Rhizome short to c. 15 cm long, erect to decumbent, narrow and very woody. Stipes of sterile fronds (6-)10-35(-45) cm long, of fertile fronds 22-55(-66) cm long, the base strongly or moderately enlarged, 4-20 mm wide in the middle, cross section one U-shaped vascular bundle (but in smaller plants more like a V-shaped form), upper part of stipe tetragonal, aerophores present on stipe and rachis, slender, hook-shaped, with pointed apex (usually short). Fronds usually covered with an amorphous gelatinous mucilage layer when dry, dorsal side of rachis usually semiterete, trigonous, grooved, or sometimes flattened, rachis 1-2(-4) mm wide in the middle part. Sterile lamina rather thin to rather thick, lower part pinnate, upper 1/3 to 1/2 pinnatifid to pinnatifid except a few free basal pinnae (or pinnatifid, the Kinabalu high elevation population), widest medially to subbasally, (10-)20-50(-66) by 5-18(-22) cm, index 2-4; pinnae 20-40(-60) pairs, widest medially (usually, rarely widest at the base in the pinnatifid form), veins of pinnae simple or paired at base or forked above base (few veins), distinct on both surfaces, margin entire (usually completely) or except the apex, most pinnae except the uppermost with equilateral base, cuneate, rounded, attenuate, or acroscopic side adnate; lowermost pinnae sessile to shortly stalked or rarely adnate but slightly constricted, usually slightly deflexed, abruptly or slightly shortened; middle pinnae (3-)4-10(-20) cm long, (0.4-)0.6-0.9(-1.4) cm wide in the middle, sessile to rarely shortly stalked, basiscopic side rounded, partly free from rachis, perpendicular to rachis, or round, base cuneate or rounded (sometimes attenuate); uppermost pinnae adnate, base unequal-to about equal-sided, basiscopically decurrent into rachis wing or rarely rounded; apex of lamina pinnatifid, terminated by a pinnatilobed, short-triangular to pinnatilobed, long-triangular or pinna-like segment (very rare, up to 10 cm long, base connected with upper shorter lateral pinnae). Fertile fronds usually longer than the sterile ones, dorsal side of rachis semiterete, flattened, or grooved, rachis 1-2(-5) mm wide in the middle part; lamina pinnate except the apex, lower pinnae shortly stalked, (8-)20-45(-65) by (3-)7-20(-26) cm; pinnae usually sparsely bearing short glandular hairs or glabrous, 15-50(-72) pairs, margin entire or subentire; 1-2(-4) cm apart; middle pinnae 4-11(-16) cm long, 2-3 mm wide, base sessile to shortly stalked; apex pinnatifid, acuminate or caudate, or terminated by a pinna-like segment, or rarely by a pinna conform to the lateral ones and often beaded at the base. Paraphyses many, yellow to dark brown. Spores yellow, with reddish tubercles.
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