Platessa arborea C. Radhakrishnan, S. Sherly & B. Karthick 2022, sp. nov

Platessa arborea C. Radhakrishnan, S. Sherly & B. Karthick sp. nov. (Figs 1–67, Fig. 9 represents the holotype) LM Description (Figs 1–60):—Valve elliptical with narrowly rounded apices. Raphe-sternum valve (RSV): Length: 6.5–9.5 µm, width: 3.5–4.5 µm, ratio length/width: 1.6–2.2 (n = 60). Axial area narrow and hardly discernible, visible only near center of the valve; central area nearly rectangular shape, bordered by 2–3 shorter striae frequently more widely spaced. Striae radiate throughout the valve, 22–24 in 10 µm. Raphe slightly filiform expanded proximal raphe endings. Sternum valve (SV): wide axial area, Length: 6.0–9.0 µm, width: 3.5–4.5 µm. Striae densely arranged 22–24 in 10 µm, parallel at centre, radiate towards apices. SEM Description (Figs 61–67):—Raphe-sternum valve (RSV): Externally, raphe filiform, slight curvature found near ends (Figs 61 & 63). Proximal raphe ends positioned in a broadened shallow groove, distal raphe located in the shallow groove with cone-shaped endings, and both ends slightly bend to the same side (Fig. 61). Striae mostly uniseriate; however found biseriate at 2–3 striae near apices. (Fig. 61). Areolae, apically slit-like throughout the valve, only at the center of the valve it is round-shaped towards axial area (Fig. 61). Internally, proximal raphe ends are strongly hooked, a very remarkable feature which is infrequent in other Platessa species, and its distal ends are curved on opposite sides, terminating in well-developed helictoglossae (Figs 64, 65). Striae lowered between raised virgae. Width of the interstriae is wider than the striae. Striae internally rectangular to round in shape and mostly covered with hymen (Figs 64, 65). An apparent depression is observed on the central nodule (Figs 64, 65). Sternum valve (SV), externally, axial area covered with several irregular shape depressions distributed on the surface of the valve (Fig. 62). Striae uniseriate, however, found biseriate at more than 10 striae near the valve mantle (Fig. 62). Interstriae is raised, and the width of striae is unequal (Fig. 62). Internally, axial area is flat and features are not easily visible (Fig. 66). Biseriate striae found near the mantle with hymenated areolae. Interstriae raised its width slightly more than the striae (Fig. 67). Holotype (designated here):—Slide #58/55, Sample #2878; deposited at the Diatom Collection, Agharkar Research Institute Herbarium (AHMA), Pune, India. Type locality:— INDIA, Sikkim, composite tree moss sample collected on 22 November 2019 on the way to Khecheopalri Lake, West Sikkim district (27.34829 °N, 88.19187 °E; elevation 1794 m a.s.l.) by Radhakrishnan Cheran. Etymology:—Named after the habitat (tree) on which it was found. In Latin, the tree is called an Arbor.
Published as part of Sherly, Sheena, Radhakrishnan, Cheran & Karthick, Balasubramanian, 2022, Platessa arborea sp. nov. (Bacillariophyceae): A new tree moss dwelling diatom from the Eastern Himalayas, India, pp. 151-158 in Phytotaxa 552 (2) on pages 152-153, DOI: 10.11646/phytotaxa.552.2.2, http://zenodo.org/record/6690982