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

357.  Astragalus scutaneus

Annual but relatively robust, with rather coarse, striate, stramineous or purplish stems 2-4.5 mm. in diameter, thinly strigulose nearly throughout with filiform or slightly flattened, straight, appressed hairs up to (0.25) 0.3-0.5 mm. long, the herbage green, the leaflets glabrous above; stems 1.5-3.5 dm. long, simple except for an occasional spur at the node preceding the first peduncle, floriferous upward from below the middle; stipules submembranous, pale green becoming scarious, pallid, and fragile, deltoid or triangular, 2.5-4 mm. long, semiamplexicaul; leaves sessile or the lowest ones very shortly petioled, 5-8 cm. long, with 17-23 lance-oblong, oblong, oblong-obovate, or (in some upper leaves) linear- oblong, truncate or retuse, flat or loosely folded leaflets 5-14 mm. long; peduncles ascending, straight or nearly so, 3.5-6.5 cm. long, shorter than the leaf; racemes densely (20) 25-40-flowered, narrowly oblong at full anthesis, the flowers early spreading and then declined, the axis somewhat elongating, 3-6.5 cm. long in fruit, the pods retrorsely imbricated; bracts membranous, triangular or triangular-acuminate, 0.8-1.8 mm. long; pedicels at anthesis spreading or arched outward, 0.4-0.6 mm. long, in fruit strongly recurved, 0.8-1.4 mm. long, persistent; bracteoles 2, minute; calyx 3-3.5 mm. long, minutely but sometimes densely strigulose with black or black and some paler hairs, the oblique disc 0.3-0.5 mm. deep, the campanulate, dorsally subgibbous, membranous, purplish tube 1.9-2.2 mm. long, 1.7-2 mm. in diameter, the subulate or triangular-subulate teeth 1.1-1.5 mm. long, the orifice oblique, the whole becoming papery, ruptured, marcescent; petals lilac or lilac-tinged, the banner purple-veined; banner recurved through 40-50°, obo- vate-cuneate or -flabellate, notched, 5.2-6 mm. long, 3.7—4 mm. wide; wings 5.2 5.5 mm. long, the claws 1.6-2 mm., the obliquely triangular or lunately oblong, obtuse or emarginate blades 4—4.2 mm. long, 1.5-2 mm. wide; keel 3.9-4.4 mm. long, the claws 1.7-2.1 mm., the half-obovate or almost half-circular blades 2.32.9 mm. long, 1.6-1.8 mm. wide, incurved through about 110° to the bluntly deltoid or sharply deltoid and subporrect apex; anthers 0.25-0.33 mm. long; pod deflexed, sessile, broadly ovate to nearly circular in dorsal view, 7-9.5 mm. long, 6-7.5 mm. in diameter, shallowly retuse at base, abruptly apiculate at apex, so strongly obcompresed as to become shield-shaped and almost leaflike, only 0.8-1.5 mm. thick, the lateral angles acute or subacute and almost winglike, slightly turned upward, the whole slightly incurved or obscurely sigmoidally arched, the thin, green, sparsely strigulose valves becoming papery, greenish-stramineous or purple- freckled, lustrous, translucent, strongly cross-reticulate, inflexed as a complete septum 0.5—1 mm. wide; dehiscence apparently through both sutures and the septum, not certainly observed; ovules 10—12; seeds olivaceous, or brown speckled with purple, smooth but dull, 1.8-2.1 mm. long.—Collections: 2 (o); representative: Pringle 11,362 (CAS, GH, MICH, US).

Sandy soil, on lake shores and perhaps elsewhere, ±6000 feet, known only from about Lake Chapala, Jalisco.—Map No. 159.—Mid-June to September.

Astragalus scutaneus (shieldlike, of the pod) Barneby in Amer. Midl. Nat. 55: 502. 1956.—"Mexico. Sandy shore at west end of Lake Chapala, Jalisco, 14 July, 1940, C. L. Hitchcock & L. R. Stanford No. 7166 ...’’—Holotypus, WS! isotypi, CAS, GH, POM, RM, RSA, UC, UTC!

The pod of the shield milk-vetch is one of the most highly modified in the genus. Its extreme stage of dorsiventral compression, narrow, almost winged lateral angles, and slightly sigmoid curvature (proceeding downward proximally and then inward distally) are mimicked on a much larger scale by the fruit of A. pterocarpus, although that is of fleshy texture and unilocular. My description of the pod as persistent on the receptacle was probably mistaken, although it seems to fall more tardily than that of A. brazoensis, and the joint with the receptacle is less clearly visible in young material. Truly ripe fruits showing the natural fall and dehiscence are still needed.

Pringle’s collection from near Guadalajara (I have not precisely identified his Constancia Station) was distributed as A. diphacus and was passed over as such by Jones, Rydberg, and L. E. James. The pod of A. diphacus, a perennial with slightly larger flowers, is didymously inflated and the valves are divided by a very much wider septum.