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The type locality for Macroptilium supinum is located 19 miles N of La Colorada, Mexico. The entire range of M. supinum ranges from Sonora and Nayarit north into Santa Cruz County, Arizona.M. supinum is a perennial herb with creeping stems arising from an elliptical caudex 3-14 cm long. Leaves are compound with three elliptical leaflets 3-8 cm long. Flowers are salmon-colored borne on erect inflorescences of 2-8. The plant also has cleistogamous (self-fertilizing) underground flowers that are primarily yellow tinged with salmon. Pods are only 8-15 mm long, and appear on both aboveground and underground inflorescences, but most seeds are produced on underground inflorescences, essentially preplanting them. Fruiting occurs following summer rains. It is the only Macroptilium/Phaseolus in the area with a caudex, and the herbaceous part of the plants die back in fall, and re emerge from dormancy in late spring. Plants also become dormant following a dry spell. Plants prefer grassland and oak woodland habitats, which are threatened by overgrazing, off-road activity, development and reservoir construction (Pena Blanca Lake). Plants are listed as Category 1 with the US Fish and Wildlife Service. M. supinum is believed to hybridize with M. heterophyllus, occasionally. Plants are highly palatable to rodents and cattle. The total U.S. population consists of approximately 1000 plants.
Desert Botanical Garden has only 116 seeds of Macroptilium supinum from two populations. Twelve plants were produced from a germination test using 25 seeds. The initial germination percentage was 88%, but some seedlings were lost due to damping off. Plants were placed in a greenhouse and were gradually nibbled away by an unknown animal, even in the greenhouse. A related species, M. atropurpureum, was grown from seed and plants were transplanted into the ground. Within 24 hours, the plants and caudexes were gone, presumably eaten by ground squirrels.
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