| 1 | Aquatic plants. |
| 1' | Land plants or epiphytes. |
| 2 | Plants floating; leaves small, simple or bilobed |
| 2' | Plants rooted in earth or on rocks; leaves larger, more divided. |
| 3 | Leaves 4-partite; sporocarps attached to stipes |
| 3' | Leaves not 4-partite; sporangia singly or in sori on lower surface of lamina. |
| 4 | Sporangia borne singly, protected by reflexed edges of narrow lamina. |
| 4' | Sporangia grouped in sori, on lower surface of lamina, not protected by reflexed edges. |
| 5 | Fern of stream-beds in deep shade; fronds pinnatifid, sori without indusia |
| 5' | Fern of open swamps; fronds bipinnatifid, sori indusiate. |
| 6 | Stipe containing 3 or more vascular bundles throughout. |
| 6' | Stipe containing one vascular bundle, or two at the base joining upwards to form a single Strand of various shape. |
| 7 | Rhizome in most cases dorsiventral, stipes jointed to it or to phyllopodia borne by rhizome. |
| 7' | Rhizome not dorsiventral, or if so the stipes not jointed to it. |
| 8 | Sori of various form (in some cases acrostichoid), not indusiate. |
| 8' | Sori indusiate. |
| 9 | Fronds pinnate; pinnae articulate to rachis, veins free |
| 9' | Fronds rarely pinnate; pinnae (if present) not articulate to rachis, or if so veins reticulate. |
| 10 | Acrostichoid; spores with perispore; veins free or uniting near margin |
| 10' | Acrostichoid or not; no perispore; where acrostichoid, veins much anastomosing. |
| 11 | Fronds simple. |
| 11' | Fronds more or less branched. |
| 12 | Sori near midrib of frond |
| 12' | Sori at ends of veins, solitary or joining many veins |
| 13 | Pinnae jointed to rachis |
| 13' | Pinnae not jointed to rachis |
| 14 | Stipes containing numerous bundles in a complex pattern, not a simple ring, in cross section. |
| 14' | Stipes containing a simple open ring of bundles. |
| 15 | Rootstock massive, short or tall; if creeping, subterranean. |
| 15' | Rootstock slender, long-creeping or high-climbing |
| 16 | Mangrove plants; distal pinnae acrostichoid, lower ones sterile. |
| 16' | Not mangrove plants; not acrostichoid. |
| 17 | Stock and stipes containing woody tissue; no stipules at bases of stipes. |
| 17' | Stock and stipes without woody tissue; stipules present at bases of stipes. |
| 18 | Veins free |
| 18' | Veins anastomosing |
| 19 | Sporangia on a separate branch attached at base of lamina, or to surface of lamina; no annulus. |
| 19' | Sporangia not on a separate branch of frond; annulus present. |
| 20 | Fronds pinnate or bipinnate, leaflets jointed to rachis. |
| 20' | Leaflets (if present) not jointed to rachis. |
| 21 | Fronds in close tufts, not spaced on a climbing rhizome; sori separate and indusiate. |
| 21' | Fronds spaced on a climbing rhizome; sori not indusiate, in most cases acrostichoid. |
| 22 | Fronds simple, sori oblique, exindusiate |
| 22' | Fronds otherwise. |
| 23 | Sori elongate, either close to midrib or to edge. |
| 23' | Sori not elongate near midrib or edge. |
| 24 | Sori close to midrib |
| 24' | Sori close to edge |
| 25 | Sporangia acrostichoid, edge of lamina in some cases reflexed. |
| 25' | Sporangia not acrostichoid. |
| 26 | Veins of sterile leaflets free. |
| 26' | Veins of sterile leaflets anastomosing. |
| 27 | Fronds bipinnate |
| 27' | Fronds simply pinnate. |
| 28 | Rhizome creeping, dorsiventral |
| 28' | Rhizome erect, radially arranged |
| 29 | Rhizome creeping, dorsiventral |
| 29' | Rhizome erect, not dorsiventral |
| 30 | Rachis grooved, groove open to admit branches or midribs of leaflets. |
| 30' | Rachis not grooved, its wings (if any) confluent with edges of lamina. |
| 31 | Rhizome-apex and bases of stipes hairy or apparently glabrous, not scaly. |
| 31' | Rhizome-apex and bases of stipes scaly. |
| 32 | Fronds repeatedly pseudo-dichotomous, a dormant apex between pairs of branches. |
| 32' | Fronds otherwise branched. |
| 33 | Apices of some or all primary rachis-branches dormant. |
| 33' | Apices of primary rachis-branches not dormant (apex of main rachis sometimes periodically dormant). |
| 34 | Rachis climbing and twining; sporangia solitary |
| 34' | Rachis trailing, not twining; several sporangia in a sorus, indusiate. |
| 35 | Sori at ends of veins, either in cups or protected by two equal or subequal flaps. |
| 35' | Sori otherwise. |
| 36 | Filmy ferns; lamina one cell thick |
| 36' | Not filmy, lamina thicker. |
| 37 | Massive rootstock, prostrate or erect and trunk-like, densely covered with long hairs. |
| 37' | Slender creeping rhizome; hairs short |
| 38 | Bases of stipes bearing prominent aerophores; fronds dimorphous, fertile with narrow acrostichoid pinnae |
| 38' | Bases of stipes lacking aerophores; fertile fronds otherwise. |
| 39 | Sporangia large, round, lacking an annulus, on deeply dissected pinnae |
| 39' | Sporangia otherwise. |
| 40 | Sporangia variously arranged on lower surface of fronds, usually along the veins, or in a submarginal groove, not indusiate. |
| 40' | Sporangia arranged otherwise. |
| 41 | Sori elongate |
| 41' | Sori small and round, or acrostichoid |
| 42 | Annulus almost apical; fronds simple or branched dichotomously; sporangia on special appendages |
| 42' | Annulus and frond otherwise. |
| 43 | Fronds pedately branched; sori superficial, indusiate |
| 43' | Fronds pinnately branched; sori at ends of veins, indusiate or not. |
| 44 | Sporangia on surface of reflexed marginal lobes |
| 44' | Sporangia not on such lobes. |
| 45 | Sori elongate along the veins of the lower surface. |
| 45' | Sori not elongate along the veins of the lower surface (in some cases elongate along a submarginal vein). |
| 46 | Sori indusiate. |
| 46' | Sori not indusiate. |
| 47 | Scales clathrate; the 2 vascular bundles at base of stipe uniting upwards to an X-shape. |
| 47' | Scales not clathrate; the 2 vascular bundles at base of stipe uniting upwards to a U-shape. |
| 48 | Unicellular slender hairs lacking |
| 48' | Unicellular slender hairs on frond and on scales |
| 49 | Fronds simple; sori in grooves (or rarely superficial); epiphytes or rock-plants. |
| 49' | Fronds usually not simple; sori not in grooves; not epiphytes or rock-plants. |
| 50 | Sori not occupying whole length of veins. |
| 50' | Sori occupying whole length of veins. |
| 51 | Lower surface of lamina not covered with powder |
| 51' | Lower surface of lamina covered with powder |
| 52 | Slender unicellular hairs lacking on lamina and scales. |
| 52' | Slender unicellular hairs present on lamina and scales. |
| 53 | Sori in marginal or submarginal grooves, or superficial and forming continuous lines parallel to midrib. |
| 53' | Sori otherwise. |
| 54 | Scales clathrate |
| 54' | Scales not clathrate |
| 55 | Sori at ends of veins (uniting several veins or not) protected by reflexed edge of lamina. |
| 55' | Sori not at ends of veins or not so protected. |
| 56 | Rachis grooved on upper surface, groove opening to admit groove of midrib of pinna. |
| 56' | Rachis not grooved in this way |
| 57 | Lower surface of frond more or less covered with white or yellow powder. |
| 57' | Lower surface of frond not so covered. |
| 58 | Sori submarginal, at ends of veins, in small projecting cups, or protected by an indusium attached on the side remote from the margin. |
| 58' | Sori otherwise. |
| 59 | Sori in cups projecting from margin |
| 59' | Sori not in such cups. |
| 60 | Sori always solitary at ends of Single veins; scales of rhizome bearing papillae on marginal cells; rhizome not protostelic |
| 60' | Sori often uniting ends of several veins; scales lacking papillae; rhizome protostelic. |
| 61 | Main rachis of fronds periodic in growth; thicket-forming ferns. |
| 61' | Main rachis of fronds continuous in growth; not thicket-forming ferns. |
| 62 | Small epiphytes (occasionally on rocks); sori superficial or immersed in pockets in substance of the lamina, not indusiate |
| 62' | Not epiphytes; sori in most cases indusiate. |
| 63 | Hairs on fronds unicellular (in rare cases very long hairs may consist of more than one cell). |
| 63' | Hairs on fronds multicellular, cells short |
| 64 | Scales confined to swollen basal part of stipe; no superficial hairs or glands on scales. |
| 64' | Scales not so confined, bearing superficial unicellular hairs or glands. |
