A Re-Evaluation of the Horkelia bolanderi (Rosaceae) Complex, with the New Species Horkelia yadonii

Abstract

The Horkelia bolanderi complex is re-evaluated, based on morphological and field studies. A new species, Horkelia yadonii, is recognized from the South Coast Ranges of California. It is a potentially threatened species growing in dry streambeds and meadows in chaparral and foothill pine woodlands. Horkelia tenuiloba is restricted to Marin, Sonoma, and adjacent Mendocino counties, where it is also a rare species occurring in moist openings in the chaparral. Horkelia clevelandzi is treated as a distinct species, which occurs in open forests in the mountains of San Diego County and Baja California. Horkelia bolanderi subsp. parryi becomes H. rydbergii, a relatively common species growing at the edge of meadows and drainages in the Transverse Ranges of southern California. Horkelia bolanderi is limited to Lake and adjacent Colusa counties, where it is a rare plant at the margins of vernal wet areas. Collections previously identified as members of the H. bolanderi complex from Santa Cruz and San Mateo counties are assigned to H. marinensis, a possible allopoly- ploid with H. cuneata subsp. sericea as the other parent. A key and map are included. Horkelia Cham. & Schldl. is a close relative of Potentilla L. that is centered in California. In general, members of Horkelia are perennial herbs with a distinct resinous fragrance from abun- dant glands. Leaves, which are primarily basal, are pinnately compound with indistinct ter- minal leaflets. In this regard they resemble many species of the related Ivesia Torrey & A. Gray, from which they differ in their characteristic flower structure. The hypanthium of Horkelia forms a flat-bottomed cup, generally about half as deep as wide, and the stamens are always 10 in number. In most species, the filaments are somewhat flattened and form a tube around the styles in early anthesis; anthers dehisce along their inner face. Petals are white (to pinkish or cream-colored) and usually ? oblanceolate-el- liptic. Sepals, which alternate with smaller bractlets, are often sharply reflexed at anthesis and purple-flecked. In contrast, Ivesia is more variable, with the hypanthium often shallow and seldom flat-bot- tomed. The five to twenty stamens generally have slender, spreading filaments and anthers that dehisce laterally. The linear to obovate pet- als are white to yellow, and the sepals are gen- erally spreading at anthesis and seldom purple- flecked.