Leptodictyum riparium (Hedw.) Warnst.

Elements in the following description are taken from Kanda (1976).

Plants yellow- to bright-green, sometimes black in submerged forms, weakly lustrous when dry. Stems prostrate or free-floating, irregularly and rather loosely branched in a single plane, in cross-section with thick-walled outer cells grading into thin-walled parenchymatous inner cells and with a small and indistinct central strand. Branches variable in length, usually with leaves more complanate than the stems. Stem and branch leaves similar, rather distant, wide-spreading, distinctly or moderately complanate, and ± twisted longitudinally when dry, extremely variable in shape and size, narrowly to ± broadly ovate-lanceolate, gradually tapered to a narrowly acuminate or a short ± acute apex, often variable in shape on a single plant, variable in length, (2.0–)2.5–3.5 mm, not channelled, very weakly decurrent and with some cortical cells often stripping off with leaves, entire; mid laminal cells linear-rhomboidal, mostly 65–95 × 9–11 µm and c. 6–9:1, not porose, the lumina much broader than the cell walls; basal cells shorter and wider; alar cells not or very weakly differentiated from adjacent basal cells, rectangular or subquadrate, plane. Costa single, rather thin, extending c. ½–⅔ the leaf. Pseudoparaphyllia and axillary hairs not seen. Paraphyllia absent.

Autoicous. Perichaetia scattered on stems, the inner leaves erect, sheathing the setae, oblong-lanceolate and costate. Perigonia gemmiform, very small (c. 0.5 mm), and scattered on stems. Setae commonly 10–15 mm (or longer in submerged forms), weakly flexuose, red, smooth; capsules short-cylindric from a weakly defined neck, strongly curved, c. 2.5 mm; mouth oblique and constricted when dry; exothecial cells mostly oblong-hexagonal, firm-walled; stomata not seen; operculum conic. Exostome pale yellow-brown, the teeth cross-striolate below on outer surface and distinctly bordered; endostomal segments from a high basal membrane, keeled and perforate, alternating with 1–2 nodose cilia. Spores 12–18 µm, very finely papillose.

Smith 2004, fig. 255, 9–12; Crum & Anderson 1981, figs. 454–457, 458 J–O.

Leptodictyum riparium is often recognised as much by its habitat and overall habit as by its few distinguishing microscopic features. Confusion is most likely with straight-leaved and submerged forms of Drepanocladus aduncus and D. brachiatus. The absence of secund leaves at the branch/stem apices in Leptodictyum riparium combined with more distinctly complanate branch leaves, less narrowed leaf bases, the tendency for leaves to twist around their longitudinal axis when dry, and lack of inflated alar cells will distinguish most collections. In all instances the alar cells of L. riparium are plane, whereas those of Drepanocladus spp. tend to be excavate. In populations which are poorly developed or heavily encrusted with unicellular algae confident determination is sometimes not possible.

Confusion is also likely with Campyliadelphus polygamus and not all material can be distinguished with confidence. Leptodictyum is confined to lowland situations while C. polygamus occurs in lowland to alpine elevations. The shoots of L. riparium are usually distinctly complanate and sometimes weakly secund distally, while those of C. polygamus are neither complanate nor secund. In L. riparium the leaves are ± evenly tapered from base to apex and the upper portions are flat (either moist or dry); those of C. polygamus are unevenly tapered and channelled above (either moist or dry). Both species can have spirally twisted leaves, but this is more distinct and constant in L. riparium. The leaves in L. riparium can also be longer (2.0–3.5 mm) than those of C. polygamus (c. 1.8–2.3 mm).

Mostly attached to wood in wet situations, less often attached to rock or soil. Often on stumps or logs beneath willows at stream or lake margins or in swamps; frequently growing in depressions which are flooded only at intervals of high water. Also occurring in clear, spring-fed streams (as at Christchurch and probably at Onehunga, N Auckland L.D.). Rarely attached to rocks "in fast flowing stream(s)" as at Waipapa River (S Auckland L.D.). Occurring from near sea level to at least 610 m (Lake Waikaremoana, Gisborne L.D.), but with most localities <100 m elevation.

Sometimes growing with aquatic species of Fissidens (F. berteroi, F. rigidulus), L. riparium is also sometimes associated with both Nitella (as at Lake Wairarapa, Wellington L.D.) and Chara (as at Lake Rotoroa, Nelson L.D.). Mixtures with Drepanocladus sp. and other aquatic mosses occur and can be perplexing.

Collections by R. Mason from a "fast flowing stream" (Waipapa River, S Auckland L.D., CHR 106224) are coarser and more robust than the bulk of examined material, with median cells up to 120 µm and c. 10:1. At Lake Rotoroa (Nelson L.D.) L. riparium has been collected at depths of between 17–22 m as part of an assemblage of 7 moss species, two thalloid hepatics, and Chara australis (de Winton & Beever 2004).

A few North I. collections, including R. Mason 7542 (Waitoa River, S Auckland L.D., CHR 106208) and K.W. Allison 4954 (Ngā Awapurua, Hawke’s Bay L.D., CHR 578605), are black in colour. However, a collection from the Waipapa River, Gisborne L.D., R. Mason 7732 (CHR 106224) is intermediate between this black form and more representative material.