Epiphytic herbs. Roots terete, not branching, glabrous, smooth. Stem elongate, leafy, pendent or patent-ascending to erect, rooting at base. Leaves conduplicate, articulate, distichous, linear, apex obtuse, apiculate. Inflorescence lateral, solitary, racemose, patent, many-flowered; rachis continuing the line of the peduncle, straight, elongate, glabrous, thicker than peduncle. Flowers resupinate about 0.4–0.6 cm wide, opening in succession, one or a few at a time, quaquaversal, green or ochre-yellow. Sepals entire, free, obtuse, lateral sepals strongly oblique. Petals shorter than sepals. Labellum rigidly attached, subentire, saccate, fleshy at apex. Column with foot hardly developed; pollinia two, spherical, stipe suborbicular, about as long as the pollinia. Capsule longer than unfertilized ovary, narrowly fusiform, almost rostrate.
There are no published chromosome counts for these species.
Bispecific Porrorhachis is embedded in Trachoma (clade E), which is an indication that the two genera should be combined; however, bootstrap support for this relationship is weak. Before making this move, more sequencing of other species of the latter are needed. Garay (1972) segregated Porrorhachis from Saccolabium based on the differences in the column, rostellum, and habit.
Porrorhachis is a genus of two species—P. galbina and P. macrosepala (Schltr.) Garay—from Java, Borneo, and Sulawesi.
Both species are epiphytes in mossy forests and scrub from 1100 to 1700 m.
No uses have been reported for Porrorhachis; it is rare in cultivation.
Group 2 (see subtribal treatment in Genera Orchidacearum vol. 6).
Nothing is known about pollination in Porrorhachis.
The generic delimitation of the genera closely allied to Pteroceras, including Porrorhachis, needs further study. Porrorhachis is only tentatively accepted here. It may be distinguished from Trachoma by the lax, elongate rachis of the inflorescence, with the flowers not opening in clusters.