Astragalus cerussatus E.Sheld.
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Authors
Rupert C. Barneby
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Authority
Barneby, Rupert C. 1964. Atlas of North American Astragalus. Mem. New York Bot. Gard. 13(2): 597-1188.
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Family
Fabaceae
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Scientific Name
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Type
"Collected on the mountain sides near Canon City, Fremont County, Colorado, by J. E. Bodin, June and July, 1890; also near Royal Gorge, Colorado, by Miss Alice Eastwood, June, 1891."—Holotypus, Bodin, June 11, 1890, MINN!
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Synonyms
Phaca cerussata (E.Sheld.) Rydb., Astragalus triflorus var. cerussatus (E.Sheld.) M.E.Jones
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Description
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Species Description - Astragalus cerussatus Low, slender, diffuse, with a short-lived perennial taproot but flowering the first season and then appearing annual, thinly villosulous throughout with fine spreading or ascending, nearly straight or crisply sinuous hairs up to 0.5-0.8 mm. long, the stems and herbage green or in youth subcinereous, the leaflets sometimes glabrescent above; stems several, ascending or decumbent with ascending tips, 0.3-2.5 dm. long, in older plants becoming somewhat indurated at base but scarcely caudiciform, branched or bearing short spurs at most of the axils, together forming densely leafy tufts or loose, low clumps; stipules thinly herbaceous, deltoid or lanceolate, 2-4 mm. long, semiamplexicaul, free; leaves 2.5-8 cm. long, shortly petioled, with subfiliform rachis and 13-21 narrowly oblanceolate to oblong-oblanceolate, retuse or sometimes obtuse, thin, flat leaflets 4-13 (18) mm. long; peduncles very slender, incurved-ascending, 2-4.5 cm. long, or some arising late in the season from axillary spurs only 1 cm. long or even less; racemes loosely (1) 3-7-flowered, the flowers at first ascending, declined in age, the axis 0.5-3.5 cm. long in fruit; bracts submembranous, ovate- or lance-acuminate, 1.5-2.5 mm. long; pedicels slender, at anthesis 1-1.2 mm., in fruit arcuate and 1.8-2.5 mm. long; bracteoles 0-2, minute when present; calyx 3.4-4.2 mm. long, villosulous like the herbage with white hairs, the disc about 0.5 mm. deep, the campanulate or obconic-campanulate tube 1.7-2 mm. long, ±1.6 mm. in diameter, the narrowly lance-subulate teeth 1.4-2.5 mm. long, the whole becoming papery, ruptured, marcescent; petals pale lilac, or whitish with lilac tips and pale purple veins in the banner; banner recurved through nearly 90°, ovate-cuneate, 5.2-6 mm. long, 3.2-4.2 mm. wide; wings 4.7-5.6 mm. long, the claws 1.8-2.3 mm., the lunately lance-oblong, obtuse blades 3.4-3.9 mm. long, 1.1-1.7 mm. wide; keel 3.6-4.3 mm. long, the claws 1.7-2.2 mm., the broadly half-obovate blades 2-2.5 mm. long, 2-2.5 mm. wide, abruptly incurved through 120° to the obliquely deltoid, obscurely porrect apex; anthers 0.3-0.4 mm. long; pod spreading or declined, sessile, readily deciduous, obliquely ovoid or ovoid-ellipsoid, bladdery- inflated, 1-2.2 cm. long, 5-14 mm. in diameter, rounded or broadly obconic at base, contracted distally into a shortly deltoid, laterally flattened beak, otherwise a trifle obcompressed, shallowly sulcate ventrally, the filiform dorsal suture more strongly convex than the ventral one, the thin, greenish or sometimes purplish valves thinly strigose-villosulous with incumbent or curly hairs 0.3-0.5 mm. long, becoming papery-membranous, stramineous, delicately reticulate, inflexed as a very narrow partial septum 0.1-0.4 mm. wide, the funicular flange ± 0.5 mm. wide; dehiscence apical, after falling; ovules 18-22; seeds olivaceous-brown, sometimes purple-speckled, smooth but dull, 1.9-2.5 mm. long.
Distribution and Ecology - Dry sandy banks and hillsides, and sandbars and shingle bars along rivers and torrents, 5400-8300 feet, rather local but forming colonies, known only from southern Colorado: along the Arkansas River from Canon City upstream to Poncha Springs and Salida, Fremont and Chaffee Counties; on the upper Rio Grande and Conejos River in Costilla and Conejos Counties; and on the La Plata River near Durango, La Plata County; reported (Rydberg, 1929, p. 352) from northern New Mexico.—Map No. 122.—April to August.
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Discussion
The powdered milk-vetch, A. cerussatus, is a pretty but inconspicuous astragalus, with diffuse stems, tiny, milky-lilac flowers, and swollen fruits of notably delicate texture. It is found most commonly on gravel bars or sandbars along the flood plain of the Arkansas River or in the beds of torrents lateral to the main stream, and in these situations the older plants must be subject to flooding in the spring thaws. The average plant of A. cerussatus is of rapid growth and short duration, coming into flower two or three months after germination of the seed and fruiting soon after. Some colonies seem to be composed entirely of annuals. Elsewhere, however, individuals may be found with flowering stems arising from a base already indurated and caudiciform in early spring, and these are doubtless in their second or third season.
The diagnostic features of A. cerussatus, which resembles in most respects several of the few-flowered, annual or short-lived Inflati, are the loosely ascending or spreading vesture of often sinuous hairs and the bladdery pod of thinly papery texture and oblique profile which is provided with a definite even though very narrow (and easily overlooked) septum. Closely related to A. Wootoni, it is difficult to distinguish in theory from all forms of that widely dispersed and polymorphic species. However, the phases of A. Wootoni known from northern New Mexico, where forms intergradient with A. cerussatus are logically to be expected, are thinly strigulose with appressed hairs and have larger, subsymmetrically ellipsoid, almost beakless pods of comparatively firm texture. It is only in the lower Rio Grande Valley in New Mexico, in trans-Pecos Texas, and adjoining Mexico that forms of A. Wootoni technically similar to A. cerussatus in their small pods and villosulous vesture are known to occur, and these have slightly but significantly shorter calyx-tube and keel-petals, and truly unilocular pods. Jones (1923, p. 104) listed A. cerussatus as a synonym of his unreasonably comprehensive "A. triflorus," believing it to be identical with Phaca triflora Kunth, our A. Wootoni var. Candollianus. However there can be no question that A. cerussatus and its central Mexican relative exist as discrete entities. The powdered milk-vetch is maintained at the rank of species for reasons expressed in the foreword to sect. Inflati.
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Objects
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Distribution
Colorado United States of America North America|