Cladium steyermarkii T. Koyama, Mem. N.Y. Bot. Gard. 18(2): 28. 1969.
Typus. Steyermark & Wurdack 1225 (holotypus in NY) from Salto de Tirica at 1758 m, Torono-tepuí, Chimantá Massif, Bolívar, Venezuela.
A sedge from Guayana Highland, hitherto known as an abberant species of Cladium, proves to represent a new genus of the Cyperaceae. Because of its spikelet structure, the genus Rhynchocladium, just described above, without doubt belongs to the tribe Cladieae. It, however, clearly differs from all known genera of Cladieae in its truly dorsiventral leaf-blades. As was pointed out by Metcalfe (Am. Jour. Bot. 56: 782-790. 1969; Anat. Monocot. V. Cyperaceae. 1971), the genera of Cladieae are well characterized by three peculiar types of leaves, i e, the pseudo-dorsiventral blades seen in Cladium, and the iso-bilateral blades seen in Machaerina, and the unifacial blades in Machaerina (part), Lepidosperma and their related genera. The true dorsiventral type of leaf blades, shown by Rhynchocladium, is a completely new feature in the tribe Cladieae. Besides the leaf character Rhynchocladium differs from its closest ally, Cladium, in the achene-type fruits that are conspicuously trigonous and are attenuate at both ends; the style of persistent nature; and the presence of well-developed hypogynous bristles. In Cladium the drupaceous fruits with contracted ends are ellipsoidal without conspicuous angles; the style is caducous; and the hypogynous bristles are always absent. Although Rhynchocladium with dorsoventral leaves causes no confusion with Machaerina with either isobilateral or unifacial leaves, the distinction between the two genera can be made also by the fact that the spikelets of Rhynchocladium are disarticulated at the base above their subtending bracteole while there is no such articulation at the base of spikelets in Machaerina. Furthermore, the fruits of Rhynchocladium show no conspicuous style-base between the body and the style in contrast to the thickened and normally hispid conical style-bases in Machaerina. The leaves of Cladium and Rhynchocladium are spirally disposed logically following 1/3 phyllotaxis, while in Machaerina its ancipitous leaves are clearly distichous.
The new generic name consists of rhynchos, beak, and Cladium alluding to the fruit with long-attenuate apex, which is exserted from the subtending glume.