Flora of North America species comparison

Species 9 (9 in the flora).

Section Horkelia encompasses the species that are most commonly encountered in heavily populated areas of California. Plants are notably glandular-viscid (unless obscured by dense vestiture) and have a distinctive resinous odor.

Previous revisions (for example, P. A. Rydberg 1908c; D. D. Keck 1938) have placed Horkelia frondosa (here treated as H. californica var. frondosa) at the beginning, implying that this is the least derived expression within the genus. Such an assumption is based on its gross resemblance to sympatric members of Drymocallis; molecular evidence (T. Eriksson et al. 1998; M. Lundberg et al. 2009; C. Dobeš and J. Paule 2010) confirms that this similarity is superficial. If, as speculated above, species composing sect. Hispidulae are relicts of the original radiation, then H. californica var. frondosa is actually one of the more derived members of the genus. Species within sect. Horkelia have been ordered here according to that interpretation.

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

Species 20 (20 in the flora).

As discussed under 9. Ivesia, recognition of the ivesioid genera including Horkelia results in a paraphyletic Potentilla, which the first author does not find sufficient to dictate generic circumscriptions. On the basis of morphology and biogeography, Horkelia itself is likely nested within Ivesia as the branch that has radiated primarily within the California Floristic Province (see discussions under Ivesia regarding discordance with M. Töpel et al. 2012, http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0050358). Ivesia in the strict sense consists of an exceptionally diverse array of well-defined species; in contrast, Horkelia is more morphologically coherent as a genus but with species less sharply defined. The primary unifying features of the genus result from a flower specifically adapted for bee pollination: a relatively deep, flat-bottomed hypanthium containing abundant nectar, to which access is restricted by the columnar arrangement of ten erect stamens with flattened filaments, at least in early flowering. Anthers open introrsely by adaxial slits so that pollen is deposited on a visitor forcing its way into the staminal column.

Other diagnostic characteristics of Horkelia include a distinctive resinous aroma in most species (except sect. Tridentatae and sect. Hispidulae) and various levels of glandularity. The terminal leaflet is most often lobed and/or confluent with lateral leaflets, in contrast to Drymocallis, which is often confused with Horkelia. Leaves range from planar with distinct leaflets to cylindric with overlapping leaflets, the latter condition often a response to drought stress in species that are normally planar-leaved. Sepals are often strongly reflexed.

Stem lengths given below include inflorescences. Leaflet descriptions are based primarily on mid rachis leaflets of the larger basal leaves present at flowering; leaflets on the smaller outermost leaves formed early in the growing season are often less divided and less hairy. Counts of leaflet marginal segments include both shallow (teeth) and deep (lobes) ultimate divisions. Flowers are often aggregated into a single corymbiform cluster or one to many more globular glomerules, some of which are tightly capitate. Hypanthia interiors are glabrous in most species, except for the hairs surrounding the torus; some taxa are also more or less pilose on the internal hypanthial walls, especially just below the rim. Sepals are described as spreading or reflexed based on their orientation at anthesis. Larger and smaller anthers often are alternating; measurements are primarily of the larger filaments, with width measured at the base. See under 8. Potentilla and 9. Ivesia for additional discussion of special terminology.

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