Flora of North America species comparison

Ivesia setosa occurs mostly to the east and south of I. baileyi, from southeastern Humboldt and Churchill counties to Elko, White Pine, and northern Nye counties, Nevada, and in the Deep Creek Range of western Utah. It represents a transition between the planar-leaved, chasmophytic members of sect. Setosae and the cylindric-leaved, matted species of flatter sites. Stems of I. setosa are more generally erect than in other species in the section, and the usually calcareous substrate is noteworthy. The deeply incised (usually not quite to base) leaflets are somewhat intermediate between the toothed leaflets of I. baileyi and the leaflets of I. shockleyi that are incised to the base into separate lobes. The individual leaflets of both I. baileyi and I. setosa are more or less flat and distichously paired; in I. shockleyi, groups of leaflet lobes are folded over onto each other, giving a verticillate appearance to the leaflet arrangement.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Species 11 (11 in the flora).

The planar-leaved, chasmophytic members of sect. Setosae found in and around the mountains centered on the Mojave Desert are hypothesized to represent the ancestral radiation of Ivesia, on the basis of morphology and distribution. A secondary radiation northward into the Great Basin is characterized by a complete sequence in size reduction, dissection, and increased number of leaflets, paralleled by a reduction in numbers of floral parts. Ivesia saxosa retains the most pleisiomorphies and could readily be accommodated within Potentilla in the strict sense, as has commonly been done. Gross morphology of I. saxosa is nearly identical to that of I. arizonica, which has been treated as a distinct genus, Purpusia, on the basis of its lack of an epicalyx, deep hypanthium, and stipelike torus. The narrow endemic I. patellifera is intermediate between these two species, and an identical habit is shared by I. baileyi and I. jaegeri, which have more dissected leaflets and more reduced flowers. All of these species are now placed in the same genus, either as Ivesia (B. Ertter 1989) or as Potentilla (J. T. Howell 1945).

Most species in sect. Setosae are more or less petrophytic (in contrast to other sections), often growing in crevices of vertical cliffs. The protection offered by these habitats often allows extended blooming periods and resultant indeterminate inflorescences that produce flowers as long as conditions are suitable, including at the height of summer when relatively few other desert species are in bloom. Stems can initially radiate in almost all directions relative to the substrate (vertical in chasmophytes, horizontal in others), but they generally succumb to gravity with increased number of flowers and become either pendent or prostrate. In particularly favorable years, blooming season and flower number can exceed the ranges given here. The small but often numerous flowers are visited by a wide diversity of potential pollinators, including ants, which are attracted to the glistening, nectariferous hypanthial disc. Most species of sect. Setosae have carunculate seeds, which may indicate that ants play a role in seed dispersal, a particularly useful adaptation for chasmophytic plants.

Since Ivesia longibracteata (sect. Ivesia) is sometimes identified as a member of sect. Setosae, it is included herein and keys out in the sixth couplet.

(Discussion copyrighted by Flora of North America; reprinted with permission.)