5a. Philadelphus microphyllus A. Gray var. microphyllus
Philadelphus argenteus Rydberg; P. argyrocalyx Wooton var. argenteus (Rydberg) Engler; P. crinitus (C. L. Hitchcock) S. Y. Hu; P. microphyllus subsp. argenteus (Rydberg) C. L. Hitchcock; P. microphyllus var. argenteus (Rydberg) Kearney & Peebles; P. microphyllus subsp. crinitus C. L. Hitchcock; P. microphyllus var. crinitus (C. L. Hitchcock) B. L. Turner; P. microphyllus var. linearis S. Y. Hu; P. microphyllus subsp. occidentalis (A. Nelson) C. L. Hitchcock; P. microphyllus var. occidentalis (A. Nelson) Dorn; P. microphyllus var. ovatus S. Y. Hu; P. minutus Rydberg; P. nitidus A. Nelson; P. occidentalis A. Nelson; P. occidentalis var. minutus (Rydberg) S. Y. Hu
Leaf blades (0.8–)1–2.2(–4) × (0.3–)0.4–1(–1.8) cm, herbaceous to coriaceous, margins entire, abaxial surface gray-white, often mottled with brown, sparsely to moderately sericeous-strigose, hairs appressed or loosely appressed, 0.2–0.6 mm, or coarser, to 0.5–1.2 mm, or when vestiture dense, with understory of slender, wavy-curved hairs, or chaotically hirsute-pilose (through introgression with var. argyrocalyx), marginal hairs often more erect, adaxial surface green, drying dark brown (cuticle thin, finely papillate), mosaic of dark brown and yellowish gray-green (cuticle forming mosaic of thin, papillate areas and thick, smooth areas), or rarely uniformly yellowish gray-green (cuticle thick, smooth), usually sparsely sericeous-strigose, hairs appressed or slightly ascending, slender, 0.3–0.7 mm, sometimes glabrate or glabrous, rarely weakly hirtellous or with some erect hairs near base. Inflorescences 1(–3)-flowered. Flowers 14–32 mm diam.; hypanthium and sepal abaxial surfaces glabrous, sparsely sericeous proximally or in 4 lines extending from pedicels to alternate sepals, or moderately sericeous throughout, hairs appressed, coarse to fine, not obscuring epidermis, or densely sericeous-strigose with understory of thinner curved-wavy hairs completely obscuring epidermis; sepals (3.5–)6.5–8 mm; petals (5.8–)8–10(–15) × (5.3–)6–8(–10) mm; stamens (26–)34–46. Capsules turbinate, 4.5–6 × 4.5–6 mm.
Flowering Jun–Aug; fruiting Jul–Nov. Arid montane scrub, hardwood or pine-oak woodlands, yellow pine-fir forests, limestone and rhyolitic substrates, canyons, open slopes, bluffs, canyons; 1600–2700 m; Ariz., Calif., Colo., Nev., N.Mex., Tex., Utah, Wyo.; Mexico (Baja California, Coahuila).
Four species that were recognized by P. A. Rydberg (1905) and Hu S. Y. (1954–1956) on the basis of hypanthium and sepal vestiture are here combined: Philadelphus argenteus and P. crinitus (densely lanate-sericeous-strigose, with underlying hairs more slender and coiled), P. microphyllus (glabrous or sparsely sericeous in four vertical lines), and P. occidentalis (uniformly sericeous-strigose, vestiture not completely obscuring the epidermis). The vestiture patterns are often mixed in populations, although regional patterns exist. Most plants in the northern range (northern Arizona, Colorado, southern Nevada, northern New Mexico, Utah, Wyoming) have sparsely sericeous-strigose adaxial leaf surfaces and a slightly denser vestiture on the abaxial surfaces; the adaxial leaf surfaces have a thin cuticle, and the leaves dry a brown color. In more arid zones (southeast Arizona, southwestern New Mexico, trans-Pecos Texas, and Baja California, Mexico), the adaxial epidermis often has a thicker, smoother cuticle or a mosaic of areas of thin and thick cuticle, and the leaves dry a gray- or olive green color.
The taxon crinitus, first named as a subspecies of Philadelphus microphyllus, is based on a population from the rhyolitic Davis Mountains in trans-Pecos Texas. Plants assigned to this taxon have densely vestitured hypanthia and sepals; longer hairs on the abaxial leaf surface (to 1.2 mm), with an underlying layer of thinner, wavy hairs; and thick adaxial cuticles. They are usually distinct from adjacent var. microphyllus. However, similar plants occur in the nearby limestone Guadalupe Mountains and again in southeastern Arizona (Huachuca and Santa Rita mountains) and in the intervening areas. Many from the latter region have been considered P. argenteus (with densely vestitured hypanthium and sepals, often similar leaf vestiture, and thick cuticles), but plants referable to P. argenteus occur scattered among plants of var. microphyllus and do not form large uniform populations; the P. argenteus form is here considered a more strongly vestitured phase within an expanded var. microphyllus, which then also includes var. crinitus.