British ferns

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L. Watson and M. J. Dallwitz

Conventional key

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1(0).
  • Plants usually bearing only a single leaf comprising two pinnae, the lower leaf-like, the upper fertile and non-laminate ... 2
  • Plants resembling duckweed - floating, with pendulous roots and tiny leaves... Azolla filiculoides
  • Plants rooted, helophytic or aquatic, with subulate leaves... Pilularia globulifera
  • Plants conventionally fern-like in form... 5

2(1).
  • The fertile spikes branched; the venation of the lamina open; the sporangia all alike, relatively massive, sub-sessile in two rows on the branches of the fertile component of the leaf ... Botrychium lunaria
  • The fertile spikes unbranched; the venation of the lamina reticulate; the sporangia all alike, relatively massive, sessile and coalescing in two rows on the spikelike fertile component of the leaf... 3

3(2).
  • The green blades with free vein endings within the network ... 4
  • The green blades without free vein endings within the network... Ophioglossum lusitanicum

4(3).
  • the sterile blades mostly 3-3.5 cm long, the sporangia 6-14 on either side of the spike ... Ophioglossum azoricum
  • the sterile blades mostly 4-15 cm long, the sporangia 10-40 on either side of the spike... Ophioglossum vulgatum

5(1).
  • The sporangia exposed, with neither indusia nor false indusia ... 6
  • The sporangia protected by indusia or false indusia... 17

6(5).
  • The rhizomes creeping ... 7
  • The rhizomes ascending... Oreopteris limbosperma
  • The rhizomes erect... 14

7(6).
  • The sori sub-orbicular ... 8
  • The sori elongated... 13

8(7).
  • The rhizomes slender; the petioles not jointed ... 9
  • The rhizomes stout; the petioles jointed and ultimately abscising at the point of attachment to the rhizome... 12

9(8).
  • Leaves conspicuously bent near the junction of rachis and petiole ... 10
  • Leaves not conspicuously bent near the junction of rachis and petiole... 11

10(9).
  • The lowermost pair of pinnae rather conspicously bent backwards away from the rest ... Phegopteris connectilis
  • The lowermost pair of pinnae not conspicuously bent backwards away from the rest... Gymnocarpium dryopteris

11(9).
  • Plants helophytic ... Thelypteris palustris
  • Plants mesophytic... Gymnocarpium robertianum

12(8).
  • The sori perceptibly sunken and creating conspicuous rows of circular impressions on the upper surface of the lamina; paraphyses present in the sporangia; the mature spores without a perispore ... Phymatodes diversifolia
  • The sori not sunken; paraphyses absent; the mature spores with a perispore... Polypodium vulgare

13(7).
  • Paraphyses present in the sporangia ... Polypodium cambricum
  • Paraphyses absent... Polypodium interjectum

14(6).
  • The sporangia at least at first aggregated in discrete sori; plants with no clear distinction into fertile and sterile leaves; leptosporangiate; the sporangial wall of only one cell layer; the sporangia dehiscing horizontally by a transverse split ... 15
  • The sporangia not aggregated into sori; plants bearing markedly different fertile and sterile leaves; eusporangiate; the sporangial wall of several cell layers; the sporangia dehiscing by a slit running across the apex from the lateral annulus... Osmunda regalis

15(14).
  • The venation of the lamina reticulate; the sori elongated; leaf blades covered underneath with rust-coloured scales; the sori becoming confluent when mature ... Ceterach officinarum
  • The venation of the lamina open; the sori sub-orbicular; leaf blades not covered underneath with rust-coloured scales; the sori remaining discrete at maturity... 16

16(15).
  • Leaves conspicuously bent near the junction of rachis and petiole ... Athyrium flexile
  • Leaves not conspicuously bent near the junction of rachis and petiole... Athyrium distentifolium

17(5).
  • The rhizomes hairy ... 18
  • The rhizomes bearing scales... 21
  • The rhizomes naked... 81

18(17).
  • The venation of the lamina reticulate; prothalli green and filamentous; leaf blades very thin and translucent - only one cell thick, lacking stomata; the rhizomes slender; the sporangia with an oblique annulus ... Trichomanes speciosum
  • The venation of the lamina open; prothalli green and fleshy; leaf blades not thin and translucent; the rhizomes stout; the sporangia with a vertical annulus... 19

19(18).
  • The false indusia continuous along the blade segments; leaves dying in the autumn; leaves distributed along the rhizomes; leaves bipinnate with conspicuously divided pinnules; the rhizomes not becoming trunk-like as in tree ferns ... 20
  • The false indusia interrupted on the individual blade segments; leaves persistent; leaves aggregated terminally; leaves tripinnate with conspicuously divided ultimate pinnules; the rhizomes becoming trunk-like beneath the crown of leaves, potentially to several meters high, covered with the blackened, persistent petiole bases and matted on the lower part with fibrous roots (tree ferns)... Dicksonia antarctica

20(19).
  • The lower pinnae linear-lanceolate with drooping tips ... Pteridium aquilinum subsp. aquilinum
  • The lower pinnae broadly ovate with upwardly curving tips... Pteridium aquilinum subsp. latiusculum

21(17).
  • Plants bearing markedly different fertile and sterile leaves ... 22
  • Plants with no clear distinction into fertile and sterile leaves... 28

22(21).
  • The sori with a true indusium ... 23
  • The sori with only a false indusium derived exclusively from the lamina margin... Cryptogramma crispa
  • The sori protected by true and false indusia combined... 26
  • The sori naked and neither indusiate nor pseudo-indusiate... 27

23(22).
  • The rhizomes creeping; the sori sub-orbicular; leaves dying in the autumn; the sori remaining discrete at maturity ... 24
  • The rhizomes erect; the sori elongated; leaves persistent; the sori becoming confluent when mature... 25

24(23).
  • The rhizomes slender ... Thelypteris palustris
  • The rhizomes stout... Dryopteris cristata

25(23).
  • The pinnae of the sterile leaves much narrowed basally, attached over only a short width ... Blechnum cordatum
  • The pinnae of the sterile leaves with broadened, contiguous bases... Blechnum spicant

26(22).
  • The venation of the lamina reticulate; leaves distributed along the rhizomes; the petioles longer than the blades; leaf blades in outline ovate-triangular ... Onoclea sensibilis
  • The venation of the lamina open; leaves aggregated terminally; the petioles shorter than the blades; leaf blades in outline oblanceolate... Matteuccia struthiopteris

27(22).
  • The sori sub-orbicular; the sori remaining discrete at maturity; plants helophytic ... Thelypteris palustris
  • The sori elongated; the sori becoming confluent when mature; plants mesophytic... Anogramma leptophylla

28(21).
  • Leaves simple ... 29
  • Leaves compound... 30

29(28).
  • The rhizomes creeping ... Asplenium septentrionale
  • The rhizomes ascending... Phyllitis scolopendrium
  • The rhizomes erect... Phyllitis scolopendrium

30(28).
  • Leaves simply divided ... 31
  • Leaves complexly divided... 36

31(30).
  • The indusia peltate; the sori sub-orbicular ... 32
  • The indusia attached on one side along a vein; the sori elongated... 34

32(31).
  • Leaf blades in outline linear ... Polystichum lonchitis
  • Leaf blades in outline elliptic... Cyrtomium falcatum
  • Leaf blades in outline ovate... Cyrtomium falcatum
  • Leaf blades in outline lanceolate... 33

33(32).
  • Generally occuring on base-rich substrates ... Polystichum lonchitis
  • Generally occuring on neutral substrates... Polystichum munitum
  • Generally occuring on acid substrates... Polystichum munitum

34(31).
  • Leaf blades leathery ... Asplenium marinum
  • Leaf blades ‘herbaceous’... 35

35(34).
  • the petioles and rachides blackish, both with very narrow brownish wings ... Asplenium trichomanes
  • the petioles brownish or blackish only near the base and green above, the rachides green and either wingless or at most only narrowly green-winged... Asplenium viride

36(30).
  • The sori sub-orbicular ... 37
  • The sori elongated... 71

37(36).
  • The sporangia marginal ... 38
  • The sporangia superficial... 41

38(37).
  • Leaves distributed along the rhizomes; the sori with only a false indusium derived exclusively from the lamina margin ... 39
  • Leaves aggregated terminally; the sori with a true indusium... 40

39(38).
  • The false indusia somewhat rectangular-elongate, not reniform ... Adiantum capillus-veneris
  • The false indusia reniform-orbicular... Adiantum raddianum

40(38).
  • Leaves rooting at their tips from a scaly bud; the longest primary pinnae near the base of the blade ... Woodwardia radicans
  • Leaves not rooting at their tips; the longest primary pinnae around the middle of the blade... Cyrtomium falcatum

41(37).
  • Leaves once pinnate, with conspicuously divided pinnae ... 42
  • Leaves bipinnate with more or less undivided pinnules... 52
  • Leaves bipinnate with conspicuously divided pinnules... 54
  • Leaves tripinnate with undivided ultimate pinnules... 63
  • Leaves tripinnate with conspicuously divided ultimate pinnules... 69

42(41).
  • Leaves persistent ... 43
  • Leaves dying in the autumn... 47

43(42).
  • The indusia peltate ... 44
  • The indusia reniform and attached at the indentation... 46
  • The indusia not reniform but flap-like from the base of the sorus, vaulted at first, later reflexed... Woodwardia radicans
  • The indusia attached on one side along a vein... Woodwardia radicans
  • The indusia consisting mainly of hairs... Woodwardia radicans
  • The indusia projecting from the margins of the very thin lamina and enveloping the sori from their bases... Woodwardia radicans

44(43).
  • Leaves rooting at their tips from a scaly bud ... Woodwardia radicans
  • Leaves not rooting at their tips... 45

45(44).
  • The longest primary pinnae near the base of the blade ... Polystichum aculeatum
  • The longest primary pinnae about a third of the distance from the base of the blade... Polystichum aculeatum
  • The longest primary pinnae around the middle of the blade... Cyrtomium falcatum

46(43).
  • Leaves rooting at their tips from a scaly bud; the longest primary pinnae near the base of the blade ... Woodwardia radicans
  • Leaves not rooting at their tips; the longest primary pinnae around the middle of the blade... Dryopteris affinis

47(42).
  • The petioles vascularised via a single strand ... 48
  • The petioles vascularised by several discrete strands... 50

48(47).
  • The rhizomes creeping; plants helophytic; the longest primary pinnae 5 to 11 cm long; the petioles not jointed ... Thelypteris palustris
  • The rhizomes erect; plants mesophytic; the longest primary pinnae up to 4 cm long; the petioles jointed and ultimately abscising in the mid-region... 49

49(48).
  • the largest pinnae oblong or ovate-oblong, 1.5-2 times as long as broad, and the rachis and the undersides of the pinnae are densely scaly with 2-3 mm long, subulate, pale brown scales and flexuous hairs; generally occuring on neutral substrates ... Woodsia ilvensis
  • the largest pinnae triangular-ovate, 1-1.5 times as long as broad and the rachis and the undersides of the pinnae scale-less or with only short scales of about 1mm long, and few hairs; generally occuring on base-rich substrates... Woodsia alpina

50(47).
  • The rhizomes creeping; the rhizomes slender; plants helophytic ... Thelypteris palustris
  • The rhizomes erect; the rhizomes stout; plants mesophytic... 51

51(50).
  • The sori 2 to 4 on either side of the mid-rib of the larger pinnules ... Dryopteris oreades
  • The sori 5 to 6 on either side of the mid-rib of the larger pinnules... Dryopteris filix-mas

52(41).
  • The indusia reniform and attached at the indentation ... 53
  • The indusia not reniform but flap-like from the base of the sorus, vaulted at first, later reflexed... Cystopteris dickieana
  • The indusia consisting mainly of hairs... Woodsia alpina

53(52).
  • Leaves persistent; the rhizomes erect; petiolar scales golden brown and often dark centred, dense; the primary pinnae decreasing markedly in length towards the base of the blade, the basal ones relatively short; paraphyses absent ... Dryopteris affinis
  • Leaves dying in the autumn; the rhizomes ascending; petiolar scales not golden brown; the primary pinnae not decreasing markedly in length basipetaly, the basal pinnae not or scarcely shorter than the longest pair; paraphyses present in the sporangia... Dryopteris submontana

54(41).
  • The indusia peltate ... 55
  • The indusia reniform and attached at the indentation... 56
  • The indusia not reniform but flap-like from the base of the sorus, vaulted at first, later reflexed... 61

55(54).
  • Leaf blades leathery ... Polystichum aculeatum
  • Leaf blades ‘herbaceous’... Polystichum setiferum

56(54).
  • Petiolar scales golden brown and often dark centred, dense; the primary pinnae decreasing markedly in length towards the base of the blade, the basal ones relatively short ... Dryopteris affinis
  • Petiolar scales not golden brown; the primary pinnae not decreasing markedly in length basipetaly, the basal pinnae not or scarcely shorter than the longest pair... 57

57(56).
  • The lowest pinna with the lowest 3 or 4 pinnules on either side all about equal in length ... Dryopteris submontana
  • The lowest pinna with the lowest pinnule on the lower side much longer than the lowest one on the upper side, and often longer than the one or two adjoining it as well... 58

58(57).
  • The lowest pinna with the basal pinnule on its lower side generally at least half as long as the pinna itself ... 59
  • The lowest pinna with the basal pinnule on its lower side nearly always less than half as long as the pinna itself... 60

59(58).
  • The rhizomes creeping ... Dryopteris carthusiana
  • The rhizomes ascending... Dryopteris expansa
  • The rhizomes erect... Dryopteris expansa

60(58).
  • The rhizomes creeping ... Dryopteris carthusiana
  • The rhizomes ascending... Dryopteris dilatata
  • The rhizomes erect... Dryopteris dilatata

61(54).
  • Leaves persistent ... Cystopteris diaphana
  • Leaves dying in the autumn... 62

62(61).
  • Leaves distributed along the rhizomes; leaf blades in outline ovate-triangular; the rhizomes slender ... Cystopteris montana
  • Leaves aggregated terminally; leaf blades in outline lanceolate; the rhizomes stout... Cystopteris fragilis

63(41).
  • The indusia reniform and attached at the indentation ... 64
  • The indusia not reniform but flap-like from the base of the sorus, vaulted at first, later reflexed... 67

64(63).
  • The lowest pinna with the basal pinnule on its lower side generally at least half as long as the pinna itself ... 65
  • The lowest pinna with the basal pinnule on its lower side nearly always less than half as long as the pinna itself... 66

65(64).
  • The rhizomes creeping ... Dryopteris carthusiana
  • The rhizomes ascending... Dryopteris expansa
  • The rhizomes erect... Dryopteris expansa

66(64).
  • The rhizomes creeping ... Dryopteris carthusiana
  • The rhizomes ascending... Dryopteris dilatata
  • The rhizomes erect... Dryopteris dilatata

67(63).
  • Leaves persistent ... Cystopteris diaphana
  • Leaves dying in the autumn... 68

68(67).
  • Leaves distributed along the rhizomes; leaf blades in outline ovate-triangular; the rhizomes slender ... Cystopteris montana
  • Leaves aggregated terminally; leaf blades in outline lanceolate; the rhizomes stout... Cystopteris fragilis

69(41).
  • The indusia reniform and attached at the indentation ... 70
  • The indusia not reniform but flap-like from the base of the sorus, vaulted at first, later reflexed... Cystopteris alpina

70(69).
  • Leaves persistent; the mature spores with a perispore ... Dryopteris aemula
  • Leaves dying in the autumn; the mature spores without a perispore... Dryopteris carthusiana

71(36).
  • The sori with a true indusium; the mature spores monolete ... 72
  • The sori with only a false indusium derived exclusively from the lamina margin; the mature spores trilete... 79

72(71).
  • The longest primary pinnae the lowermost ... 73
  • The longest primary pinnae near the base of the blade... 74
  • The longest primary pinnae about a third of the distance from the base of the blade... 75
  • The longest primary pinnae around the middle of the blade... 76

73(72).
  • Primary pinnae 3 to 5 on each side of the leaf ... Asplenium ruta-muraria
  • Primary pinnae 6 to 25 on each side of the leaf... Asplenium adiantum-nigrum

74(72).
  • The petioles vascularised via a single strand; leaves not rooting at their tips; the sori becoming confluent when mature; the mature spores with a perispore ... Asplenium onopteris L.
  • The petioles vascularised by several discrete strands; leaves rooting at their tips from a scaly bud; the sori remaining discrete at maturity; the mature spores without a perispore... Woodwardia radicans

75(72).
  • Leaves once pinnate, with conspicuously divided pinnae ... Asplenium marinum
  • Leaves bipinnate with more or less undivided pinnules... Asplenium obovatum
  • Leaves bipinnate with conspicuously divided pinnules... Asplenium obovatum

76(72).
  • Leaves once pinnate, with conspicuously divided pinnae ... 77
  • Leaves bipinnate with more or less undivided pinnules... Asplenium obovatum
  • Leaves bipinnate with conspicuously divided pinnules... 78
  • Leaves tripinnate with undivided ultimate pinnules... Athyrium filix-femina

77(76).
  • Leaf blades leathery ... Asplenium marinum
  • Leaf blades ‘herbaceous’... Asplenium viride

78(76).
  • Leaves persistent ... Asplenium obovatum
  • Leaves dying in the autumn... Athyrium filix-femina

79(71).
  • Leaves distributed along the rhizomes; the leaves not as in Pteris (q.v.) ... 80
  • Leaves aggregated terminally; the leaves compound-pinnate, with long, narrow pinnae of which only the lowest on each side bears a single pinnule... Pteris cretica

80(79).
  • The false indusia somewhat rectangular-elongate, not reniform ... Adiantum capillus-veneris
  • The false indusia reniform-orbicular... Adiantum raddianum

81(17).
  • The sporangia marginal; prothalli green, flat and strap-shaped; leaves simple; leaf blades very thin and translucent - only one cell thick, lacking stomata; plants mesophytic ... 82
  • The sporangia superficial; prothalli green and fleshy; leaves compound; leaf blades not thin and translucent; plants helophytic... Thelypteris palustris

82(81).
  • The valves of the indusia orbicular and toothed ... Hymenophyllum tunbrigense
  • The valves of the indusia ovate and entire... Hymenophyllum wilsonii

To view the illustrations with detailed captions, go to the interactive key. This also offers full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2004 onwards. British ferns (Filicopsida). Version: 4th January 2012. http://delta-intkey.com’.

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