Argyrochosma dealbata

Stem scales brown. Leaves 3--15 cm. Petiole chestnut brown, 0.50--0.75 mm diam. Blade deltate, 3--5-pinnate proximally, somewhat herbaceous, abaxially covered by whitish farina, adaxially glabrous or sparsely glandular; rachis rounded to slightly flattened adaxially. Pinna costae straight or nearly so, branches not arising from prominent angles. Ultimate segments not articulate, dark color of stalks continuing into segment bases abaxially; segment margins recurved, often partially concealing sporangia; veins often visible adaxially. Sporangia submarginal, borne on distal 1/3 of secondary veins, containing 64 spores. 2 n = 54.

Sporulating summer--fall. Calcareous cliffs and ledges; 100--600 m; Ark., Ill., Kans., Ky., Mo., Nebr., Okla., Tex.

The distinctions between Argyrochosma dealbata and A . limitanea are subtle but apparently absolute. Although both occur in Texas, their ranges do not overlap. Despite the morphologic similarities, isozyme analyses indicate that A . dealbata , a diploid species, was not involved in the origin of polyploid A . limitanea .