Tetraphidopsis pusilla (Hook.f. & Wilson) Dixon

Plants yellow- or brown-green, very small and slender, usually on twigs or small branches. Primary stems not seen, apparently lacking. Secondary stems red-brown and clustered, variably branched but often simple (branches, if present, arising in leaf axils), mostly 3–15(–40) mm, with smooth, red-brown rhizoids restricted to base of the stem or to branch bases, in cross-section lacking a central strand. Leaves in several rows, not complanate, spreading at c. 45 (when moist), erect-appressed when dry, not secund, lanceolate, acute (sometimes narrowly acute), neither twisted nor reflexed at apex, smooth, narrowly recurved or plane at margins, entire or weakly crenulate above, mostly (0.8–)1.0–1.2(–1.5) × 0.25–0.35 mm but usually smaller on lower stem; mid laminal cells elongate-rhombic, not porose, mostly 27–39 × 7–9 µm, with cytoplasm retracted from cell walls in dried material; cells at insertion shorter and weakly pigmented; alar cells short-rectangular to ± quadrate, numerous but merging gradually with interior cells and not forming a distinct group; cells at basal margins usually not pigmented. Costae single, ill-defined, usually c. ¼ the leaf length, occasionally longer. Paraphyllia absent. Pseudoparaphyllia present, foliose. Gemmae usually present, clavate, mostly 4–6 cells and 90–105 µm long, aggregated in dense, ± ovoid clusters (clusters c. 0.25–1.0 mm long) and borne at the apex of terminal flagelliferous shoots.

Apparently dioicous. Perichaetia scattered and appearing lateral, in leaf axils on secondary shoots, the inner leaves ovate-lanceolate and ecostate, expanding after fertilisation (to c. 1.8 mm) and clasping the seta base. Male plants not dwarfed, with numerous scattered or sometimes terminal gemmiform and yellow-brown perigonia. Setae c. 3 mm, straight, pale; capsules erect, symmetric, ellipsoid, c. 1 mm; exothecial cells ± irregular, mostly rounded and short oblong; annulus not seen; operculum short rostrate, much shorter than the capsule. Peristome double; exostome teeth c. 275–300 µm, weakly shouldered, neither furrowed nor bordered, transversely striate below, finely baculate above on the outer surface, trabeculate but otherwise smooth on the inner surface; endostome lacking a basal membrane and cilia, the segments c. ⅔ the height of the teeth, straight or sometimes appendiculate. Calyptra cucullate. Spores single-celled, dimorphic, the larger variable in size, often ovoid, c. 30–45 µm in greater dimension, papillose.

Dixon 1912, pl. 21, figs 25–41 (as T. novae-seelandiae); Sainsbury 1955, pl. 52; Malcolm & Malcolm 2003, p. 65.

The minute size, the terminal clusters ("capitula") of gemmae and the preference for small branches and twigs as a substrate make this species virtually unmistakeable in a N.Z. context.

Mostly on twigs and small branches, much less often on tree trunks. Most commonly at forest and stream margins in moist environments. Occurring on at least 13 genera of woody dicots, as well as Cordyline and Ripogonum. Frequently associated bryophyte species include: Calyptopogon mnioides, Crosbya straminea, Cryphaea spp., Daltonia splachnoides, Ephemeropsis trentepohlioides, Neckera pennata, Papillaria flexicaulis, various Orthotrichaceae, as well as Dendroceros validus, Metzgeria hamata and various leafy hepatics. Occurring from near sea level to c. 760 m (near Tangiwai, Wellington L.D.).

The majority of populations of this minute species are sterile, with shoots <15 mm high, and bear a characteristic terminal ovoid to spherical cluster of gemmae. A short portion of the shoot immediately below the gemmae usually bears reduced leaves (i.e. is flagelliferous) but is otherwise suggestive of the gemmae clusters seen in the well-known but unrelated northern hemisphere species Aulacomnium androgynum.

Capsules are relatively rare (as Sainsbury correctly noted) and plants with capsules only rarely found. Fruiting shoots, when found, are frequently elongate (from c. 15–40 mm) and much branched. The original Colenso collection is fruiting, and Hooker & Wilson’s protologue of Meteorium pusillum reflects this. Spores are apparently dimorphic. The larger and presumably viable spores are variable in size and are mixed with smaller (mostly c. 12–15 µm) spores that are perhaps non-viable. However, the presumption that the smaller spores are not viable needs experimental confirmation, as does the possibility that the different-sized spores may germinate to form male and female plants.

Sainsbury’s (1955) suggestion that T. pusilla is "very rare in the South [Island]" is not correct, although it is certainly more frequent in wetter parts of this island. Nearly all Canterbury L.D. collections are from the Banks Peninsula and localities near the Main Divide. All collections (excepting one from Mt Watkin) from Otago L.D. are from relatively damp localities in the vicinity of Dunedin and along the south-eastern Otago coast. Numerous records are known from Westland L.D.