Aridarum velutandrum

General description: 

Very diminutive mat-forming obligate rheophytes to 3 cm tall. Stem very short, at most ca. 1.5 cm long, ca. 2 mm in diam., naked. Leaves many together, forming a dense sward; petiole 0.5–1 cm long, ca. 1 mm diam., adaxially canaliculate, sheathing at the extreme base; petiolar sheath with wings extended into a very narrowly triangular ligular portion 1 cm long drying dark red-brown; blade very stiff, adaxially very dark green, paler abaxially, acicular, 0.5–2.5 cm long x 1–2 mm wide, base very narrow cuneate, apex sharply pointed, for ca. 0.5 mm, margin thickened; midrib abaxially prominent, adaxially impressed, primary lateral almost indistinguishable, running to a thick marginal vein; secondary and tertiary venation mostly completely obscure in living material, forming a faint tessellate reticulum in dry material. Inflorescence solitary; peduncle somewhat stout, exceeding the petioles, ca. 5 mm long x 1 mm diam., terete, pale green; spathe more or less ovoid, not constricted, ca. 1.5 cm long and apically beaked for ca. 2 mm; lower part funnel-shaped, green, persistent; limb inflating and gaping to create a slit at pistillate anthesis, then sooncaducous, glistening white, apical beak medium green. Spadix bluntly spindleshaped, ca. 8 mm long, ca. 3 mm diam.; pistillate flower zone ca. 0.5 mm long, reduced to one or incompletely 2 whorl of pistils, ca. 2 mm diam.; pistils crowded, subglobose, ca. 0.5–1.5 mm diam., green; stigma sessile, discoid, centrally impressed, about the same width as the ovary, green; interpistillar staminodes confined to a row along the spathe/spadix adnation, shortly stipitate, globose, deep yellow, darker on the top, about the height of the pistils; sterile interstice defined a partial to complete whorl of staminodes, these pubescent, globular, 0.5–0.6 mm in diam., medium yellow; staminate flower zone ca. 5–9 mm long, comprised of 1–2 rows of fertile flowers; staminate flowers crowded, each comprised of two stamens arranged in longitudinally aligned pairs, truncate, deeply excavated with the thecae together on the inner (with respect to the stamen pairs) side of the anther, ellipsoid to ellipsoid-oblong from above, 0.8–1 mm long x 0.4–0.6 mm wide, pubescent, medium yellow; thecae separated by a ridge forming an incomplete septum in the cavity or this weak or absent, very shortly horned, with the horns on edge of the anther; appendix ca. 1–4 mm long, rounded; appendix staminodes more or less irregularly globular to ellipsoid, mostly flattopped, lowermost ones more or less excavated, ca. 0.5–0.8 mm diam., pubescent, medium yellow. Fruiting spathe funnelform, pale greenish; fruits not seen.

Diagnostic description: 

Aridarum velutandrum and A. orestum both have pubescent staminate flowers. Aridarum velutandrum is, however, readily distinguished by having very short, naked stems (vs. stems elongated and clothed in conspicuous fibrous-netted cataphyll, ligule, and leaf base remnants), and very stiff leaf blades that are sharply V-shaped in crosssection, and furnished with a sharply pointed tip (vs. leaf blades by the softly leathery, almost flat with a tubular leaf tip).


Aridarum velutandrum is known only from Similajau N.P. (Bintulu, Sarawak), where it is restricted to the very hard sandstones of the Sungai Anchau.


Aridarum velutandrum occurs as a rheophyte on in slightly shaded very hard river sandstones under humid lowland forest between 30 and 60 m asl.

Taxonomic Notes: 

Aridarum velutandrum and A. orestum are unique in the genus in having pubescent staminate flowers. It is highly probable that they are closely related and possibly the result of a vicariance event separating a lowland (A. velutandrum) and a highland (A. orestum) population. Molecular analyses is required but will necessitate re-collection of A. orestum since attempts to retrieve DNA from the alcohol-treated prior to pressing Type material have comprehensively failed.

Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith