Monographs Details: Astragalus robbinsii (Oakes) A.Gray var. robbinsii
Authors:Rupert C. Barneby
Authority: Barneby, Rupert C. 1964. Atlas of North American Astragalus. Mem. New York Bot. Gard. 13(1): 1-596.
Family:Fabaceae
Synonyms:Phaca robbinsii Oakes, Tragacantha robbinsii (Oakes) Kuntze, Astragalus labradoricus var. robbinsii (Oakes) M.E.Jones, Atelophragma robbinsii (Oakes) Rydb.
Description:Variety Description - Stems slender, ascending, (11) 15—27 cm. long; leaves 3.5—8.5 cm. long, with 7-13 leaflets 6-18 mm. long, minutely strigulose beneath with scattered hairs up to 0.25-0.4 mm. long; peduncles (5) 7-15 cm. long; racemes (6) 10-21-flowered, the axis (1) 3-10 cm. long in fruit: calyx 4-5 mm. long, the tube 3-3.5 mm, the teeth 0.8-1.8 mm. long; petals whitish, immaculate; banner 8-8.5 mm. keel 6-6.7 mm. long; stipe of the pod (2) 3-3.5 mm. long, the body 1-1.5 cm. long, (3.5) 4-5 mm. in diameter, compressed-triquetrous, flattened dorsally. the beak not over 1 mm. long, the valves minutely and sparsely black strigulose with hairs 0.2-0.4 mm. long, the septum ± 0.3 mm. wide; ovules 7-8.

Distribution and Ecology - Limestone ledges near and above high water, associated with Potentilla fruticosa, along the lower Winooski (Onion) River near Burlington, Vermont, the one known station obliterated by a dam erected in 1894.—Map No. 5.—May to early June.

Discussion:

The typical form of the Robbins milk-vetch, known from only one population extinct for almost seventy years, differs (or differed) from other New England and long-stipitate Cordilleran varieties in its small, pallid flowers and relatively short, minutely and sparsely pubescent pod. If the circumstances of its discovery and naming had not focused a particular attention upon it, and it had been discovered only recently in the Rocky Mountains, it would probably have been passed over as one more minor variant of what is here called var. minor (including the eastern A. Blakei). As one looks eastward today from the dam that links the cities of Burlington and Colchester and backs up the waters of the Winooski River over the limestone ravine, which formerly provided a cool retreat in the lowlands for a plant of boreal- montane affinities, the prospect is dominated by the profile of Mount Mansfield, the type- locality of A. Blakei, only a few miles distant. It seems most probable that var. Robbinsii had its origin through seeds brought down from the hill country to the east, and that it was able to persist in this one favored station because of a chance mutation adapted to a lowland calcareous habitat.

The Robbins milk-vetch was formerly thought to be widely dispersed in New England, but all records have been traced back to other varieties, of which the critical characters were only appreciated in modern times. The history of Phaca Robbinsii was recounted in detail by Rydberg (in Torreya 24: 98, sequ. 1924), the first to circumscribe the entity in its present narrow sense. After its discovery in 1829, A. Robbinsii was collected in 1841 by J. Carey, in 1846 by Oakes, and several times between 1877 and 1891 by C. G. Pringle, A. J. Grout, and Rev. James Blake. It was apparently last seen in the living state by W. W. Eggleston on June 15 and 18, 1893. The plant recently reported as A. Robbinsii from coastal Nova Scotia (Schofield in Rhodora 57: 308. 1955) represents an outlying population of var. minor.

Distribution:Vermont United States of America North America|