Monographs Details: Astragalus humillimus Freyn & Sint.
Authors:Rupert C. Barneby
Authority: Barneby, Rupert C. 1964. Atlas of North American Astragalus. Mem. New York Bot. Gard. 13(2): 597-1188.
Discussion:311.  Astragalus humillimus

Diminutive, tufted perennial, cinereously strigulose with straight, truly appressed, largely dolabriform hairs up to 0.2-0.3 mm. long, the leaflets pubescent on both sides, the closely and repeatedly forking, suffruticulose caudex beset with scaly persistent stipules and rigid, subspinescent petioles; stems of the year ± 0.5-1 cm. long, the internodes very short, concealed by imbricated stipules; stipules submembranous, becoming papery, deltoid-ovate, 1-1.5 mm. long, fully amplexicaul but free, pubescent dorsally, the tips recurved; leaves 8-15 mm. long, shortly petioled, with stiff, subterete, tapering rachis and 7-11 (13) obovate-cuneate or oblong-elliptic, obtuse, sessile but articulate, loosely folded, thick-textured, finally deciduous leaflets 0.7-2 mm. long; peduncles 2-4 mm. long; racemes very shortly 1-3-flowered; calyx ± 3 mm. long, the tube 2.2 mm., the subulate teeth ± 0.8 mm. long; petals not seen, "pallid" (ex char., but probably purplish); pod apparently spreading, sessile, tardily disjointing from the receptacle, oblong-ellipsoid, ± 4.5 mm. long, 2 mm. in diameter, carinate by the prominent sutures, the strigulose valves becoming stiffly papery and stramineous; dehiscence apical and downward through both sutures; ovules apparently 4 ("9" acc. Gray); seeds not seen.—Collections: typus only.

Sandstone rock ledges in the piñon belt, sometimes submerged in drifting sand, apparently very rare and local, known only from the type-station on the Mesa Verde, Montezuma County, Colorado.—Map No. 140.—May to July.

Astragalus humillimus (smallest of all) Gray ex Brand. in Bull. U. S. Geol. Surv. 23: 235. 1876.—"Growing upon sandstone rock of the Mesa Verde, near the edge of Mancos Canyon."—Holotypus, Brandegee 1087, collected in July, 1875, GH! isotypi, MO, NY, UC, US!—Tragacantha humillima (Gray) O. Kze., Rev. Gen. 945. 1891. Phaca humillima (Gray) Rydb. in Bull. Torr. Club 32: 665. 1905.

The typus of the singular little Mancos milk-vetch, A. humillimus, was collected in high summer when the flowers were past and all but the last' fruits had fallen. I have seen only one perfect pod, but this is, as Gray described it, laterally compressed and carinate by both sutures. It is, in fact, like that of the much larger-flowered A. siliceus, while that of A. cremnophylax, with which A. humillimus was confused by Tones (1923, p. 82, Pl. 6), is obtusely trigonous, with a salient ventral keel opposed to a distinctly flattened or even depressed dorsal face. The two species of Grand Canyon and Mesa Verde are otherwise similar, except for the petioles of A. humillimus which ripen into stiff, persistent structures quite analogous to the so-called spines that form the armature of the Asiatic tragacanths. In habit of growth A. humillimus resembles A. jejunus, a species of southern Wyoming in which the caudex is also beset with a thatch of stiffly marcescent petioles. The two species were associated by Rydberg (1929, p. 359) in a section of Phaca characterized by this feature, but the stipules of A. jejunus are connate and its pod is bladdery and of extremely thin texture. They differ further in the mode of hair-attachment, and the similarity in growth-form is probably fortuitous.

Brandegee left unusually precise records of the habitat of A. humillimus. It should be easy to find again, but it has nevertheless eluded repeated search on Mesa Verde and along the Mancos canyon to the east. It is to be sought in crevices of rimrock pavement and on sand- filled ledges of shelving rock masses, the ecological niches in which several specialized Basin astragali have found a congenial home. Care should be taken to distinguish two other dwarf astragali found in similar places on the Mesa, A. calycosus var. scaposus and the endemic A. deterior. Both have flaccid petioles and longer oblong or narrowly ellipsoid pods, grooved dorsally and partially or fully bilocular.