Didymodon nevadensis, a New Species for North America, with Comments on Phenology

Abstract

Didymodon nevadensis Zand. is described as a new taxon from North America, reported from southern British Columbia, southern Nevada, northern Texas, and southern Chi- huahua. The new species is distinguished from its closest relative (D. brachyphyllus (Sull.) Zand. = D. reedii Robins., syn. nov.) by the absence of a conical cell ending the costa, a wider costa at midleaf than below, the presence of a characteristic bulging ventral costal pad of cells, costal guide cells in two layers, and leaf margins loosely revolute. Study of extensive populations at the type locality in southern Nevada showed that this species is confined there to gypsiferous ridges and is presently represented only by female plants. While studying the distribution of the rare Cali- fornia bearpoppy (Arctomecon californica Torr. & Frem.), a candidate for the federal list of threatened species in Nevada, one of us (GMS) observed large p9pulations of an associated moss. Subsequent ex- ploration of three prominent gypsum formations in the remote southeastern corner of Nevada revealed this species to be locally abundant and restricted in distribution to gypsum substrates. At the behest of LRS, the senior author examined the plants and found they compared well with previously puzzled over, morphologically less well developed, frag- mentary specimens from Canada, Texas, and Mex- ico. This indicated that the plants deserved recog- nition as a new species with scattered but wide dis- tribution; LRS then examined the populations at the type locality.