Synonym(s): Darea microphyllum Sm.; Allantodia scandicina Kaulf.; Asplenium aspidioides sensu Hillebr., non Schltdl.; As. aspidioides sensu A. Heller, non Spreng.; As. multisectum Brack.; As. poiretianum Gaudich.; As. vexans Underw.; Athyrium poiretianum (Gaudich.) C. Presl
Endemic 'iikolea (P. & E.)
Greek mikros, small,+ phyllon, leaf, in reference to the small ultimate segments.
Plants medium-sized, terrestrial. Rhizomes forming small caudices, erect, 15-30 cm tall. Fronds herbaceous, 25-90 cm long, forming rosettes at tips of rhizomes. Stipes straw-colored to pale brown, bases thickened, scales at bases sparse, linear-lanceolate, red-dish brown. Blades 3-pinnate-pinnatifid to 4-pinnate, ovate or ovate-oblong, dark green. Pinnae alternate, stalked; costae and costules bearing short, white, fleshy, translucent spines, up to 1 mm long, at bases on ad-axial surface (very noticeable on live plants, obscure and scalelike when dry). Ultimate segments sessile to short-stalked, up to 2 mm long, 0.3-1.5 mm wide, bases wedge-shaped, uneven-sided with 2 to several deeply cut, toothed lobes. Sori short, linear, 1-2 mm long, at base of each ultimate segment close to midribs and sinuses, on only one side of veins. Indusia firm, opening toward costules.
Locally common in mesic to wet forests, 500-2,320 m, all major islands. Athyrium microphyllum is variable in size and in the width of the ultimate segments, and it has many diverse local forms including one found on Namolokama Peak on Kaua 'i with sori extending prominently beyond the segment tips.
In the field Athyrium microphyllum usu-ally has small, erect stems and a growth habit resembling that of small tree ferns. The species is easily identified by the erect, white, fleshy, translucent spines on the upper surface of the pinnae. and pinnule midribs. These are quite prominent on living plants and found on no other Ha-waiian fern. Athyrium microphyllum also has finely divided foliage and short, linear sori close to the midribs and sinuses.