Jones’s waxydogbane

Cycladenia jonesii Eastwood, Leafl. W. Bot. 3: 159. 1942

Stems glabrous. Leaves gla­brous. Corollas pubescent abax­ially or marginally with hairs that are not inter­woven, sparsely pubescent adaxially, lobes 4–5 mm.

Flowering summer; fruiting summer. Steep slopes of barrens in desert scrub and pinyon-juniper woodlands; of conservation concern; 1300–2600 m; Ariz., Calif., Utah.

Variety jonesii is known from Utah (Emery, Garfield, Grand, and Kane counties) and Arizona (Mohave County), where it occurs on gypsiferous and nongypsiferous soils in barrens associated with desert scrub communities and pinyon-juniper woodlands, and eastern California (Inyo Mountains), where it occurs on steep shale slopes.

In Utah, plants of var. jonesii exhibit a floral dimorphism, with corollas 18–21 mm or 23–28 mm in length, within a single population (S. L. Welsh et al. 2003).

S. D. Sipes and V. J. Tepedino (1996) found that although var. jonesii is self-compatible, most reproduction is vegetative. Because a pollen vector is required, the limited fruit set observed may be due to low visitation rates by potential pollinators, or to the possibility that the original pollinator is no longer consistently found within the distribution range of the plant. These authors also observed limited fruit set in flowers that were cross-pollinated by hand, raising the possibility that fruit production could also be limited by soil nutrient availability.

Allozyme studies (S. D. Sipes and P. G. Wolf 1997) have shown var. jonesii to be well differentiated genet­ically from the typical variety despite the morpholog­ical similarity. However, the results of two unpublished studies (M. P. Last 2009; H. K. Brabazon 2015) suggest that plants from the Inyo Mountains, California, here referred to var. jonesii, are genetically similar to popula­tions of var. venusta and may be better assigned to that variety or to an undescribed taxon.

Cycladenia humilis var. jonesii is listed as threatened by the United States Fish and Wildlife Service and is in the Center for Plant Conservation’s National Collection of Endangered Plants.

SELECTED REFERENCES Sipes, S. D. and V. J. Tepedino. 1996. Pollinator lost? Reproduction by the enigmatic Jones cycladenia, Cycladenia humilis var. jonesii. In: J. Maschinski et al., eds. 1996. Southwestern Rare and Endangered Plants: Proceedings of the Second Conference: September 11–14, 1995, Flagstaff, Arizona. Fort Collins, Colo. Pp. 158–166. [U.S.D.A. Forest Serv., Gen. Techn. Rep. RM-283.] Sipes, S. D. and P. G. Wolf. 1997. Clonal structure and patterns of allozyme diversity in the rare endemic Cycladenia humilis var. jonesii (Apocynaceae). Amer. J. Bot. 84: 401–409.