Rhynchostegium

Rhynchostegium is a genus of pleurocarpous mosses belonging to the family Brachytheciaceae.^[2] The genus has a cosmopolitan distribution across different climatological regions except the polar regions, mostly in tropic to north temperate regions.^[2][1] The genus contains both aquatic and terrestrial species.^[1][3] The genus was named for their rostrate opercula.^[1] The type species of this genus is Rhynchostegium confertum (Dicks.) Schimp.^[1]

The genus name comes from the Greek rhyncho- (beaked) and stegos (a lid), which refers to the rostrate operculum of the sporophyte.^[1]

The genus was first described by Bruch and Wilhelm Philippe Schimper in 1852.^[2][1]

Terrestrial species of Rhynchostegium live in moist to wet or shaded habitats, on rock, soil, tree base, tree stem, and logs.^[1][4][5]

Aquatic species live by or in running water, including streams, springs, rivers, beds of waterfalls, and seepy cliffs.^[3][6]

Rhynchostegium are small to large mosses that form either loose tuft or extensive mats on the substrate, with irregular or regular branching.^[1][3] The younger plants are generally deep green or light green; aging plants could become whitish, brownish, or paler green.^[1][3] Stems are creeping and lack hyaloderm, with acute to acuminate pseudoparaphyllia.^[3] Stem leaves are erectopatent or erect.^[1][3] Branch leaves are similar in morphology to stem leaves but smaller and sometimes narrower.^[3] Leaves are commonly straightly to homomallously arranged; subimbricate, subcomplanate, or complanate arrangement are sometimes seen, especially in branch leaves.^[1] Leaf base, decurrent or not, varies from ovate to ovate-cordate, occasionally lanceolate, and the narrowing from gradual to abrupt, towards a short- or long-acuminate apex, where sometimes a differentiated long acumen or apiculus is present.^[1] The leaves have a single costa that generally smoothly ends 35-75% up the leaf, and more often in branch leaves in an abaxial spine.^[1] Leaf surfaces vary from flat to slightly concave and not to strongly longitudinally plicate, with little to some pores and linear laminal cells.^[1][3] Leaf margins are serrate to serrulate.^[1][3] Axillary hairs constitute of 3-7 cells,^[3] with 1-3 upper cells.^[1] Alar cells are slightly enlarged, and either undifferentiated or quadrate to elongate-rectangular.^[1][3]

Rhynchostegium are autoicous.^[3] Covered by a naked calyptra is a rostrate to long-rostrate operculum attached to a red-brown to brown, oblong-cylindric, weakly curved capsule, which is inclined or horizontal to a red-brown, smooth seta that has abruptly contracted perichaetial leaves at the base, with acumen straight to reflexed.^[1][3] An annulus separates the operculum.^[3] The peristome is xerochastic and perfect, which the red to orange-red exostomes have reduced trabeculae and cross-striolae at the base of the teeth; in rare cases the exostomes are narrow and yellow.^[1][3] The broadly or narrowly perforated endostomes and developed to vestigial cilia are supported by a low or high basal membrane.^[1] Spore diameters range between 9-16 μm.^[3]

Allelopathy has been studied on Rhynchostegium pallidifolium, which usually form pure colonies in their natural habitat.^[7][8] Methanol extract of R.pallidifolium represses the seedling of cress, alfalfa, lettuce, ryegrass, timothy, and Digitaria sanguinalis in a concentration-dependent manner.^[7] A combination of ESI-MS and ¹H NMR analyses identified the inhibitory chemical as 3-hydroxy-β-ionone.^[7] Further study showed a minimal 3-hydroxy-β-ionone concentration of 1 μM for the inhibition of cress hypocotyl growth, and 3 μM for cress root growth, while the endogenous concentration.^[8] The presence of 3-hydroxy-β-ionone in their natural substrate and the growing medium suggested secretion to the environment, which may imply an important role of 3-hydroxy-β-ionone in competition with other plants and the forming of pure colonies.^[8]

Acetone extract of Rhynchostegium riparioides showed antibiotic activity on some Gram-negative bacteria, including Escherichia coli, Proteus mirabilis, Entero-bacter cloacae and Pseudomonas aeruginosa.^[9]

Ethanolic extract of Rhynchostegium vagans showed similar effect on some Gram-negative bacteria and fungi, with performance superior to chloramphenicol and fluconazole.^[10]

Rhynchostegium riparioides is used in monitoring of heavy metals concentration in freshwater in multiple regions around the world,^[11][12][13] such as copper,^[14][15] zinc.^[16] R. riparioides as a neutrophilous species has been used in monitoring water acidification.^[17]

The World Flora Online lists 221 species of Rhynchostegium.^[18]