Orthotrichum casasianum
Orthotrichum casasianum is a species of moss in the family Orthotrichaceae. It is endemic to the Spanish province of Álava, in the Basque Country. It grows in less than 200 trees on the banks of the Bayas river. The species is critically endangered, and was added in 2013 to the Basque Catalog of Threatened Species.
Description
Orthotrichum casasianum is a small epiphytic moss forming loose, olive-green cushions up to about 20 mm across, with individual shoots 6–10 mm tall. The leaves are erect and become slightly when dry, measuring 2.0–2.8 mm long by 0.4–0.6 mm wide. They are linear- to linear-oblong, with margins for much of their length. Leaf tips are broadly rounded or, often concave, and typically bear a minute one- or two-celled . Upper leaf cells are thick-walled and either smooth or bear very low, whereas basal cells are larger, transparent, thin-walled and rectangular.Reproductive structures occur on autoicous plants. Sporophytes have short 0.4–0.6 mm long, carrying emergent urn-shaped capsules up to 2 mm in length. When dry, these capsules are cylindrical- with eight distinct longitudinal ribs and stomata around the upper half. The double comprises an outer layer of sixteen teeth fused into eight pairs; each tooth shows a pattern of low papillae externally and is almost smooth internally. The inner peristome consists of sixteen smooth, markedly incurved segments. The protective is naked, smooth and distinctly folded.
Distribution and habitat
At the time of its original publication, this species was known only from two well-preserved riparian woodlands along the Bayas River in Álava, northern Spain. It grows as an epiphyte on the bark of Alnus glutinosa, Sambucus nigra, Fraxinus excelsior and Corylus avellana in humid, eurosiberian-climate forests that experience periodic flooding. Similar humid woodland habitats in the Cantabrian Mountains and western Pyrenees may harbour additional populations.Taxonomy
Orthotrichum casasianum was first formally described in 1999 by F. Lara, R. Garilleti and V. Mazimpaka following the study of fruiting specimens collected from riparian woodlands in Álava, northern Spain. The authors placed it within the family Orthotrichaceae, and specifically assigned it to subgenus Pulchella on account of its emergent, ribbed capsules and the characteristic structure of its peristome. The specific epithet honours the distinguished Iberian bryologist Creu Casas for her contributions to the study of mosses in Spain.Within subgenus Pulchella, O. casasianum is most closely allied to the Holarctic scanicum complex—especially O. scanicum and O. lewinskyae—sharing features such as a cylindric-urn-shaped capsule with hemicryptoporous stomata and a double peristome. It can, however, be readily distinguished by its consistently brown, capsules; broadly rounded, often concave leaf apices tipped with a tiny hyaline apiculus; and the faint reticulation of its teeth rather than the dense papillae seen in similar taxa. Although superficially similar to the variable O. pallens, O. casasianum differs in its slender, loosely foliose shoots, more incurved leaf tips when dry, and the uniform colouration of its capsule and seta.