Family

Rhachidosoraceae

Nomenclature

Rhachidosorus siamensis S.Linds., Thai For. Bull. (Bot.) 40: 102. 2012. – Type: Thailand, Chiang Mai, Fang District, Doi Pha Hom Pok National Park, trail from Kew Lom Ranger Substation to Pang Mong Kon, 1750 m altitude, 20º3'56'' N, 99º8'6'' E, in shady gully of small stream in lower montane forest on granite bedrock, 2 October 2009, D.J. Middleton, S. Lindsay & P. Suksathan 5051 (holotype QBG; isotypes BKF, E, P).

Description

Terrestrial. Rhizome ascending, 2–3 cm diameter (when fresh), producing a mass of fibrous roots; scales on rhizome and stipe very variable in shape and size, brown or golden brown, weakly clathrate with elongate cells, narrowly triangular to linear, often rather crisped, 2–14 x 0.1–2.3 mm. Fronds to 113 cm long.  Stipes and rachis mid to dark green above, very dark green to black beneath (distinction less obvious when dry), stipe 20–36 cm long, densely scaly at base, becoming sparser towards rachis and into rachis. Lamina mid to dark green and slightly shiny above, much paler beneath, 47–78 x 18–36 cm, bipinnate to tripinnate, ovate in outline, apex acuminate, formed from progressively smaller and closer pinnae without a distinct pinna; rachis ± terete but with a narrow wing on either side visible from above, each with a thickened margin, with linear scales and short glandular hairs; 18–22 pairs of free pinnae, 3.5–12 cm apart near base, triangular, apex acuminate, largest pinnae towards base but not basal, 9.8–20 x 2.9–8.7 cm, stalked for 4–8 mm; axes winged, wings with thickened margins which are sometimes also slightly inrolled, lowest pinnae reflexed or not; ultimate segments ovate to elliptic, dentate, apex obtuse to acute, glabrous above, with many scattered short glandular hairs on veins and lamina beneath; venation clearly visible, veinlets not reaching margin and with enlarged endings visible above.  Sori oblong or narrowly elliptic, mostly on acroscopic veins, one per pinnule lobe, basal lobe often larger and then with sori on both acroscopic and basiscopic veins and opening towards each other, 2–4 mm long, more or less equidistant from costule/costulet (depending on whether frond is bipinnate or tripinnate) and segment margin; indusia pale green, often translucent, margin slightly darker and minutely irregular to fimbriate. Spore surface muriform .

Distribution in Thailand

NORTHERN: Chiang Mai.

Wider Distribution

Endemic to Thailand.

Ecology

On clay bank in shady gully of small stream in lower montane forest on granite bedrock at 1750 m.

Proposed IUCN Conservation Assessment

Least Concern (LC). Although this species is only known from the type collection it was collected in a well protected national park where there are no obvious current threats.

Voucher specimens - Thailand

D.J. Middleton, S. Lindsay & P. Suksathan 5051, Chiang Mai, Fang District, Doi Pha Hom Pok National Park (BKF, E, P, QBG).