Endemic

Latin tres, three, + pinna, winged, feath-ered, + -fid, divided into many parts, in reference to its 3-pinnate quality.

Plants extremely variable, usually epi-geous, growing on and in moss mats at base of trees. Rhizomes long-creeping, slender, 1-3.5 mm diam. including scales, rhizomes not touching ground. Fronds (8-)12-25(-50) em long. Stipes 1-2 cm apart, 0.8-1.4 mm diam., narrowly winged bilaterally, pale brown. Blades 2-pinnate-pinnatifid to 3-pinnate (occasionally 2-pinnate), ovate-lanceolate to deltate, with many small, red, or reddish brown glands. Pinnae spreading, close (often overlapping) to well separated, 1-pinnate to fully 2-pinnate over entire pinna, basal pinnae reduced or not. Pinnules spatulate or oblong, broad or narrow, with up to 10 pairs of ultimate segments; acroscopic basal pinnule often very enlarged and again pinnatifid to 1-pinnate; basal pinnules more divided than more distal ones; tips blunt or acute. Ultimate segments linear, oblanceolate to acutely spatulate. Veins with tips projecting beyond sori. Sori abaxial, subapical, seldom as wide as ultimate segments.

Occasional to locally common epigeous fern growing in moss mats at the base of trees (usually not epiphytic but seldom touching the ground) in very wet rain forests (occasionally in transition to mesic forests on Kaua'i), 600-1,950 m, all major islands.

This extremely variable species has many localized, semi-isolated, and apparently stable forms. Some plants are 2-pinnate with no sign of the acroscopic basal pinnule being again divided, but others are fully 3-pinnate out to the last pinnule. The number of pinnae varies greatly and they may be well separated or overlapping. The pinnules and ultimate segments can vary from delicate, linear, narrow, and obtuse to broad, spatulate, and acute; intermediates of these forms can be found. However, in all variants the rhizome is always thin and long-creeping, and the habitat remains the same.

A robust form found in the Ko'olau Range on O'ahu (and occasionally elsewhere) has much more leathery fronds, stout stipes, winged rachises and costae, basal pinnae that are not reduced in size, and broad ultimate segments with acute tips. The extreme plants of this form are quite distinct and have been named Adenophorus hillebrandii.

Adenophorus tripinnatifidus needs further study using common-garden, DNA, and isozyme studies, electron microscopy of spores, and a review of the holotypes and associated nomenclature. Adenophorus tripinnatifidus may need to be divided into two or more species or varieties to explain the observed diversity.

Adenophorus tripinnatifidus may be recognized by its slender, long-creeping rhizomes and usually 3-pinnate fronds. The upward-facing pinnules at the pinna bases may be the only pinnules again divided.