Astragalus platytropis

Dwarf or diminutive, closely tufted, acaulescent or nearly so, with a taproot and pluricipital crown, this forking in old plants into a suffruticulose caudex commonly beset with a thatch of persistent petioles and stipules (on shifting rock slides the caudex sometimes buried and elongating, rarely up to 1.6 dm. long), strigulose throughout with fine, straight, appressed hairs up to 0.35-0.6 mm. long, the herbage silvery- or gray-silky, the inflorescence commonly black-hairy; stems of the year mostly reduced to crowns, exceptionally up to 2 cm. long but the internodes even then seldom surpassing the stipules, exceptionally 1 or 2 of them up to 1 cm. long; stipules subherbaceous becoming papery and brownish, 1.5—5.5 mm. long, mostly several-nerved, the lowest amplexicaul and usually shortly connate, the rest semiamplexicaul, with ovate or lance-acuminate, erect or spreading blades; leaves (1) 1.5-9 cm. long, with slender, wiry petioles and (5) 7-15 moderately distant or crowded, elliptic, obovate, oblong-obovate, or oval, acute, obtuse, or rarely retuse, flat or loosely folded leaflets (2) 4-11 mm. long; peduncles slender or subfiliform, 1.5-6.5 (8) cm. long, ascending at anthesis, arcuate-decumbent or prostrate in fruit; racemes subumbellately (2) 4—9-flowered, the axis scarcely elongating, 2—6 mm. long in fruit; bracts membranous, pallid or brownish, ovate or lance-acuminate, 0.6—2 mm. long; pedicels straight, ascending, at anthesis 0.7—1.4 mm., in fruit a trifle thickened, 1-1.9 mm. long; bracteoles 0-2, minute when present; calyx (3) 3.3-.6 mm. long, densely strigulose with black or mixed black and white, rarely all white hairs, the symmetric disc 0.5—0.8 mm. deep, the tube 2-2.7 mm. long, 1.7-2.2 mm. in diameter, the subulate, linear-subulate, or triangular, acute or obtuse teeth (0.6) 1—2 mm. long; petals whitish or sordidly suffused with lilac or dull lavender or leaden-purple; banner gently recurved through 30-45°, oblong-ovate beyond the short, cuneate claw, shallowly notched or entire, 7.2-9.5 mm. long, 4.4-5.2 mm. wide; wings (nearly as long to 1 mm. longer than the banner) 7.6—9.4 mm. long, the claws 2.5—3.1 mm., the obliquely oblong, obtuse or subemarginate, straight or slightly incurved blades 5.4—7.4 mm. long, 2.1—2.7 mm. wide; keel nearly as long or slightly longer than the banner, 7.8—8.6 mm. long, the claws 2.6—3.3 mm., the broadly oblong-oblanceolate blades 5.7-6.5 mm. long, 2.4-3 mm. wide, abruptly incurved through about 90° to the obtusely deltoid or broadly rounded apex, appearing truncate in profile; anthers 0.45-0.55 mm. long; pod loosely ascending (humistrate), broadly ovoid, ovoid-ellipsoid, or subglobose, bladdery-inflated, 1.5—3.3 cm. long, 1-1.8 (or when flattened apparently to 2.2) cm. in diameter, obtuse at base, contracted at apex into a very short, conical, or laterally compressed and deltoid beak, the body slightly obcompressed, shallowly sulcate along the ventral or both sutures, the thin, purple- speckled or -mottled, strigulose valves becoming papery, brownish, finely cross- reticulate, inflexed as a septum 1.8—3 mm. wide produced ± half way across the cavity to unite with a funicular flange 2—4.5 mm. wide; dehiscence apical, after falling; ovules 26—34; seeds brown or orange-brown, smooth or minutely pitted, 1.8-3.2 mm. long.—Collections: 28 (ii); representative: Tweedy (from Beaverhead County) undated (F, GH, NY); Alexander & Kellogg 4546 (UC); J. & M. Linsdale 321, 505 (CAS, RSA); Clokey 5618 (CAS, GH, NA, NY, RSA, SMU, UC, WS); Ripley & Barneby 4028 (CAS, RSA); Roos & Roos 5961 (NY, RSA).

Dry rocky crests, gravelly ridges and screes, descending from near timber line to openings in the forest belt, commonly on limestone but occasionally on granite or in pumice-sand, mostly 8600—11,750 feet, local, western Utah (Deep Creek Mountains, Tooele County) southwest through the higher mountains of Nevada (West Humboldt, Ruby and Spruce Mountains, Elko County; Egan and Schell Creek Ranges, White Pine County; Toiyabe Mountains, northern Nye and southern Lander Counties; Mt. Irish (at "6200 ft."), Lincoln County; Wassuk Range, Mineral County; and Sheep and Charleston Ranges, Clark County) to eastcentral California (Sweetwater and White Ranges, Mono County; Inyo Mountains, Inyo County); also rarely in dry sandy or rocky soil of sagebrush hillsides, 5550-7000 feet, probably on limestone, on the upper Pahsimeroi River in eastcentral Idaho and in adjoining Beaverhead County, Montana.—Map No. 137.—July to August southward, fruiting in June in the northern lowland stations.

Astragalus platytropis (with broad keel) Gray in Proc. Amer. Acad. 6: 526. 1865.— "With the preceding species," i.e., A. ineptus and A. Whitneyi, these found on a "dry rocky mountain near Sonora Pass, [California], in loose gravel and sand near the summit (alt. 10,000 ft.)."—"Holotypus, collected by W. H. Brewer in 1863, GH! isotypi, K, US!—Tragacantha platytropis (Gray) O. Kze., Rev. Gen. 947. 1891. Phaca platytropis (Gray) Rydb. in Mem. N. Y. Bot. Gard. 1 (Fl. Mont.): 246. 1900. Cystium platytrope (Gray) Rydb. in Bull. Torr. Club 40 : 50. 1913 ("platytropis").

The broad-keeled milk-vetch, A. platytropis, is a strongly marked astragalus, easily recognized by its tufted, silvery-gray foliage, subcapitate racemes of small, luridly purplish flowers with petals of nearly equal length, and disproportionately large and swollen, red-speckled or mottled pod. The partition within the fruit is composed in about equal parts of genuine septum, arising as an invagination of the endocarp from the dorsal suture, and of a flange formed by a web of tissue connecting the exceptionally long seed-funicles. As a consequence of the unusual development of the flange, the seeds are disposed in two rows lengthwise along the middle of the pod’s cavity, not approximate to the ventral suture as happens in the ordinary inflated legume.

The contemporary populations of A. platytropis are spatially isolated to a degree that must prevent any exchange of pollen between them, and it is not surprising to find that somewhat different modes of variation have become established in different parts of the discontinuous range. In some stations the leaflets are all elliptic and acute, in others (especially southward) all obovate and obtuse. In stable habitats the plants are truly cespitose and stemless, but on mobile scree or talus the caudex-branches elongate as they become buried by shifting gravel. The calyx-teeth are most commonly subulate or linear-subulate, but occasionally become shortened and obtusely triangular. Otherwise neither flower nor fruit shows much variation, and the fundamental features of the species must have become fixed at a comparatively early date.

The type-locality of A. platytropis, A. ineptus, and A. Whitneyi, vaguely indicated as a mountain near Sonora Pass, was probably not in the Sierra proper but more likely in the Sweetwater Mountains, the rugged profile of which can be viewed from the Sierra crest at Sonora Pass across the trench of the upper Walker River Valley. All three species are known to occur in the Sweetwaters, but A. platytropis, essentially a Basin species, has not been collected in the Sierra in modern times.