1. Philadelphus serpyllifolius A. Gray, Smithsonian Contr. Knowl. 3(5): 77. 1852.
Thyme-leaf mock orange Thyme-leaf mock orange
Philadelphus serpyllifolius var. intermedius B. L. Turner
Shrubs, 5–10(–20) dm. Stems light reddish brown, weathering gray and striate, stiffly divaricately to loosely branched, moderately strigose and appressed villous-sericeous; internodes (0.5–)1.3–3(–4.5) cm; short-shoot spurs sometimes present; axillary buds exposed. Leaves: petiole (1–)2–4(–5.5) mm; blade gray-white abaxially, green adaxially, lance-ovate or oblong-ovate to broadly ovate, (0.5–)1.2–2.5(–3) × (0.2–)0.4–0.8(–1.4) cm, herbaceous to subcoriaceous, margins entire, plane, often drying revolute, abaxial surface sparsely to moderately sericeous-strigose, hairs ± appressed, coarse to slender, 0.6–1.1 mm, with dense understory of white, slender, curled-crisped hairs, adaxial surface sparsely to moderately sericeous-strigose, hairs scattered, appressed, coarse, 0.5–1.2 mm, (these (4–)5–9 per mm of width in populations that have been called var. intermedius), also hirsute, hairs slender, usually erect, rarely appressed, 0.1–0.3(–0.4) mm. Inflorescences: flowers solitary, produced from previous year's long shoots. Pedicels 1–2 mm, strigose-sericeous to villous. Flowers: hypanthium strigose-sericeous or villous; sepals ovate, 3–5.5 × 2–3.2 mm, apex obtuse, acuminate to ± caudate, abaxial surface strigose-sericeous or villous, adaxial surface sparsely sericeous but villous along margins; petals white, oblong-ovate to ovate, 4–9(–11) × 3–6 mm; stamens 14–22; filaments sometimes proximally connivent, (1.5–)2–4.5 mm; style 1, clavate, 2.4–3.2 mm, slender base 0.5–0.7 mm; stigmatic portion 1.3–1.7 × 0.9–1.1 mm, apex slightly lobed. Capsules turbinate-globose, 4–5.2 × 4–5 mm, sepals persistent on distal 1/3, capsule distal surface usually impressed in 4(–8) vertical lines. Seeds not caudate, to 1.1 mm.
Flowering Apr–Jun; fruiting May–Nov. Rocky igneous and limestone slopes, bluffs, pinyon-oak-juniper zones; 1100–1900(–2300) m; Tex.; Mexico (Coahuila).
In two populations of Philadelphus serpyllifolius from Brewster County (Glass Mountains, Sierra Madera), the adaxial leaf vestiture has both long and short appressed hairs (the shorter hairs not erect as in typical P. serpyllifolius). These populations also show variation in development of the understory of tightly coiled hairs on the abaxial leaf surface, with some collections lacking the understory of coiled hairs. These have been recognized as var. intermedius.