From the Latin difformis, meaning irregularly, unevenly or differently formed, which probably refers to different fronds from the same plant being dissected to different extents. This is especially noticeable between fertile and sterile fronds, with fertile fronds being more dissected than sterile fronds.

Fern with semi-erect or spreading fronds. Rhizome erect, scaly. Stipe and rachis brown on underside, green above, sparsely scaly; stipe 2–8 cm long, 0.5–1.5 mm diam. Lamina somewhat dimorphic, the sterile less divided than the fertile, ovate, usually 2-pinnate at base but rarely 1-pinnate in smallest specimens, 5–28 cm long, 2.3–15 cm wide, glossy green, thick and fleshy, sparsely scaly. Primary pinnae in 5–15 pairs, ± oblong to narrowly ovate; longest pinnae below middle, 1.2–9 cm long, 0.8–3.5 cm wide; apices acute to obtuse and toothed; bases usually divided to midrib. Secondary pinnae ovate, elliptic or obovate (or sometimes linear near the pinna apices), to 2 cm long, to 1.2 cm wide, with cuneate and shortly stalked bases, ± entire or toothed margins and blunt and toothed apices. Sori to 6 mm long. Perispores pale with fimbriate broad ridges and fenestrate lacunae; exospores 40.5–46.7 × 28.7–31.5 µm (means).

Asplenium difforme and the similar A. decurrens Willd. both have thick and fleshy fronds and inhabit rocks between the high tide mark and the first line of shrubs and trees along the coast, which distinguishes these species from all other Australian Aspleniaceae. Asplenium difforme is usually 2-pinnate at the base, whereas A. decurrens is almost always pinnate (see A. decurrens profile). In addition to this difference there are almost always some sori in A. difforme that are close and parallel to the margin with the indusia opening toward the closest margin, whereas in A. decurrens the sori are always remote from the margin with the indusia opening away from the closest margin. The two species can also be distinguished geographically. Asplenium difforme occurs from Kiama north to Noosa Heads in Queensland, whereas Asplenium decurrens occurs further south from Bowen Island, near Nowra in New South Wales.

Native.

Occurs along the coast from Noosa Heads, near Gympie, Queensland, to Kiama, New South Wales; also Norfolk Island.

Grows on rocks between the high tide mark and the first line of shrubs and trees along the coast. On Norfolk Island a common plant on sea cliffs (Mills 2007).

Thick and fleshy fronds have evolved independently several times in Asplenium (e.g. A. decurrens, A. difforme, A. marinum) as an adaptation to exposed coastal conditions with salt spray (Pangua et al. 2009).

Qld: Noosa National Park, beach E of Devil's Kitchen, D.J. Ohlsen 336 & L.R. Perrie (MELU); Point Lookout, Stradbroke Island, C.W. Harman 53a (BRI).

N.S.W.: Cabbage Tree Island, E of Port Stephens, 12 Jan. 1969, P. Martensz s.n. (CANB, NSW).

N.Is.: Duncombe Bay, M. Lazarides 8063 (CANB, K, NSW); along Rooty Hill Rd, R.D. Hoogland 11298 (CANB, K); point S of Emily Bay, P.S. Green 1878 (K); Cascade, A. Cunningham 28 (K); Phillip Island, R.M. Laing s.n. (CHR).

Asplenium difforme R.Br., Prodr. 151 (1810); Asplenium obtusatum var. difforme (R.Br.) Benth., Fl. Austral. 7: 747 (1878); A. marinum var. difforme (R.Br.) F.M.Bailey, Proc. Roy. Soc. Queensland 1: 15 (1884). Type: Botany Bay, 1770, J. Banks & D. Solander s.n.; holo: BM 001079722 n.v., BM 001079723 n.v., mounted on two sheets.

Previously this species has been regarded as a variety of Asplenium obtusatum G.Forst. and is also very similar to the coastal New Zealand taxon A. appendiculatum (Labill.) C.Presl subsp. maritimum (Brownsey) Brownsey which it is distinguished by its more obtuse pinnae and slightly dimorphic fronds. Phylogenetic analyses of chloroplast DNA have shown that A. difforme is most closely related to the other Norfolk Island species A. dimorphum Kunze, which confirms treating A. difforme distinct from A. obtusatum and closely related species, and A. appendiculatum (Ohlsen et al. 2014).

F.M. Bailey, Lithograms of the Ferns of Queensland 106 (1892), as A. marinum var. difforme; S.B. Andrews, Ferns of Queensland 59, fig. 4.5A (1990), as A. obtusatum var. difforme; P.G. Wilson, in G.J. Harden (ed.), Flora of New South Wales 1: 55 (1990); K. Mills, The Flora of Norfolk Island 2: 17 (2007); K. Mills, The Flora of Norfolk Island 3: 15 (2007); P. Coyne, Norfolk Island's Fascinating Flora: 138 (2011).

Andrews, S.B. (1990). Ferns of Queensland. (Queensland Department of Primary Industries: Brisbane).

Bailey, F.M. (1892). Lithograms of the Ferns of Queensland. (Queensland Department of Agriculture: Brisbane).

Brownsey, P.J. (1998). Aspleniaceae, in McCarthy, P.M. (ed.), Flora of Australia 48: 295–327. (ABRS/CSIRO: Australia).

Coyne, P. (2011). Norfolk Island's Fascinating Flora. (Petaurus Press: Belconnen, A.C.T.).

Laing, R.M. (1915). A revised list of the Norfolk Island flora, with some notes on the species. Transactions and Proceedings of the New Zealand Institute 47: 1–39.

Mills, K. (July 2007). The Flora of Norfolk Island 2. Epiphytes and mistletoes. (The Author: Jamberoo, N.S.W.).

Mills, K. (July 2007). The Flora of Norfolk Island 3. The coastal species. (The Author: Jamberoo, N.S.W.).

Ohlsen, D.J., Perrie, L.R., Shepherd, L.D., Brownsey, P.J. & Bayly, M.J. (2014). Phylogeny of the fern family Aspleniaceae in Australasia and the south-western Pacific. Australian Systematic Botany 27: 355–371.

Pangua, E., Belmonte, R. & Pajaron, S. (2009). Germination and reproductive biology in salty condition of Asplenium marinum (Aspleniaceae), a European coastal fern. Flora, doi: 10.1016/j.flora.2008.09.007.

Wilson, P.G. (1990). Aspleniaceae, in Harden, G.J. (ed.), Flora of New South Wales 1: 53–56. (New South Wales University Press: Kensington).

Submitted 2 June 2020. Published 30 July 2020. Revised edition adapted from Brownsey (1998).