A Reconsideration of the Genus Pityrogramma (Adiantaceae) in Western North America

Abstract

The silverback and goldback ferns of the genus Pityrogramma Link are striking, graceful plants of small to medium size, well known to fern fanciers for their beautiful white to yellow farinose indument. Tryon & Tryon (1982) classified the in the tribe Taenitideae of their broadly circumscribed Pteridaceae Reichb. (= Adiantaceae (C. Presl) Ching; see Pichi Sermolli, 1986) and the taxa were last revised taxonomically by Tryon (1962), who recognized 14 species. Among the goldback and silverback ferns, the complex of species indigenous to the southwestern United States and adjacent Mexico has excited the greatest controversy, both with respect to species number and affinities of the group. Tryon (1962) followed Weatherby's (1920) disposition of the taxa as four varieties of a single species, P. triangularis (Kaulf.) Maxon. Alt & Grant (1960), however, recognized three distinct species based on Weatherby's varieties, P. pallida (Weath.) K. & V. Grant, P. viscosa (D. Eaton) Maxon, and P. triangularis (with only two varieties). The California Flora (Munz & Keck, 1968) followed the single species scheme in the general text, but recognized three species in the supplement, and also included mention of two other varieties of P. triangularis described since the work of Alt & Grant (1960). At the generic level there has been growing recognition that the southwestern complex is anomalous within Pityrogramma. Tryon (1962) pointed out that this departs in several morphological characters from the relatively uniform central group of species in the genus and suggested that this specialized relative of Pityrogramma proper might merit generic recognition. He chose to treat the taxa as a single genus, however, to emphasize the similarities (rather than the differences) between the groups. More recently, Tryon & Tryon (1982) suggested that the southwestern complex might be better classified in the tribe Cheilantheae rather than the tribe Taenitideae, but postponed formal taxonomic segregation from Pityrogramma until the group's affinities became better known. During studies leading to treatments for the genus Pityrogramma for the Flora of North America Project and the Ferns and Fern Allies of the Southwestern United States (Windham & Yatskievych, in prep.) we also concluded that the P. triangularis complex should be segregated from Pityrogramma. This paper is intended to review the rationale for distinguishing these two morphologically