Monographs Details: Astragalus hypoxylus S.Watson
Authors:Rupert C. Barneby
Authority: Barneby, Rupert C. 1964. Atlas of North American Astragalus. Mem. New York Bot. Gard. 13(2): 597-1188.

320.  Astragalus hypoxylus

Low, perennial, with basally indurated stems, strigulose with subappressed hairs up to 0.2 mm. long, the herbage cinereous, the leaflets bicolored, gray beneath, yellowish-green and glabrous above, white-ciliate; stems of the season very short, up to 4 cm. long, simple above the immediate base, decumbent; stipules deltoid, rather firmly herbaceous becoming papery and brownish, 1-1.5 mm. long, thinly strigulose dorsally; leaves 1.5-4 cm. long, with slender petioles and 11-13 obovate, obtuse, flat or loosely folded leaflets 2-4.5 mm. long; peduncles slender, 5-7 cm. long, greatly surpassing the leaves, prostrate and radiating in fruit; racemes densely and shortly 8—18-flowered, the flowers and fruits both ascending, the axis little or not elongating, 1-2.5 cm. long in fruit; bracts lanceolate or triangular-subulate, 1.3-2.5 mm. long; pedicels ascending, in fruit straight, thickened, persistent, ± 1.5 mm. long; bracteoles 0; calyx ± 6 mm. long, strigulose- villosulous with white hairs, the disc scarcely oblique, the campanulate tube 3.2 mm. long, 2 mm. in diameter, the lance-subulate teeth ± 2.8 mm. long, the whole becoming membranous, ruptured, persistent; petals purplish (or at least lilac- tinged); banner recurved through 45° (?), 7.5-8 mm. long; wings a little shorter, the claws nearly 3 mm. long, the blades lanceolate, little incurved; keel 5.8-6.2 mm. long, the claws ± 3.1 mm., the half-obovate blades 2.9—3.2 mm. long, 1.8 mm. wide, abruptly incurved through 95° to the bluntly deltoid apex; pod ascending, sessile on and deciduous from the convex receptacle, narrowly lanceolate in profile, 7—9 mm. long, 2—2.5 mm. in diameter, very slightly incurved, obtuse at base, contracted at apex into a short, glabrescent cusp, triquetrously compressed, with rounded lateral angles, carinate ventrally by the suture, deeply sulcate dorsally, the green, loosely strigulose valves becoming stiffly papery, stramineous, obscurely reticulate, mflexed as a complete septum ± 1 mm. wide; ovules 6 (possibly more?); dehiscence and seeds not seen.—Collections: 1 (the typus).

Habitat not recorded, known only from the type-locality, "near Fort Huachuca, in southwestern Cochise County, Arizona; to be sought in the mountains about the head of the San Pedro and Santa Cruz Rivers in southeastern Arizona and adjoining Sonora.—Map No. 142.—June and July, probably flowering in early spring.

Astragalus hypoxylus (woody below) Wats. in Proc. Amer. Acad. 18: 192. 1883.— Collected at Maloney’s Ranch, in the Huachuca Mountains, Southern Arizona, by J. G. Lemmon, in July, 1882."—Holotypus (Lemmon 2656), GH! isotypus, UC!—Hamosa hypoxyla (Wats.) Rydb. in Bull. Torr. Club 54: 336. 1927.

The type-specimen of the Huachuca milk-vetch, A. hypoxylus, which has no root and only faded and mostly broken flowers, is hardly sufficient for analysis; nevertheless it demonstrates what seems to be a very distinct species. The epithet hypoxylus refers to the ligneous lower stem which is, perhaps, not altogether typical and not part of a true caudex as the term is used in these pages; it suggests rather the growth of early spring which has become hardened and has given rise to short, new herbaceous shoots under the stimulus of early summer rains. The typus of Jones’s A. madrensis shows that a similar condition may occur late in the year in A. nothoxys. Around Fort Huachuca A. nothoxys is locally abundant and must be sympatric with A. hypoxylus, if the locality data are correct. The two species are similar in many technical features, including the form of the fruit, and in superficial matters of pubescence and of coloring of the flowers and leaves. Differential points to be looked for in A. hypoxylus are the crowded racemes elevated on very slender peduncles well beyond the foliage and the much shorter, few-ovulate pod. Further the calyx of A. hypoxylus is more deeply cleft so that the teeth are longer in proportion to the more shallowly campanulate tube; and the keel-tip lacks the terminal cusp which is nearly always characteristic of A. nothoxys.

A collection from Puebla (Purpus 2477) referred to A. hypoxylus by Jones (1923, p. 278) was identified by the same author (op. cit., p. 275) as A. luisanus. Like the holotypus of A. luisanus it belongs to A. hypoleucus Schau.