The fronds are usually characteristically circinate (that is, coiled) when young.
The spores are of one, or more rarely of two different kinds; they are produced in sporangia which are aggregated into sori or scattered over the lower surface of the frond. The position, shape, structure and degree of protection of the sorus is important in the identification of ferns. The spores produce a small independent prothallus which is delicate and mostly less than 1 cm diameter. After fusion of the gametes produced by the sex organs on the prothallus the independent sporophyte, or fern plant, develops.
| | Key to the class FILICOPSIDA | |
| 1 | Plants rooted in soil or mud, or epiphytic | 2 |
| Plants free-floating | 12 |
| 2 | Fertile fronds borne as an offshoot of the sterile fronds, the two parts markedly different; vernation not circinate | OPHIOGLOSSACEAE |
| Fertile and sterile fronds borne separately on rhizome, or all fronds fertile; vernation circinate
Back to 1 | 3 |
| 3 | Sporangia borne on the surface of the fronds, often covered by an indusium or by the recurved margin of the frond; never completely enclosed by a modified frond (sporocarp) | 4 |
| Sporangia completely enclosed in a sporocarp; sterile leaves filiform or with four leaflets
Back to 2 | MARSILEACEAE |
| 4 | Stipes with an expanded, stipule-like base; sporangia relatively large, ± sessile, rounded to oval in shape, splitting along a central line | 5 |
| Stipes with base lacking stipule-like structures; sporangia smaller, on short stalks, splitting vertically or laterally
Back to 3 | 6 |
| 5 | Stipule-like structures fleshy, trunk short (to 80 cm) and massive, with a single crown | MARATTIACEAE |
| Stipule-like structures thin; trunk taller, usually with 2 or more crowns
Back to 4 | OSMUNDACEAE |
| 6 | Sori on narrow segments borne in pinnate groups at tips of the fertile fronds, or projecting from the margins of the pinnae of fertile fronds | SCHIZAEACEAE |
| Sori not borne on such projections
Back to 4 | 7 |
| 7 | Aquatic fern with spongy stipes; fronds strongly dimorphic | PARKERIACEAE |
| Terrestrial or epiphytic ferns
Back to 6 | 8 |
| 8 | Arborescent ferns | 9 |
| Non-arborescent ferns
Back to 7 | 10 |
| 9 | Trunk and base of stipes scaly; indusia absent or scale-like | CYATHEACEAE |
| Trunk and base of stipes hairy; sori located in 2-lipped cups, the outer lip formed from the recurved margin of the lamina, the inner lip the true indusium
Back to 8 | DICKSONIACEAE |
| 10 | Fronds pseudodichotomously branched with dormant apical buds | GLEICHENIACEAE |
| Fronds not branching in this manner
Back to 8 | 11 |
| 11 | Sori ± enclosed by the strongly revolute margins of the pinnae, the fronds long and narrow; rhizomes hairy | PLATYZOMATACEAE |
| Sori marginal or superficial, margins of fronds often recurved but never strongly revolute; rhizomes sometimes hairy
Back to 10 | 13 |
| 12 | Leaves c. 20 mm long, covered in long papillae | SALVINIACEAE |
| Leaves small, 1–2 mm long, imbricate in 2 rows
Back to 1 | AZOLLACEAE |
| 13 | Sori restricted to the marginal or submarginal region of the lamina, if submarginal usually covered by a false indusium | 14 |
| Sori superficial
Back to 11 | 15 |
| 14 | Rhizomes covered with hairs or bristles | 17 |
| Rhizomes covered with scales
Back to 13 | 20 |
| 15 | Fronds simple | 29 |
| Fronds pinnatifid or more compound
Back to 13 | 16 |
| 16 | Sori elongated along the veins or sporangia scattered over much of the surface of the lamina | 35 |
| Sori circular
Back to 15 | 41 |
| 17 | Sporangia borne on a columnar or filiform receptacle within a 2-valved or cupular involucre | HYMENOPHYLLACEAE |
| Sporangia borne on the surface of the lamina
Back to 14 | 18 |
| 18 | Sori circular, indusia 2-lipped with the inner lip the true indusium | 19 |
| Either sori circular and indusium false and formed from the recurved margin or sori linear and with true indusia present or absent
Back to 17 | DENNSTAEDTIACEAE |
| 19 | The 2 lips free, inner membranous and obscure, indumentum on stipe base of long hairs; lamina ± leathery.(Calochlaena) | DICKSONIACEAE |
| The 2 lips fused to form a cup, indumentum on stipe base of very short, dense hairs; lamina soft and membranous
Back to 18 | DENNSTAEDTIACEAE |
| 20 | True indusia present | 21 |
| Indusia false (formed by recurved lamina margin) or absent
Back to 14 | 23 |
| 21 | Indusia cup-like, attached at the base and sides.(Davallia) | DAVALLIACEAE |
| Indusia flap-like
Back to 20 | 22 |
| 22 | Sori borne along single veins | ASPLENIACEAE |
| Sori borne across the tips of a number of veins
Back to 21 | LINDSAEACEAE |
| 23 | Epiphytes, leaves simple, linear, rhizome scales clathrate | VITTARIACEAE |
| Terrestrial plants, leaves compound; rhizome scales not clathrate
Back to 20 | 24 |
| 24 | Fronds simply pinnate | 25 |
| Fronds at least 2-pinnate or 2-pinnatifid
Back to 23 | 26 |
| 25 | Pinnae leathery, veins not readily visible; sori continuous in broad bands along each margin of the pinna.(Pellaea) | SINOPTERIDACEAE |
| Pinnae herbaceous, veins readily visible; sori continuous in narrow bands
Back to 24 | PTERIDACEAE |
| 26 | Recurved margin covering and bearing the sorus, the whole structure being ± reniform | ADIANTACEAE |
| Recurved margin not bearing the sorus
Back to 24 | 27 |
| 27 | Pinnae rarely >5 cm long; sori and indusia short, sori becoming confluent (indusia sometimes absent).(Cheilanthes) | SINOPTERIDACEAE |
| Pinnae usually >5 cm long; sori and indusia linear
Back to 26 | 28 |
| 28 | Rhizomes tufted or short-creeping.(Pteris) | PTERIDACEAE |
| Rhizomes long-creeping.(Histiopteris)
Back to 27 | DENNSTAEDTIACEAE |
| 29 | Sori circular | 30 |
| Sori elongated or confluent and covering whole areas of the frond
Back to 15 | 31 |
| 30 | Fronds articulate on the rhizome, scales of rhizome peltate and/or clathrate | POLYPODIACEAE |
| Fronds not articulated, scales of rhizome neither peltate nor clathrate
Back to 29 | GRAMMITACEAE |
| 31 | Indusia present | 32 |
| Indusia absent
Back to 29 | 33 |
| 32 | Indusia parallel with the primary vein | BLECHNACEAE |
| Indusia running along the secondary veins
Back to 31 | ASPLENIACEAE |
| 33 | Sporangia borne on the much narrowed tail-like apex of the fertile frond.(Belvisia) | POLYPODIACEAE |
| Apex of fronds not very narrow and tail-like
Back to 31 | 34 |
| 34 | Sporangia scattered, becoming confluent; stipe and lamina bearing stellate hairs.(Pyrrosia) | POLYPODIACEAE |
| Sori oblique, usually remaining distinct; stipe and lamina bearing simple or branched hairs
Back to 33 | GRAMMITACEAE |
| 35 | Bracket epiphytes with 2 kinds of fronds, green fertile fronds plus brown or green sterile nest-fronds.(Platycerium) | POLYPODIACEAE |
| Plants usually terrestrial, fronds of one kind but may be dimorphic
Back to 16 | 36 |
| 36 | Indusia present | 37 |
| Indusia absent
Back to 35 | 39 |
| 37 | Sori parallel with the primary and secondary veins of the frond, fronds 1-pinnatifid to 1-pinnate | BLECHNACEAE |
| Sori oblique with respect to main vein of the segment, or a main vein lacking; fronds more compound, usually at least 2-pinnatifid
Back to 36 | 38 |
| 38 | Rhizome scales not clathrate; indusia often double (i.e., one on each side of a vein), especially at the base of the segment | ATHYRIACEAE |
| Rhizome scales clathrate; indusia always single
Back to 37 | ASPLENIACEAE |
| 39 | Fronds pinnate, segments with distinct petiolules; sporangia covering the entire lower surface of the fertile pinnae.(Acrostichum) | PTERIDACEAE |
| Fronds otherwise; sporangia, at least when first formed, along veins
Back to 36 | 40 |
| 40 | Rhizome scales clathrate, fronds and stipes covered with long hairs.(Pleurosorus) | ASPLENIACEAE |
| Rhizome scales not clathrate, fronds glabrous or the lower surface covered with a waxy powder
Back to 39 | TAENITIDACEAE |
| 41 | Fronds delicate and membranous.(Cystopteris) | ATHYRIACEAE |
| Fronds herbaceous to leathery
Back to 16 | 42 |
| 42 | Fronds of 2 kinds (green fertile fronds plus brown nest-fronds), veins anastomosing.(Drynaria) | POLYPODIACEAE |
| Fronds of 1 kind, but may vary in shape
Back to 41 | 43 |
| 43 | Rhizomes covered with hairs.(Hypolepis) | DENNSTAEDTIACEAE |
| Rhizomes covered with scales
Back to 42 | 44 |
| 44 | Rhizome scales peltate, stipe often articulate on the rhizome | 45 |
| Rhizome scales not peltate, stipe not articulate on rhizome
Back to 43 | 46 |
| 45 | Fronds pinnatifid (or simple), veins anastomosing; sori large, usually 2 mm or more diam.(Microsorum) | POLYPODIACEAE |
| Fronds 1-pinnate or more divided, veins free; sori small, <2 mm diam
Back to 44 | DAVALLIACEAE |
| 46 | Fronds pinnatisect.(Doodia) | BLECHNACEAE |
| Fronds at least 1-pinnate
Back to 44 | 47 |
| 47 | Fronds 1-pinnate to 2-pinnatifid (sometimes pinnatifid at apex) | 48 |
| Fronds 2-pinnate or more compound
Back to 46 | 49 |
| 48 | Pinnae ± oblong in outline, never divided completely into pinnules at the base, veins anastomosing at the bases of the lobes and running to the fork between the lobes | THELYPTERIDACEAE |
| Pinnae ovate to lanceolate in outline, veins free or copiously anastomosing
Back to 47 | DRYOPTERIDACEAE |
| 49 | Fronds with lamina membranous, sori without indusium, rhizome scales finely ciliate.(Macrothelypteris) | THELYPTERIDACEAE |
| Fronds with lamina usually leathery, sori indusiate (except for 1 sp.), rhizome scales entire or finely toothed
Back to 47 | DRYOPTERIDACEAE |
