Sentry Milkvetch

Perennial, Herbs, Stems woody below, or from woody crown or caudex, Taproot present, Nodules present, Stems prostrate, trailing, or mat forming, Stems less than 1 m tall, Stems solid, Stems or young twigs sparsely to densely hairy, Stems silvery, canescent, tomentose, cobwebby, or wooly, Stems with 2-branched hairs, dolabriform, Leaves alternate, Leaves petiolate, Stipules conspicuous, Stipules membranous or chartaceous, Stipules persistent, Stipules free, Leaves compound, Leaves odd pinnate, Leaf or leaflet margins entire, Leaflets opposite, Leaflets 5-9, Leaflets 10-many, Leaves hairy on one or both surfaces, Flowers in axillary clusters or few-floweredracemes, 2-6 flowers, Inflorescence axillary, Bracts very small, absent or caducous, Flowers zygomorphic, Calyx 5-lobed, Calyx glabrous, Petals separate, Corolla papilionaceous, Petals clawed, Petals white, Petals pinkish to rose, Petals blue, lavander to purple, or violet, Petals bicolored or with red, purple or yellow streaks or spo ts, Banner petal narrow or oblanceolate, Wing petals narrow, oblanceolate to oblong, Wing tips obtuse or rounded, Keel petals auriculate, spurred, or gibbous, Keel tips obtuse or rounded, not beaked, Stamens 9-10, Stamens diadelphous, 9 united, 1 free, Filaments glabrous, Style terete, Style persistent in fruit, Fruit a legume, Fruit unilocular, Fruit freely dehiscent, Fruit oblong or ellipsoidal, Fruit exserted from calyx, Fruit beaked, Fruit glabrous or glabrate, Fruit hairy, Fruit 3-10 seeded, Seeds cordiform, mit-shaped, notched at one end, Seed surface smooth, Seeds olive, brown, or black.

Astragalus cremnophylax is a rare species of milkvetch known by the common names sentry milkvetch and cliff milkvetch. It is endemic to Arizona, where the three varieties grow in three separate locations. The rarest variety, var. cremnophylax, is a federally listed endangered species which grows only on the rim of the Grand Canyon. The two other varieties are known from the Buckskin Mountains and Marble Canyon.

This plant is a resident of the Kaibab Limestone, where it grows in cracks in the bare limestone. The habitat is a pavement of dry, exposed, white rock that takes full sun and brisk, dry winds. Plants occur in rock crevices that have a few centimeters of soil, if any, and accumulate a minute amount of moisture at times. The species sometimes grows alongside Petrophytum caespitosum, which occupies a similar niche.

The rare var. cremnophylax was threatened with extinction when its tiny populations were trampled by tourists creeping to the edge of the Grand Canyon. Fences have been placed at strategic locations there to protect the plant.