Mesotus celatus Mitt.

Plants robust, grey- or pale green, creeping, with erect branches. Primary stems prostrate, elongate (to at least 100 mm), beset with dense yellow-brown, smooth, and much-branched rhizoids. Stem leaves widely spreading from a sheathing base, short-acuminate from the very broadly ovate base, costate, (1.7–)1.9–2.8 × 0.9–1.2 mm, with scattered rhizoids arising from nematogon cells in the lower base. Branches (secondary stems) straight, at or nearly at right angles from substrate, variable in length (commonly 20–60 mm), in cross-section lacking a central strand and the outermost 3–5 layers of cells strongly incrassate and pigmented, central cells often with large, dark inclusions when dry. Branch leaves lanceolate and evenly tapered or lanceolate from an oblong base, sigmoidally curved and rugose when moist (the intact shoots appearing like a pin-wheel in end view when fresh due to the curvature of the leaves), slightly narrowed near insertion, bordered in the basal ⅓ to ½, deeply concave and with numerous mostly abaxial rhizoids below, keeled above, sharply serrate above, concolourous at apex, 4.0–6.5 × 1.0–1.3 mm (under cover slip), spirally twisted, contorted, and rugose when dry; mid and upper laminal cells highly irregular in outline and orientation, strongly incrassate, with multiple rounded papillae obscuring the lumen (the papillae becoming smaller toward the leaf apex), mostly 10–14 µm in greater dimension and 0.5–2:1; marginal cells elongate, smooth, paler than adjacent laminal cells, and strongly porous, forming a broad border extending to about mid leaf; cells of the leaf base elongate, thick-walled, smooth, very strongly porose, mostly orange, filling nearly the whole ovate base and transitioning rapidly into the upper laminal cells, intermixed with numerous shorter, oblong, and non-pigmented "nematogonous" cells, which give rise to adaxial (and fewer abaxial) rhizoids; alar cells strongly differentiated, pigmented, mostly oblong, smooth, scarcely porose, and forming a large elliptic group extending c. 6–10 cells up the margin and c. ½ to the costa base. Costa lustrous in surface view, convex and protruding abaxially, c. 0.1 the widest part of the leaf, subpercurrent, papillose abaxially near apex, smooth below, in mid leaf cross-section with scattered guide cells and abaxial and adaxial stereid groups.

Pseudautoicous. Perichaetia terminal on erect branches, overtopped by lateral innovations (but often with clusters of lateral archegonia 1 mm or less below the base of mature capsules), the inner leaves apparently expanding after fertilisation, becoming abruptly narrowed from an oblong and ± plicate base to a long subula, paler than adjacent vegetative leaves, c. 6 mm (at capsule maturity). Dwarf male plants minute and difficult to locate, c. 0.4–0.6 mm, embedded in axillary tomentum of vegetative leaves, the ♂ leaves c. 250–370 µm, serrate, mostly broadly acute, ecostate, with the cells papillose in upper portions. Setae single, <0.5 mm; capsules immersed and inconspicuous, symmetric, oblong-cylindric or broadly obovoid, c. 2.5 mm, brown-green or brown, smooth when moist, weakly wrinkled/sulcate and constricted at mouth when dry; exothecial cells mostly oblong but variable in length, firm-walled (with transverse walls thinner than longitudinal); stomata apparently absent; annulus of 1–2 rows of inflated quadrate cells, not falling; operculum short rostrate-conic, c. 0.8 mm; columella stout. Peristome teeth inserted c. 100 µm below the rim, yellow-brown, undivided, irregularly divided, or perforate in uppermost ⅓, broadly lanceolate, c. 300–400 × 90–110 µm, with conspicuous irregularly thickened trabeculae on outer surface and very faint abaxial vertical striolations and a faint zig-zag line on the inner surface. Calyptra mitrate, lobed at base, c. 1 mm. Spores dimorphic, some (apparently aborted) 12–25 µm, irregular in shape and lacking chlorophyll, the remainder multicellular (to c. 20-celled) due to endosporic germination, variably spherical, ellipsoid, to trapezoidal in a single capsule, chlorophyllose, mostly 75–135 µm in greater dimension, with age becoming extremely irregular in outline, larger, and often forming cohesive masses that adhere to the columella.

Brotherus 1924, fig. 170; Allen 1987, figs 1–3; Beever et al. 1992, fig. 25, pl. 8; Malcolm & Malcolm 2003, p. 43; Malcolm & Malcolm 2006, p. 281.

Primarily on trunks and larger branches of southern beech (especially Lophozonia menziesii, but also on Fuscospora fusca and F. solandri s.l.); less often on other tree species, including Coprosma cf. propinqua, Griselinia littoralis, Hoheria sp., Myrsine divaricata, Pseudopanax anomalus, and Weinmannia racemosa as well as Dacrycarpus dacrydioides and Dacrydium cupressinum. One epilithic collection from an "exposed boulder in open part of [Fuscospora truncata] forest" at Mt Furneaux in Marlborough L.D. (P.J. Brownsey s.n., 2 Jan 1992, WELT M028208) has been confirmed. On the South I. this species ranges from near sea level (Lake Paringa, Westland L.D.) to c. 1200 m (Mt Richmond, Marlborough L.D.). It is frequently associated with a wide range of epiphytic taxa, including Cladomnion ericoides, Dicnemon calycinum, D. semicryptum, Holomitrium perichaetiale, H. trichopodum, Lepyrodon australis, Macromitrium longipes, Papillaria flavolimbata, Weymouthia cochlearifolia, as well as Herbertus alpinus, Jamesoniella monodon, Lepicolea scolopendra, Paraschistochila tuloides, Plagiochila spp., and Porella elegantula.

The few records of M. celatus from the North I. seem to accurately reflect its occurrence there. Mesotus celatus is a widespread and relatively common species throughout Nelson, Marlborough, Westland, Otago, and Southland L.D.; it is relatively infrequent in Canterbury, with the exception of a few localities (e.g., Lewis Pass) near the Main Divide. The absence of records from Stewart I. probably reflects the absence of southern beech species on that island.

Mitten (1882, p. 52) described M. acutus, providing a confusing diagnosis emphasising the dense areolation and the more strongly serrulate ("magis serrulatis") margins relative to M. celatus. No collector or locality data (apart from "Australia") is given for the type specimen, which has not been examined. Although Allen designated a holotype for M. acutus Mitt. in NY (ex herb. Borrer) and placed it in the synonymy of M. celatus (his placement is followed here), the uncertainty concerning the type’s provenance and the question of whether M. celatus occurs (or occurred) on either mainland Australia or Tasmania remains uncertain. There are apparently no modern collections of Mesotus from either region. Dalton et al. (1991) placed M. acutus in their "doubtful and excluded records" for Tasmania. Dalton has also (pers. comm., July 2008) specifically and unsuccessfully searched for Mesotus on that island. Streimann & Klazenga (2002, p. 116) considered the occurrence of Mesotus on either mainland Australia or Tasmania doubtful. Mesotus celatus is here considered a N.Z. endemic genus and species.

The perichaetia in this species were interpreted as terminal and subtended by an innovation by Lindberg (1872). Allen (1987, p. 446) opined that the terminal perichaetia are "laterally displaced by branch innovations". Lindberg (1872) also interpreted the innovation as arising from the axil of the uppermost vegetative leaf and termed the fruit "pseudolateral". Clusters of lateral archegonia that are not associated with any modified leaves often occur a short distance (c.1 mm) below the base of the seta.

Epiphytic dwarf ♂ plants in this species are, in my experience, extremely difficult to locate. The few calyptrae seen have weak multiple lobes at their base, but are not "deeply four-lobed" as described by Allen (1987). In more mature capsules the germinated and multicellular "spores" tend to form cohesive protonemal masses, which are often adherent to the columella. The shapes of these cell masses become highly irregular (rather than spherical to trapezoidal when the capsules are green), and in some cases the boundaries between the spore-derived cellular clusters become obscured.

The species that is most likely confused with M. celatus is Sclerodontium pallidum, which has a similar overall habit, coloration, and bordered and rugose leaves with papillose cells. In the field, M. celatus branches exhibit a very distinctive pin-wheel-like aspect in end view due to the sigmoid curvature of the individual leaves. Mesotus celatus leaves lack hair-points, while in S. pallidum some hair-pointed leaves are nearly always present. The immersed and inconspicuous capsules, the multicellular spores, and the multiple and unbranched nature of the laminal papillae of Mesotus also serve to distinguish it, as do its epiphytic habitat and more southern distribution.

Mesotus shares a creeping habit with species of Macromitrium, but no N.Z. Macromitria has the pin-wheel aspect of the branches, dimorphic leaves, immersed capsules, or robust stature of Mesotus.