Thermopsis rhombifolia (Nutt. ex Pursh) Richardson
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Authority
Isely, Duane. 1981. Leguminosae of the United States. III. Subfamily Papilionoideae: tribes Sophoreae, Podalyrieae, Loteae. Mem. New York Bot. Gard. 25 (3): 1-264.
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Family
Fabaceae
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Scientific Name
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Description
Species Description - Usually rhizomatous, sometimes caudex-based pubescent or glabrate, erect, slightly branched herb with solitary or clustered stems 2-10 dm. Pubescence subappressed-villosulous, primarily of lower surfaces of leaves and often scant. Petioles (.5-) 1-4 cm; leaflets broadly ovate, obovate to oblong-oblanceolate, 28 cm, 1.5-6 r. Stipules (1—)2—5 cm long, to 2.5 cm wide, foliaceous, persistent, the lower as large as associated leaflets or smaller, ovate to oblong-lanceolate, reduced upwards. Raceme terminal, usually solitary, short or long, loosely or densely flowered, elongating to .5-3 dm in fruit; bracts conspicuous at anthesis, usually equaling or exceeding pedicels, 6— 10(— 15) mm, ovate to lanceolate, obtuse, acute, or acuminate. Pedicels 3-10 mm, ascending. Calyx 8-11 mm; lobes 2-3.5(-4.5) mm, ca. .5-1 r. Corolla 18—22(—25) mm. Ovary stipitate 1-2 mm; body narrowly oblong, villosulous with hairs .2-.5 mm; ovules ca. 9-15. Legumestraight and stiffly erect or (e portion of range) divaricate, and slightly (0°-45°) or much (90°-180°) decurved, even annular, 4-6 cm x 5-8 mm; valves thinly coriaceous, sparsely or abundantly puberulent, sometimes glabrate at maturity, expressed over seeds. Seeds 3-15.
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Discussion
n = 9, 18 (see varieties). I have divided western Thermopsis, a complex of confluent taxa, into two species for reasons discussed in the generic introduction. Of the two, T. rhombifolia is the eastern montane and intermountain complex. Were it not for the existence of the geographically and morphologically intermediate tetraploid, Thermopsis rhombifolia var divaricarpa, T. rhombifolia in the historic sense, contrasting with the others in its modest stature and strongly decurved or annular pods, could perhaps be maintained as a separate species of the eastern slopes of the Rockies and adjacent high plains. But var divaricarpa links var rhombifolia to the regionally diverse var montana (including T. montana and T. pinetorum of Larisey), and all three merge. The northwestern var ovata, confluent with var montana, is also related to T. macrophylla var venosa. Thermopsis rhombifolia is usually evidently or incipiently rhizomatous. Two or three stems arise together from contiguous vertical rhizomes, these clusters being spaced at varying distances along horizontal rootstocks (probably roots). But, in some specimens, the perennial stem is essentially a superficial crown with little evidence of caudex or rhizome development. I do not know whether this variation in habit is largely mediated by growth conditions or is genetically controlled.
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Distribution
Nw plains, both slopes of Rockies and Intermountain states: w North Dakota to e Oregon and Washington, s to Arizona and New Mexico; n to s Saskatchewan and Alberta. Said to be slightly cult.
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