Chiastocaulon
Epiphytic or lithophytic, dioicous. Specialised asexual propagules absent. Stems differentiated into creeping stems with reduced leaves and stems with normal leaves, diverging from the substrate and sparingly branched or rarely frequently irregularly to flabellately branched (not in Victoria), with two ranks of lateral leaves and sometimes vestigial underleaves; branches emerging from stems abaxially or laterally (not in Victoria) and with a collar of tissue at base, or abaxial to leaves with reduced width and sometimes fewer lobes and without a collar of tissue at base (not in Victoria). Lateral leaves reniform, rotund or broadly ovate to lanceolate, oblong (not in Victoria) or obovate (not in Victoria) in outline, transverse to succubous, alternate or opposite (not in Victoria), remote to imbricate, secund in direction of abaxial stem, horizontally spreading and directed toward the stem apex (not in Victoria) or transversely oriented, dentate to lobulate at the apex, dentate to lobulate along the abaxial margin and sometimes adaxial margin (not in Victoria), or entire on all margins away from apex (not in Victoria), plane or recurved at base, and sometimes revolute on adaxial margin and forming a distinct cnemis (not in Victoria). Leaf cells quadrate, circular, oblong, rectangular or elliptic, usually longer toward central leaf base, smooth or faintly striolate, thin-walled with coarse nodulose and often confluent trigones throughout leaf (not in Victoria) or coarse nodulose and confluent trigones mostly toward central base and usually more evenly thick-walled toward margins, or firm-walled throughout and without trigones (not in Victoria), with 2–10 oil bodies; oil bodies ellipsoid to ovoid, homogenous (not in Victoria) or granular-botryoidal. Rhizoids on creeping stems with reduced leaves, arising from leaf and underleaf bases, usually absent from stems with normal leaves. Androecia becoming intercalary, sometimes forming the bases of branches (not in Victoria), with 3–4 pairs of leaf-like bracts or with up to around 30 pairs of strongly differentiated and saccate bracts (not in Victoria), each with 1 or 2 (not in Victoria) antheridia. Sporophytes terminal on stems with normal leaves, developing within a perianth; perianth obloid to ellipsoid or obovoid to obconic (not in Victoria), with prominent adaxial and abaxial keels and with bilabiate and dentate, laciniate or rarely entire (not in Victoria) mouth; capsule ellipsoid, 4–7-stratose; elaters bispiral; spores globose, papillate or granulate.
Twenty species shared between New Zealand, eastern Australia, and India through to the tropical Pacific, with one species extending north to Japan; one species, C. biserialis (Lindenb.) S.D.F.Patzak, M.A.M.Renner, Schäf.-Verw. & Heinrichs, in Victoria.
Chiastocaulon in its traditional circumscription comprised a single species, C. dendroides (Nees) Carl (Söderström et al. 2016) and has been alternatively treated as a subgenus of Plagiochila (e.g. Schuster 2021). However, phylogenetic analyses of nuclear and chloroplast DNA sequences demonstrated that Chiastocaulon, as well as Acrochila R.M.Schust., both genera with alternate lateral leaves, evolved from Plagiochilion S.Hatt. ancestors, which are distinctive among the Plagiochilaceae for their opposite leaves (Patzak et al. 2016; Renner et al 2016). Acrochila and Plagiochilion have accordingly been subsumed into a broader Chiastocaulon to address their non-monophyletic status (Patzak et al. 2016), which is followed here. Under this circumscription, Chiastocaulon encompasses Plagiochilaceae species with branches that mostly emerge from the abaxial side of stems and have a collar of tissue at their base (i.e. ventral-intercalary branching) and have leaf cells without papillae.
Patzak, S.D.F., Renner, M.A.M., Schäfer-Verwimp, A., Feldberg, K., Heslewood, M.M., Peralta, D.F., de Souza, A.M., Schneider, H. & Heinrichs, J. (2016). A phylogeny of Lophocoleaceae-Plagiochilaceae-Brevianthaceae and a revised classification of Plagiochilaceae. Organisms Diversity & Evolution 16: 481–495.
Renner, M.A.M., Heslewood, M.M., Patzak, S.D.F., Schäfer-Verwimp, A. & Heinrichs, J. (2016). The genera Chiastocaulon, Cryptoplagiochila and Pedinophyllum (Plagiochilaceae) in Australia. Australian Systematic Botany 29: 358–402.
Schuster, R.M. (2021). Austral Hepaticae Part III. Nova Hedwigia Beiheft 120. Cramer in der Gebrüder Borntraeger Verlagsbuchbehandlung: Stuttgart.
Söderström, L., Hagborg, A., von Konrat, M., Bartholomew-Began, S., Bell, D., Briscoe, L., Brown, E., Cargill, D.C., Costa, D.P., Crandall-Stotler, B.J., Cooper, E.D., Dauphin, G., Engel, J.J., Feldberg, K., Glenny, D., Gradstein, S.R., He, X., Heinrichs, J., Hentschel, J., Ilkiu-Borges, A.L., Katagiri, T., Konstantinova, N.A., Larraín, J., Long, D.G., Nebel, M., Pócs, T., Puche, F., Reiner-Drehwald, E., Renner, M.A.M., Sass-Gyarmati, A., Schäfer-Verwimp, A., Moragues, J.S., Stotler, R.E., Sukkharak, P., Thiers, B.M., Uribe, J., Váňa, J., Villarreal, J.C., Wigginton, M., Zhang, L. & Zhu, R. (2016). World checklist of hornworts and liverworts. Phytokeys 59: 1–828.
Spinning