323b. Astragalus Emoryanus var. terlinguensis
Peduncles mostly dimorphic, the early or lower ones, sometimes all, filiform and only 1.5-30 mm. long, bearing 1-3 (4) flowers, and much shorter than the leaf, the later ones commonly better developed and up to 3-5.5 cm. long; herbage cinereously hirsutulous with rather stiff, straight and spreading or incurved- ascending hairs up to (0.5) 0.6-0.75 mm. long; pod as given in the key.— Collections: 22 (o); representative: Warnock 7656 (SMU, SRSC, WIS, TEX), 47,023 (SMU, SRSC); McVaugh 7829 (SMU, SRSC, TEX); Hinkley 2346, 2435 (NY).
Dry rocky hillsides and sandy or gravelly washes, 2500-4000 feet, locally plentiful around the Big Bend of the Rio Grande and tributary streams, especially Maravillas and Terlingua Creeks, Brewster County, Texas, and adjoining Coahuila, and (perhaps somewhat isolated) in the Van Horn Mountains, in Culberson and Hudspeth Counties, Texas.—Map No. 146.—February to April.
Astragalus Emoryanus var. terlinguensis (Cory) Barneby in Amer. Midl. Nat. 55: 494. 1956, based on A. terlinguensis (of Terlingua Creek) Cory in Rhodora 39: 419. 1937.—"Type: Terlingua Creek above the mouth of Alamo de Caesario Creek (about 18 miles south of Terlingua), 13 April 1936, Cory 18584 (Gray Herb.)."—Holotypus, labeled: "Terlingua Creek, 8 miles n. of Terlingua," the date and number as cited, GH!
The extreme form of the Big Bend milk-vetch, A. Emoryanus var. terlinguensis, is remarkably distinct from var. Emoryanus, the hirsutulous pubescence, short, few-flowered peduncles, transparently membranous fruiting calyx, and especially the short, relatively plump pod all contributing something to its individuality. As pointed out elsewhere (Barneby, 1956, l.c.), the short peduncle is a juvenile character, also found low on the stems of var. Emoryanus as well as in several genuine Leptocarpi and some species of other groups. I have specimens from Brewster County, furthermore, in which the calyx and vesture of var. terlinguensis are combined on the same plant with the developed peduncles and long pods proper to var. Emoryanus, and these seem decisively intermediate between the two forms.
The dispersal of var. Emoryanus into two elongate lobes, divided by the Big Bend country and var. terlinguensis, forms a pattern unmatched elsewhere in American Astragalus and one which is a rarity in the higher plants. It is possible but not very probable that exploration of Coahuila will show var. Emoryanus to be distributed more continuously east and west than is known at present. Lacking such evidence, I must suppose that var. terlinguensis, the derived and more specialized of the two forms, represents a dominant mutant which has arisen near the center of the species-range and there has almost fully replaced the parental type.