from the Faeroes, Norway and north-west France (Fig. 2). In Norway it is found along the west coast from Lista to Stad with one outlying station further north at Stemshesten (630 N) (see map and comments in Faegri

6 30 24 70 18' 12 18'E 12 o 24: 30 36 42' 48' 54I. _ 60-- 6 - t 7 2 5?

60'

630'w1 - 0r 6

FIG. 2. The distribution of Hymenophyllum wilsonii Hooker in Europe. 0, Native occurrence; x, probably extinct. Reproduced by permission of the Committee for Mapping

the Flora of Europe.

1960). In France it has been recorded only from four localities in Finistere and Manche and in these it is less abundant than H. tunbrigense (Doignon 1947). The only extra- European populations which are fairly certainly conspecific with those in Europe are those in the Atlantic islands (Azores, Madeira, Canaries).

The altitudinal range in Britain is from sea level to 3300 ft (1000 m) on Macgillicuddy's Reeks in south-west Ireland (Hart in Scully 1916). In England to 2000 ft (610 m) in west Yorks; in Wales to 2000 ft (610 m) on Plynlimmon and at least 2850 ft (864 m) on Snowdon. In Caernarvonshire, the maximum number of localities, grouped in 500 ft intervals, is at 500-1000 ft (152-305 m) (see histograms in Evans 1964). In Scotland to 2500 ft (762 m) in Breadalbane and 2012 ft (613 m) in the Hebrides (Harris). In the


Faeroes thirteen out of the fifteen known stations are under 300 m (Hansen 1966). In Norway to at least 350 m (Lid, in Faegri 1960). The French localities all appear to be at low altitudes (Doignon 1947).

II. Habitat. (a) Climatic and topographical limitations. H. wilsonii is found on rock outcrops, on moss-covered boulders and among screes on shaded or exposed surfaces (often, but by no means always, vertical or steeply inclined), on the trunks and branches of trees and in turf among grasses and bryophytes. Its habitat range thus overlaps that of H. tunbrigense but extends into considerably more open conditions where it is more strongly illuminated and more subject to desiccation in dry weather. Like H. tunbrigense it often grows in crevices, deep holes among boulders, etc., but in general it appears less tolerant of deep shade and grows better in moderate shade or in the open provided the atmosphere is not too dry. Thus in contrast to H. tunbrigense it often grows in boulder- strewn ravines without permanent streams or on small rocky ridges in quite open woodlands. Where the two species of Hymenophyllum occur in the same locality, they are very rarely found intermingled; usually there are separate cushions, carpets or zones of each, H. wilgonii in general occupying the more exposed and well-illuminated situations and H. tunbrigense the more sheltered and darker ones. In north Wales, west Scotland, etc. it is common to find stream valleys in which H. tunbrigense is confined to the lowermost sheltered parts while H. wilsonii extends far into the upper and more open parts: the zone of overlap may be quite narrow. On trees there is a similar difference in the distribution of the two species (see below).

Like H. tunbrigense, it is usually absent in the spray zone of waterfalls, but seems rather more tolerant of running water and is occasionally found on rocks liable to flooding, e.g. at Rhaeadr Du, Merioneth.

The range of habitats occupied by H. wilsonii clearly depends partly on factors affecting atmospheric humidity, particularly the amount and distribution of the rainfall, exposure to wind and the frequency of cloud and fog: these factors would probably be best expressed in terms of potential evapo-transpiration, but adequate data are lacking. In areas of low humidity it tends to be restricted to sheltered gullies, etc., while in the most humid areas it may grow on rocks or in turf on hillsides fully exposed to sun and wind, e.g. on the Rhinogau, Merioneth, on Slieve More, Achill island (west Ireland).

In Caernarvonshire there is an interesting difference between the distribution of H. wilsonii in Snowdonia with an annual rainfall of over 60 in. (150 cm), where it occurs abundantly in a wide range of habitats and altitudes, and in the Lleyn Peninsula where the rainfall is 40-60 in. (100-150 cm): here it is confined to very small north-facing sites on rocks or screes near the summits of the hills (Yr Eifl, Carn Madryn, Moel-y-Gest, etc.) at about 200-550 m. This may be partly because much of Lleyn is cultivated and suitable ravines are lacking, but it probably also depends on the tendency of clouds to persist on the hill tops.

In Norway there is a relation between habitat range and climate similar to that in Britain (Faegri 1960, pp. 76-7). In the outer archipelago it grows even in exposed situations, covering large areas in pure stands. On the mainland it becomes increasingly exacting with distance from the coast, becoming restricted to moist and sheltered situations. Faegri concludes that the humidity of the air is the decisive controlling factor. In the fjord region of south-west Norway H. wilsonii is rather common on steep north-facing mountain slopes (Lye 1966).

In the British Isles H. wilsonii occurs as an epiphyte on trees only in very humid areas (south-west and south Ireland, Merioneth, west Scotland). Details of the distribution of


the two Hymenophyllum species in Derrycunihy Wood, Killarney, are given by Richards (1938). H. wilsonii occurs in large mats on the trees to over 30 ft (9 m) above the ground. In the part of the wood where the trees are tallest and the illumination at ground level lowest (Type 1) it is not found lower than 9 m, but in parts where the trees are smaller and the shade less deep (Types 2, 2a and 3), H. wilsonii descends on the tree bases and also grows on boulders. H. tunbrigense grows mainly on tree bases and boulders in all parts of the wood and never ascends far up the trunks. Where both species of Hymeno- phyllum occur on the same tree, H. wilsonii occupies a higher zone than H. tunbrigense and there is very little overlap.

A similar distribution of the two species can be seen at Coed Ganllwyd, Merioneth. The occurrence of H. wilsonii as an epiphyte at higher levels than H. tunbrigense is in

accordance with the evidence (pp. 254-5) that the former has higher light requirements and greater drought tolerance than the latter. The variation in the level to which H. wilsonii descends (and to which H. tunbrigense ascends) in Derrycunihy Wood is closely parallel to the variation in the levels at which epiphyte synusiae occur in different types of tropical rain forest (Richards 1952, pp. 121-2). Shreve (1911) observed similar variations in the height distribution of filmy ferns in the Blue Mountains in Jamaica.

Temperature. H. wilsonii has a more northerly range than H. tunbrigense in Britain and in Europe generally; it is absent from south-east England, the part of the British Isles with the highest summer temperatures. There is no experimental evidence that H. wilsonii is less tolerant of high summer temperatures and, as suggested under H. tunbrigense (X), the absence of H. wilsonii in south-east England may be due to a lack of habitats combining adequate illumination with a sufficiently humid microclimate. H. wilsonii is tolerant of low winter temperatures (see Vc) and it is unlikely that the normal temperature conditions exceed its limits of tolerance anywhere in the British Isles.

Rainfall and potential evapo-transpiration. All known British stations for H. wilsonii are in areas receiving 40 in. (1000 mm) or more annual rainfall, with at least 200 rainy days.

All British stations for H. wilsonii probably lie within the area with less than 3 in. (7 6 cm) annual average potential water deficit on Green's map (1964) (cf. H. tunbrigense, II).

Microclimate. Evans (1964) collected many data on temperature, relative humidity and vapour pressure deficit for sites in North Wales where H. tunbrigense and H. wilsonil grow near together. The results are consistent in showing that though H. wilsonii is often found in sites warmer and less humid than any occupied by H. tunbrigense the microclimate ranges of the two species frequently overlap.

(b) Substratum. Like H. tunbrigense, H. wilsonil grows on rocks very varied in geological age, texture and chemical characteristics. In Britain and in Norway (Faegri 1960) it is absent on typical limestones, such as Carboniferous limestone, whether because of their physical or their chemical properties, but it grows on basic volcanic tuffs, e.g. at Cwm Idwal and by Afon Dulyn (Conway Valley), Caerns. and Cader Idris, Merioneth. In the Carboniferous limestone area of the Burren, Co. Clare (Ireland) it occurs in one locality, as an epiphyte on Corylus avellana and on sandstone but not on limestone rocks (Webb 1963). In south-west Norway Hymenophyllum wilsonii is found over a pH range of 3 9- 4*3 (Lye 1966).

H. wilsonji frequently grows on dead plant material ranging from dead moss to peat and raw humus. As an epiphyte on living plants it usually occurs on Quercus petraea: in Derrycunihy Wood, Killarney, it has been observed also on Ilex aquifolium and the


stems of Calluna vulgaris and Vaccinium myrtillus. On Foula (Shetland) it is common on the bases of Luzula sylvatica and Juncus squarrosus tussocks (Hawksworth 1969).

III. Communities. Lists of species for communities including Hymenophyllum wilsonji are given in Table 1 (see under H. tunbrigense).

Because of its greater tolerance of exposure and temporary desiccation, H. wilsonji has more numerous and varied associates than H. tunbrigense and occurs in a wider range of plant communities. In the British Isles besides growing in the same woodland and moorland associations as the latter, it is not uncommon in various other communities, e.g. that of base-rich rock ledges and crevices on mountains (chomophyte and chasmo- phyte communities of Tansley 1949) where the associates include arctic-alpine species such as Saxifraga oppositifolia, S. stellaris and Thalictrum alpinum: it also occurs in bare rock communities where nearly all its associates are non-vascular plants. In St Kilda HIymenophyllum wilsonli occurs in Callunetum (Turrill 1927). Because of its greater tolerance for exposed and well-illuminated situations, H. wilsonii is more often associated with lichens than is H. tunbrigense.

It commonly grows intimately mixed with bryophytes and seems to compete success- fully with them; two of its most constant associates are Diplophyllum albicans and Rhacomitrium lanuginosum. One case was observed on a boulder scree at a high altitude in north Wales in which it had been overgrown by Sphagnum quinquefarium and laboratory experiments on competition between the two Hymenophyllum species and Leuco- bryum glaucum indicate that under very moist conditions Hymenophyllum wilsonji is less successful than H. tunbrigense (Evans 1964).

As has been stressed, the ecological range of the Hymenophyllum species overlaps, though their tolerances with respect to light and humidity are different. They usually occupy discrete areas and rarely grow in mixture. Where, as sometimes on trees in south-west Ireland and on rock faces in sheltered ravines in north Wales, they grow in close contact, the competitive balance between the two species must be determined by their different compensation points and drought resistance (see VId).

Gaume (1944) gives details of associated species in Finistere. In Norway its associates are chiefly bryophytes, including, as in Britain, Atlantic species such as Breutelia chrysocoma, Campylopus atrovirens, Isothecium myosuroides and Plagiothecium undulatum (Lye 1966). In south-west Norway Hymenophyllum wilsonii, together with various bryophytes, plays a part in a cyclical succession on rock surfaces. It may act as a pioneer in colonizing bare rock. Later the heavy carpets of H. wilsonii and bryophytes often peel off, leaving a bare surface to be re-colonized (Lye 1967).

IV. Response to biotic factors. In wooded habitats H. wilsonii is less affected than H. tunbrigense by tree felling and may survive in a depauperate condition for some time after the tree cover has been removed. It can survive light burning and regenerate from the rhizomes.

V. (a) Gregariousness. Forms colonies or patches which in north Wales and Ireland (Praeger 1934, 1950) may reach 10-20 ft2 (1-2 m2). In general the patches are smaller than in H. tunbrigense and do not exceed 2 ft2 (02 m2). The leaves tend to be more diffusely distributed than in H. tunbrigense and when the plant is growing among bryophytes and grasses they may be widely scattered. It is not known whether the patches are always clones or can arise from more than one gametophyte.

(b) Performance in various habitats. Performance varies greatly with the humidity of the habitat. In exposed sites, e.g. cracks in the rock face at Idwal Slabs, Cwm Idwal, Caerns., the leaves may be no more than 05 cm long, but in moist sheltered places the


plant may form loose hanging masses up to 10 cm deep with individual leaves up to 20 cm long.

Sori are most abundant on small leaves in conditions drier than the optimum for vegetative growth; they are usually absent on etiolated or very luxuriant leaves. The number of sori per leaf rarely exceeds fifteen, but several groups may be found on one leaf, corresponding to flushes of growth.

(c) Effect offrost, drought, etc. There is no evidence that low temperatures by themselves are harmful and the extensive damage to the leaves observed in Snowdonia (Evans) and in Scotland (Ratcliffe, personal communication) in the unusually long frost of early 1963 was probably due to the combination of severe frost and prolonged drought. In Nor- way there was no trace of frost damage during the severe winters of the early 1940's, even at its highest stations near Bergen at c. 250 m (Faegri 1960). Though droughts may do little physiological harm, they may bring about the peeling off of colonies from rock surfaces and thus lead to their destruction.

Observations show that H. wilsonji is less liable to damage by drought than H. tunbrigense.

VI. (a) Morphology of sporophyte. Similar in most respects to that of H. tunbrigense. The differences between the two species in the rate at which water is lost from and taken up by the leaves from a saturated atmosphere suggests that there might be differences in the fine structure of the cuticle or the outer cell walls, but no such differences have been detected with the light microscope.

(b) Mycorrhiza. No information. (c) Perennation and vegetative reproduction. Herbaceous chamaephyte, perennating by

means of its rhizome system. Reproduction is probably by spores (see VIIIa), and by fragmentation of parts of the rhizome or whole carpets. Clowes (in Moore & Lindley 1855) stated that the leaves were persistent for several years. Observations in north Wales on marked leaves in the field confirm that they are normally long-lived with growth flushes once or twice a year, depending on weather conditions; some leaves persist for at least 5 years. Vegetative spread over rock surfaces is slow; in a set of thirty-seven pioneer rhizomes marked in Merioneth not one grew more than 2 cm in 12 months (all were subsequently killed by droughts). Longevity of clones or colonies unknown.

(c) Chromosomes. n = 18; the chromosomes are 'the largest yet recorded in the Pterido- phyta' (Manton 1950, material from Borrowdale, Cumberland).

(d) Physiological data. The comparative physiology of H. wilsonii and H. tunbrigense (mostly based on Evans 1964) is discussed under the latter (VId). The ecologically most significant facts which have been established experimentally are: (i) the leaf cells of H. wilsonii are considerably more drought-resistant than those of H. tunbrigense; (ii) leaves of H. wilsonii both lose water to a dry atmosphere and gain water from a saturated atmosphere more slowly than those of H. tunbrigense; (iii) the gametophyte of H. wilsonji is more drought resistant than that of H. tunbrigense; (iv) at temperatures between 10 and 200 C, the leaves of H. wilsonli have a higher compensation point than those of H. tunbrigense, but at 5? C the compensation point is about the same in both species.

VII. Phenology. No differences have been observed in the phenology of H. wilsonji and H. tunbrigense. The leaves of the former appear to be longer-lived than those of the latter, but this seems to be due to the greater susceptibility of H. tunbrigense to damage by drought. There is no information on the phenology of the gametophyte of H. wilsonii.

VIII. (a) Reproduction of the sporophyte. In a sample from north Wales (thirty sori from five leaves) the average number of sporangia per sorus was twenty-five. The 'typical'


number of spores per sporangium is 128 (Bower 1926). The potential spore output per sorus is thus less than half that of H. tunbrigense (about 3000-4000, compared with 7500-15000). Production of sori is, however, more frequent than in H. tunbrigense: fertile leaves are relatively more numerous per colony (or per individual plant) and patches without at least some fertile leaves are rare. Though spores are produced in abundance, the evidence suggests that effective reproduction, as in H. tunbrigense, is mainly vegetative: neither free-living gametophytes nor very small sporophytes which might have arisen recently from gametophytes have been seen, though the occurrence of H. wilsonji on young trees, Calluna vulgaris stems, etc. in south-west Ireland is circumstantial evidence that reproduction from spores takes place. Apospory has not been observed.

(a) (b)

FIG. 3. Gametophytes of Hymenophyllum wilsonii Hooker. (a) x 70; (b) x 160.

(b) Discharge and dispersal of spores. As in Hymenophyllum tunbrigense. (c) Viability and germination of spores. The spores show a high percentage germination

under laboratory conditions. Germination often begins within the sporangium and germinating spores can sometimes be found within the indusium, but the older gametophytes have not been seen in the field. In culture, growth of gametophytes is not as slow as in H. tunbrigense.

(d), (e) Morphology and reproduction of gametophyte. Germinating spores may reach a three-celled stage before leaving the sporangium. Gametophytes in culture are shown in Fig. 3. The largest prothalli, which are strap-shaped, have numerous rhizoids and reached a length of 4 mm in 12 months. One prothallus produced a number of archegonia but subsequently died. Apogamy has not been reported and gemmae have not been observed.

(f) Hybrids. None reported. IX. (a) Animal feeders or parasites. Traces of feeding by small animals have been ob-

served on plants in north Wales and woodlice and slugs have been seen eating the leaves, but the amount of damage is very small.

(b) Plant parasites. None observed. Epiphyllous lichens and hepaticae are sometimes found on the leaves (see under H. tunbrigense IXb).


(c) Diseases. None known. X. History. First record: described by Ray (1686) as 'Adiantum petraeum perpusillum

anglicum foliis bifoliis vel trifidis Newtoni' from plants sent to him by N. Newton from Buzzard Rough Crag near Wrenose [Wrynose] in the Lake District.

Interglacial finds of Hymenophyllum spores, some of which may belong to H. wilsonii, are mentioned under H. tunbrigense (X). The only fossils definitely identified as H. wilsonji are two spores and three sporangia in fine detritus mud containing Salix and Pinus wood from Pollen Zone VIc at Wybunbury Moss, Cheshire (H. J. B. Birks, in litt.).

The conjectural history of Hymenophyllum wilsonii has already been discussed above under H. tunbrigense (pp. 257-8). It has a less fragmented area than the latter and no outlying relict stations in Europe far outside its range in the British Isles, north-west France and west Norway. Though there is no direct evidence that it formerly occupied a much larger area than it does now, it nevertheless seems plausible that in some moist climatic period (the post-glacial climatic optimum or earlier) it extended further east than at present. If this were so, its absence in the Sussex Weald and in continental Europe (except in France and Norway) would be difficult to account for except by assuming that owing to its high compensation point (and relative intolerance of deep shade), which excludes it from the kind of habitats in which H. tunbrigense has been able to survive, suitable refuges for it do not exist. In 1961 one of us (G.B.E.) attempted to test its ability to grow on apparently suitable sandstone rocks at two localities in Sussex and Kent by transplanting plants from north Wales. The plants failed to establish and afterwards died in a severe drought.

Though H. wilsonii has disappeared from some of its localities, there is no evidence that its area is diminishing seriously at the present time. Because it is less dependent on an extremely moist and sheltered environment than H. tunbrigense, it has suffered less from deforestation and destruction of its habitats; also because it is relatively abundant in many parts of its area and perhaps rather less conspicuous than H. tunbrigense, it has been a less easy victim to collectors.

Faegri (1960, p. 77) supposes that it reached Norway from the west by direct immigra- tion across the North Sea, but if, as suggested above, its continental range formerly extended further east than now, this is not necessarily true.

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