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All symbols on the above map represent herbarium collections that in early 2013 were labeled as either E. sericeus or E. nuttallianus. Examination of over 540 specimens from NMC, NMCR, UNM, SRSC and TEX/LL lead me to conclude that west of the Trans-Pecos many of these incorrectly identify E. discolor specimens as E. nuttallianus – many of the yellow squares on the map. Most commonly confused were those specimens with hairs on the upper leaf surface — essentially collections at SRSC & TEX/LL, perhaps following L.H. Shinners treatment in D.S. Correll & M.C. Johnston, Manual of the Vascular Plants of Texas (1970). (All Mexican collections were at TEX/LL). New Mexico collections, especially those at UNM, are generally correct. These appear to follow W.C. Martin & C.R. Hutchins, A Flora of New Mexico (1981), which uses habit and sepal features to distinguish the two species.
Plants of the high plains sand areas along the Texas/New Mexico border also presented difficulties (inverted orange triangles). I consider these to represent a distinct species, E. arenarius (with epithet from Latin arena ‘sand’). These plants are sympatric with E. nuttallianus in the sands areas, but their habit and sepal form are very different, somewhat similar to E. sericeus. My interpretation is that collectors who based identification on pubescence saw them as E. nuttallianus, but collectors familiar with E. nuttallianus saw them as different, the only option with available keys at the time being E. sericeus.
One collection of interest is the isotype specimen of E. oreophilus Greene, green box with yellow center, SW New Mexico. There are numerous isotype collections of this plant, and identifications vary a great deal. It is identical with the heavily pubescent E. discolor plants of the area (SW New Mexico, N Mexico, Trans-Pecos) in venation, phyllotaxis, sepal form and hair type — thus E. discolor (or E. sericeus var. discolor van Oostroom).