Plagiothecium Schimp.

The following generic description draws on Buck & Ireland (1985) and Ireland (1986).

Plants mat-forming, light-, dark-, or yellow-green, often glossy. Stems prostrate or occasionally ascending, irregularly branched or rarely simple, mostly complanate-foliate, sometimes ± julaceous, in cross-section with central strand present or indistinct, with the outermost layer of cells enlarged and thin-walled. Branches prostrate or ascendant. Stem and branch leaves similar, soft, imbricate, complanate to julaceous, erect or spreading, ovate to ovate-lanceolate, often asymmetric, concave, entire or serrulate at apex, decurrent; upper laminal cells linear or linear-rhomboid, smooth (in N.Z. species), shorter in leaf apex, below broader, shorter, and sometimes weakly porose, not pigmented at base; alar cells differentiated, usually inflated, ± decurrent. Costa short and double, occasionally single forked or simple. Asexual propagulae often present in leaf axils (but absent in N.Z. species), cylindric or fusiform, uniseriate. Paraphyllia and pseudoparaphyllia lacking.

Autoicous or occasionally dioicous. Perichaetia and perigonia often clumped near stem base. Setae elongate, smooth; capsules erect to cernuous, oblong-cylindric or ovoid, often striate when dry; exothecial cells thin- or thick-walled; stomata present in neck; annulus differentiated, composed of inflated hyaline cells in N.Z. species; operculum conic or rarely ± rostrate. Peristome hypnoid. Calyptra cucullate, smooth, falling early. Spores nearly round to broadly elliptic, smooth or minutely ornamented.

A genus of about 90 species according to Buck & Ireland (1985), occurring mainly in temperate regions and at high elevations in the tropics. Eventual revision will probably substantially reduce the number of accepted species.

The genus Plagiothecium in N.Z. presents taxonomic difficulties greater than the single species recognised here would suggest. While no member of this genus seems to be discussed by Wilson (1854), Hooker’s Handbook (1867) reported a single collection of the northern hemisphere P. (Hypnum) denticulatum from southern beech forest in Canterbury. Brotherus (1916) proposed a second species, and Dixon (1929) chose to recognise both P. denticulatum (L.) B.S.G. and P. novae-seelandiae Broth. from N.Z., differentiating them on the basis of their sexualities and leaf apical shapes. Sainsbury (1955) recorded only one species, for which he used the name Plagiothecium denticulatum (Hedw.) B.S.G. Ireland (1992) reviewed the Australasian material and concluded that P. denticulatum "probably occurs only in the Northern Hemisphere" and that Australasian material so-named is better considered as P. novae-seelandiae Broth.; he also recorded the South American Plagiothecium lucidum from N.Z. Ochyra (2002) subsequently synonymised P. novae-seelandiae with the Australian P. lamprostachys (Hampe) A.Jaeger.

The characters used by Ireland (1992) to differentiate his two alleged N.Z. species are difficult to apply across a broad range of material, although extreme forms have a very different gestalt. The differing appearance provides some support for Ireland’s (and Dixon’s) interpretation regarding the presence of two distinct taxonomic entities. Among the characters in Ireland’s key are the width of the leaf decurrency, the width of mid laminal cells, and the presence or absence of flagelliferous branches; however, a large fraction of herbarium collections/populations are intermediate with respect to these characters. Many intermediates have markedly complanate shoots and branches, and distinctly asymmetric leaf bases (allegedly features of P. lucidum) associated with leaves that are broad at apices and not or only weakly wrinkled when dry (allegedly features of well-developed P. lamprostachys). Such intermediates are widely distributed regionally. The size of laminal cells is also variable and does not correlate well with either of Ireland’s species concepts. The many morphological intermediates blur the distinction between the two alleged species and consequently only a single N.Z. species of Plagiothecium is accepted here.

Not all relevant types have been studied, and the taxonomic conclusions of Ochyra (2002) are adopted here. He recognised one species of Plagiothecium from Australasia, and considered P. lamprostachys (Hampe) A.Jaeger to be the earliest applicable name. Klazenga (2012) also accepted only P. lamprostachys in his treatment for Australia.

Plagiothecium helvolum is an unpublished name applied by C. Müller in herb. to an R. Helms collection. Its "type" is referable to Sauloma tenella. This name is not considered further.

Plagiothecium laetum B.S.G. was recorded from Campbell I. by Vitt (1974), but there can be little doubt that his collections (not seen) represent P. lamprostachys. Plagiothecium laetum is a northern hemisphere species and is not considered further.

Plagiothecium lucidum (Hook.f. & Wilson) Paris. This name, based on a Chilean type, has been applied to N.Z. mainly by Ireland (1992). The reasons for rejecting it as a N.Z. species are outlined above.