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The Key

1	Frond undivided	2
	Frond divided	3
2	Fronds occur singly, lamina ovate, with a fertile stalk arising from near the base	Common Adder's Tongue Ophioglossum vulgatum
	Fronds occur in clusters and the parallel-sided lamina has a heart-shaped base with linear sori on the underside	Hart's Tongue Fern Asplenium scolopendrium
3	Fronds are less than 10cm long and irregularly and variably sub-divided	4
	Fronds are more than 10cm long, OR regularly and more or less symmetrically divided, or both	7
4	Fern growing as a spreading mat in moist habitat, fronds very thin (translucent), with sporangia enclosed in cup-like paired indusia	5
	Fern growing in a tuft on rock or wall, fronds are opaque, sporangia are NOT in cup-like paired indusia	6
5	Fronds 2-5cm long, lamina ovate to obovate (sometimes narrowly), leaf-lobes flat, roundish sori held in the plane of the frond, edges of the indusia toothed	Tunbridge Filmy Fern Hymenophyllum tunbrigense
	Fronds 2-10cm long, lamina ovate to lanceolate or even parallel-sided (usually narrowly), leaf-lobes bent away from the substrate, oval sori held out from the plane of the frond, edges of the indusia untoothed	Wilson's Filmy Fern Hymenophyllum wilsonii
6	Fronds irregularly 1- or 2- forked, ultimate segments very narrow, usually on hard rocks in mountainous areas	Forked Spleenwort Asplenium septentrionale
	Fronds irregularly and variably 1- or 2- divided on the same plant, or even occasionally undivided, and the ultimate stalked segments are fan-shaped, diamond-shaped or rounded with finely toothed margins, usually growing on limestone or other basic rocks or on wall mortar	Wall Rue Asplenium ruta-muraria (Typically, this fern is more or less regularly 2-divided, and appears in the key again at 29 )
7	Fronds once-divided	8
	Fronds divided two, three, or more times	17
8	Plant dimorphic, with markedly different fertile and vegetative fronds	9
	Fertile and vegetative fronds not markedly different	10
9	Fronds arise singly, and are fleshy, 5-25cm long, with fan-shaped pinnae and a fertile spike arising from near the base	Moonwort Botrychium lunaria
	Fronds in clumps, hard, glossy, 20 to 50 cm long, pinnules parallel sided and decreasing in length markedly to the base of the frond, vegetative fronds held horizontally, fertile fronds thinner and erect	Hard Fern Blechnum spicant
10	Fronds in clumps, hard, toothed, prickly, with mitten-shaped pinnae, dark glossy green, indusia round with central depression, small to medium-sized fern on limestone or other basic rocks in mountains	Holly Fern Polystichum lonchitis
	Fronds not as above	11
11	Fronds arising in clumps from a short rhizome	12
	Fronds arising singly from a long creeping rhizome, frond base truncate, sori without indusia	15
12	Undersurface of the frond densely covered with scales, rust-coloured when mature and surrounding the linear sori, pinnae rounded and attached to rachis by the whole of the base, grows on lime-rich rocks or wall mortar	Rustyback Fern Asplenium ceterach
	Fronds not as above	13
13	Frond length less than 7 times the maximum width, the pinnae are conspicuously glossy on the upper surface, thick and fleshy, fronds mostly15 to 20 cm, but can be larger, normally occurs only on rocks close to the sea	Sea Spleenwort Asplenium marinum
	Frond length more than 8 times the maximum width, pinnae not fleshy, not restricted to maritime habitats	14
14	Rachis shiny dark brown or black (except sometimes at the tips of new fronds), usually 8 to 20 cm long and up to 2 cm wide, many roundish pinnae	Maidenhair Spleenwort Asplenium trichomanes
	Rachis green, pinnae conspicuously toothed, frond usually 5-15 cm long and up to 1.4 cm wide, grows on basic rocks in upland areas	Green Spleenwort Asplenium viride
15	Small fern, fronds usually 10-25 cm in length, parallel-sided in the lower part of the frond, sori round, with a thin amber line (annulus) over each sporangium, visible with a x 10 lens, usually on acidic substrate	Common Polypody Polypodium vulgare
	Fronds obovate or triangular, sori usually oval	16
16	Fronds of mature plants 15-60 cm in length, oval in outline, with longest pinnae in central part of frond, lowest pinnae often inflexed. Oval sori, without a visible dark annulus	Western Polypody Polypodium interjectum
	Fronds 5-40 cm in length, triangular or broadly oval in outline, pinnae often with serrated margins, lower pinnae markedly inflexed. Sori oval, with short brown annulus, usually on basic substrate	Southern Polypody Polypodium cambricum
	N.B. The three Polypodium species are very variable, and may be difficult to identify on gross morphological grounds alone. In addition, they all hybridise to give intermediate forms. See Page (1997) for details of how to separate them microscopically.
17	Fronds are divided only twice	18
	Fronds are divided at least three times	30
	N.B. In Athyrium filix-femina, some individuals have twice-divided fronds and others have three-times-divided fronds. In Cystopteris fragilis, the fronds are twice-divided but the margins of the pinnules of some individuals are so deeply cut that they appear to be three-times-divided. These two species appear in the key twice, under both 'twice-divided' and 'three-times-divided'
18	Large fern of moist habitats, with large (visible without a lens) spherical sporangia congregated at the frond apex where there are no expanded pinnules, and not on the back of the expanded pinnules in the lower part of the frond	Royal Fern Osmunda regalis
	Small sporangia in sori on the back of the expanded pinnules	19
19	Small to medium-sized fronds arising singly from a creeping rhizome in moist woodland or rocky upland slopes, stipe longer than the obovate or triangular blade, two lowest pinnae conspicuously angled backwards towards the base, round marginal sori without indusia	Beech Fern Phegopteris connectilis
	Fronds not as above	20
20	Medium to large lanceolate fronds arising from a creeping rhizome, found in wet fens and marshes. Distinct fertile fronds formed later in the season are taller, more erect, and have narrower pinnae than the sterile fronds. The small, naked, round sori are marginal on the pinnae	Marsh Fern Thelypteris palustris
	Fern not as above	21
21	Medium to large, usually upland fern, fronds clustered on the crown of short upright rhizome, conspicuous white scales on the very short stipes, fronds lanceolate, tapering to very small pinnae at the base, round naked sori situated marginally on the pinnules, minute yellow glandular hairs on undersurface of lamina, may give off lemon scent when young fronds are rubbed	(Lemon-Scented) Mountain Fern Oreopteris limbosperma
	Fern not as above	22
22	Medium to large fern with chaffy scales on the stipe, larger pinnules mitten-shaped with a prominent 'thumb', indusia circular, centrally attached	23
	Fern not as above	24
23	Medium-sized fern, fronds narrowly lanceolate, rather hard and leathery-textured, glossy; stipes short (rarely more than one-fifth of the frond length), base of each pinnule forms an acute angle where it tapers to the pinna mid-rib, pinnule stalks flat	Hard Shield Fern Polystichum aculeatum.
	Large fern with arching obovate fronds, soft-textured, stipes long (rarely less than one-fifth of the frond length), base of each pinnule forms an obtuse angle where it joins the distinct cylindrical stalk	Soft Shield Fern Polystichum setiferum
	N.B. The last two ferns can be variable and not always easy to separate, they also hybridise to form Polystichum x bicknellii, intermediate between its parents. Hard shield fern tends to be more winter-hardy than soft shield fern.
24	Medium to large fern with large chaffy stipe scales, sori round with kidney-shaped indusia (i.e. round indusium with radial attachment) .	25
	Sori without kidney shaped indusia	27
25	Bi-pinnate medium to large ferns, usually robust, with dark spot at the junction of the pinna mid-rib and rachis, most easily visible on the undersurface of the frond	Scaly Male Ferns Dryopteris affinis complex
	N.B.The Scaly Male Ferns are an extremely variable group of ferns that form hybrids with the Common Male Fern. For those wanting to separate the D. affinis taxa, there is a guide on the BPS website under Publications / Special Publications, No. 13.
	Bi-pinnate ferns without dark spot at the pinna - rachis junction	26
26	Large fern with arching fronds arising usually from a single upright rhizome, pinnules usually toothed. Indusia aglandular, often with edges spreading outwards, lost soon after spores are shed, sori distributed over most of the pinnule area	Common Male Fern Dryopteris filix-mas
	Medium to large fern, with erect fronds arising from a much-branched rhizome, bluntly-lobed pinnules have margins crisped upwards. Sori are limited to the half of the pinnule closest to the pinna midrib, indusia with minute glands	Mountain Male Fern Dryopteris oreades
27	Medium to large fern with scattered narrow scales along the stipe, sori are curved, comma- or "J"-shaped, with persistent, fringed, indusia, usually on neutral to acidic substrate	Common Lady Fern Athyrium filix-femina (See also 30 below)
	Small to medium-sized fern of basic rocky habitats or walls, sori not as above	28
28	Fronds thin and delicate, with round sori exposed when the inflated, pear-shaped (bladder-like) indusia shrink and detach before spore release	Brittle Bladder Fern Cystopteris fragilis
	Fronds shiny or leathery, linear sori covered by elongate indusia	29
29	Fronds less than 10cm long, twice divided, sometimes rather irregularly, with fan-shaped, diamond-shaped or rounded pinnules with finely toothed margins, indusia with fringed margins	Wall Rue Asplenium ruta-muraria
	Fronds up to 30cm long, regularly divided, sometimes approaching 3-divided at the base, lamina (narrowly) triangular, with toothed ovate or lanceolate pinnules, indusium margins entire	Black Spleenwort Asplenium adiantum-nigrum
30	Fronds up to 120cm long, lamina lanceolate to ovate, sori curved, comma- or "J"-shaped, with persistent fringed indusia	Common Lady Fern Athyrium filix-femina
	Lamina obovate or triangular, sori circular or linear	31
31	Sori circular	32
	Sori linear	38
32	Indusia kidney-shaped, often visible after spore release	33
	Indusia absent or pear-shaped, often not visible by time of spore release	36
33	Medium to large fern of moist habitats, fronds almost erect, in irregular groups arising from a semi-creeping rhizome, facing in various directions. Pinnae rotated to a near horizontal position. Scales on the stipe uniformly pale or only darker at the base	Narrow Buckler Fern Dryopteris carthusiana
	N.B.This fern frequently hybridises with the Broad Buckler Fern (see 34) where the two parent species grow close together. The hybrid, D. x deweveri, is distinguished by its intermediate morphology and abortive sporangia.
	Fern arises from a short erect rhizome to form a distinct "shuttlecock", fronds 3-4 times divided, pinnae not or very little rotated, fronds all facing towards the centre at least at the base	34
34	Large fronds arch outwards from a pronounced shuttlecock, lamina obovate or triangular, pinnule margins turn downwards, stipe scales have a conspicuous dark central stripe	Broad Buckler Fern Dryopteris dilatata
	Pinnule margins flat or turn upwards to give a crisped appearance, stipe scales are uniformly pale, or only darker at the base	35
35	Fronds widely-spreading, lamina triangular or obovate with many minute glands on the under-surface. Lower part of the stipe purplish, stipe scales have a slightly darker patch only at the base if at all, margins of the lobed pinnules turned upwards to give a crisped appearance	Hay-scented Buckler Fern Dryopteris aemula
	Fronds semi-erect, lamina obovate and delicately dissected, not glandular. Stipe scales uniformly pale or only slightly darker in the centre, deeply lobed pinnules are flat	Mountain Buckler Fern Dryopteris expansa
36	Small to medium-sized unbranched fronds, lamina obovate, 3- to 4- divided, circular sori exposed when the inflated pear-shaped (bladder-like) indusia shrivel and disappear, often before spore release	Brittle Bladder Fern Cystopteris fragilis
	Fronds with three distinct triangular segments, naked sori	37
37	Medium-sized fern restricted to limestone rocks, fronds grey-green with many microscopic glands	Limestone Oak Fern Gymnocarpium robertianum
	Small to medium-sized fern of damp woods and rocky slopes, bright green fronds are not glandular	Common Oak Fern Gymnocarpium dryopteris
38	Large fern with fronds 50-200+ cm long, lamina branched with broadly triangular segments, the sori, if present are marginal and protected by the inrolled pinnulet margin	Bracken Pteridium aquilinum
	Small fern with fronds less than 50cm, lamina triangular	39
39	Grows on lime-rich rocks or wall mortar, fertile fronds similar to the sterile fronds, linear sori and indusia not marginal	Black Spleenwort Asplenium adiantum-nigrum
	Forms tufts or cushions on non-calcareous mountain screes and rocks, conspicuously dimorphic with fertile fronds more erect and more finely divided than the parsley-like sterile ones, linear sori marginal, protected by the inrolled pinnulet margins	Parsley Fern Cryptogramma crispa