Astragalus missouriensis var. amphibolus

Dry hillsides, plains and valley floors, with piñon, juniper, or sagebrush, on granitic, volcanic or sandstone bedrock, 5800-8200 feet, locally plentiful in scattered stations on the west slope of the southern Rocky Mountains in Colorado, from the Grand River in Garfield County south through the Gunnison, Mancos, and San Juan Valleys, east to the headwaters of the Rio Grande in Conejos County, west into the foothills of the La Sal and Abajo Mountains in Grand and San Juan Counties, Utah.—Map No. 94.—May to July.

Astragalus missouriensis var. amphibolus (ambiguous, combining features of A. missouriensis and A. amphioxys) Barneby in Amer. Midl. Nat. 37: 447. 1947.—"Mancos, Montezuma Co., Colorado, Baker, Earle & Tracy No. 56... "—Holotypus, collected June 22, 1898, NY! isotypi, CAS, MINN, ND, POM, RM!

The var. amphibolus is locally abundant on the sagebrush plains extending east from Monticello into Colorado, and it is here most certainly, as probably throughout its range, a fixed self-perpetuating entity. However it seems likely that it arose in the first place as a hybrid involving var. missouriensis, with which it is sympatric in a small sector of its range, and A. amphioxys, found in the form of var. vespertinus at the western periphery of its area, although at lower elevations. We have at present too few examples of the fully ripe fruit to determine how often the pod remains attached to the receptacle after dehiscence. Pods persistent on peduncles of the preceding year, a feature characteristic of var. missouriensis, have been seen in var. amphibolus from the Abajo Mountains and the Mancos Valley, but a plant from Ouray County, Colorado, collected in July, had already shed its fruit in the manner of A. amphioxys. If the hypothesis of a hybrid origin is correct, variation of this nature might be expected in different populations of the variety.