Wijkia extenuata (Brid.) H.A.Crum

Plants extremely variable in habit, branching, and colour, medium-sized, forming densely interwoven mats. Stems creeping, irregularly pinnately or bipinnately branched, short or elongate (to c. 100 mm or more), with scattered fascicles of smooth brown rhizoids, in cross-section with no hyaloderm, several outer layers of thick-walled cells, and no central strand. Branches simple or subpinnately branched, often tapered to a slender and cuspidate tip, occasionally with numerous microphyllous and flagelliform branchlets (in var. caudata). Stem and branch leaves differentiated. Stem leaves erect-spreading, broadly ovate to ± oblong, abruptly tapered to a piliferous, weakly twisted, and serrulate apex (up to ⅓ the total leaf length), becoming less toothed towards the base, concave, plane or nearly so at margins, c. 1.8–2.0(–2.5) × 0.7–0.8 mm (under cover slip); mid laminal cells (upper third of leaf) linear to linear-rhombic, mostly 45–60 × c. 4 µm, and c. 10–15:1, firm-walled, seriately papillose abaxially or occasionally smooth, porose; basal cells shorter, strongly porose, and orange-pigmented in a few rows across base; alar cells strongly differentiated, c. 8 cells inflated and hyaline or pigmented at extreme alar corners, with a few irregular or oblong supra-alar cells. Branch leaves smaller, erect-spreading or appressed, ovate-lanceolate or lanceolate, ± evenly tapered to a narrowly acute apex, sharply serrulate above by projecting cell ends (usually more serrulate than stem leaves), or rarely crenulate or entire, concave, smooth, not clasping, often weakly twisted in apex, (0.7–)1.1–1.4 × 0.25–0.35 mm; mid laminal cells (upper third of leaf) linear-rhombic, firm-walled, seriately papillose abaxially (with up to 5 low, rounded papillae) or occasionally smooth, usually porose; alar cells usually fewer than in stem leaves, often 4–6 strongly inflated in extreme alar corners.

Dioicous. Perichaetia scattered on stems, large (c. 2.5 mm) and conspicuous (apparently lengthening after fertilisation), the inner perichaetial leaves lanceolate and tubulose, sheathing the seta, serrulate above, tapered to a very slender and often ± reflexed acumen, with strongly porose laminal cells. Perigonia scattered on branches, gemmiform, and c. 0.7 mm long, the inner bracts broadly ovate. Setae red-brown, smooth, long and slender, twisted to the left above, 30–45 mm; capsules ± horizontal, oblong-cylindric, asymmetric and curved, narrowed below the mouth and weakly striolate when dry, with a short and ill-defined neck, not constricted below the mouth when moist, 2.0–2.5 mm; stomata few in neck, superficial; annulus apparently absent; operculum high-conic, lacking a rostrum; exothecial cells isodiametric to oblong, moderately thickened in corners but with the longitudinal walls thicker than transverse walls (± "subcollenchymatous"). Peristome as per genus; endostomal cilia paired and nodose (few seen). Calyptra cucullate, smooth. Spores spherical, mostly (12–)15–24 µm, green, smooth, sometimes germinating in capsule.

Confusion is most likely to occur between the present species and Sematophyllum subhumile var. contiguum, q.v.

Distribution and ecology as for varieties.

This is an extremely variable species with numerous forms (not given taxonomic recognition here), which can cause confusion until the range of malleability is recognised. Necessarily, the description above is based on representative material. When well-developed, the distinctly dimorphic leaves easily differentiate W. extenuata from any other N.Z. species of Sematophyllaceae.

In the most common and representative form of the species, the distinction between the stem and branch leaves is obvious, the stem leaves being abruptly tapered to a piliferous and serrulate apex and the branch leaves distinctly smaller, ± ovate-lanceolate or lanceolate, more evenly tapered and sharply serrulate above. The laminal cells of the branch leaves are usually distinctly porose and abaxially papillose. The abaxial papillae are more easily seen near the apex of branch leaves by mounting a small, intact piece of the branch. The papillae, which can be seen only under a compound microscope, can then be viewed in profile. Capsules are uncommon, reflecting the dioicous nature of the species. When fresh, well-developed plants often have a grey-green colouration and an oily sheen. While difficult to describe, this colouration is distinctive in the field.

One very common variant consists predominantly of branches. In most instances of this "branch-only" growth form, the branch leaves are strongly toothed, and the laminal cells porose and papillose abaxially, thus allowing ready assignment to the present species. There is an obvious gradation between representative and "branch-only" growth forms. Whitehouse 29179a (from Lake Waikareiti, Gisborne L.D., CHR 265425) is an example of such transitional material. "Branch-only" growth forms do not develop the flagelliform branches that are a conspicuous feature of Wijkia extenuata var. caudata.

As with many epiphytic species, W. extenuata can sometimes assume a pendent habit (e.g. K.W. Allison 2294 from Rotorua, S Auckland L.D. (CHR 579505) and J.-P. Frahm 11-8 from Wills Hut Track, Otago L.D., CHR 503325). In this pendent growth form the leaves are narrowly lanceolate or ovate-lanceolate and ± evenly tapered to a slender and nearly entire acumen. The branch leaves in such material are longer (to at least 1.6 mm) and finer than usual; the laminal cells here are often weakly porose and can be either smooth or papillose abaxially.

Populations also occur in which abruptly tapered stem-like leaves predominant. Such "stem-only" forms are often terrestrial (H. & W. Frey 94–143, near Murchison, Nelson L.D., CHR 458214). "Stem-only" forms obtain their most extreme expression in alpine grasslands where the plants are strongly yellow-pigmented, the leaves only weakly denticulate near their apices, the laminal cells smooth but porose (C.D. Meurk s.n. from Temple Basin, CHR 481544).

The type collection of Hypnum extenuatum Brid. was made by Labillardière in Nova Hollandiae fide Müller (1850–1851, vol. 2, p. 392), with subsequent collections by Sieber and J.D. Hooker.