2. Holodiscus microphyllus Rydberg, Bull. Torrey Bot. Club. 31: 559. 1904.
Holodiscus discolor (Pursh) Maximowicz subsp. microphyllus (Rydberg) A. E. Murray; H. discolor var. microphyllus (Rydberg) Jepson
Shrubs, erect to spreading, ˂often tangled and tightly condensed where exposed or with open growth where protected˃, 3–10(–20) dm. Stems: long shoots (not predominant, depending upon seasonal conditions) 2–10(–20) cm, 1st year slightly angled or round, green to straw yellow, glabrous or canescent, pilose, or tomentose, sessile- and stipitate-glandular, 2d or 3d year epidermis exfoliating, periderm shiny maroon or dark red, aging into light to dark gray or nearly black bark; short shoots (usually predominant) 0.2–3 cm; water-sprout shoots very rare, 5–15 cm. Leaves: petiole often absent, sometimes obscured by decurrent leaf base; blade maximal width usually from middle distally, chartaceous to coriaceous; long shoots (not predominant): blade obtrullate or broadly obovate to ± ovate, 1–3 × 0.5–1.5 cm, coriaceous, base acute, cuneate or decurrent, margins entire proximally, serrate in distal 1/4–1/2, secondary teeth infrequent, primary and secondary veins slightly protruding abaxially, venation weakly percurrent, apex obtuse, sometimes acute, abaxial surfaces glabrous, glabrate, or pilose, tomentose to villous, sericeous, sessile- or stipitate-glandular, adaxial glabrous or glabrate, puberulent, or short-hirsute, ± sessile-glandular; short shoots (predominant): in fascicles of 3–8, blade obtrullate, 0.3–1.5(–3) × 0.2–1 cm, base cuneate, margins serrate, secondary teeth absent, primary and secondary veins usually protruding abaxially, venation very weakly percurrent, apex obtuse, abaxial surfaces ˂whitish (waxes and cutin)˃, glabrous or strigose on veins and tomentose to villous or sericeous (in intercostal regions) to densely ˂long, white˃ sericeous, ± sessile- and stipitate-glandular, adaxial glabrous, glabrate, puberulent, velutinous, villous, with scattered longer hairs, usually stipitate- or sessile-glandular; water-sprout shoots: blade 2–3 × 1–2 cm. Inflorescences 10–50-flowered, 2–10 × 1–7 cm; branches puberulent to tomentose, ± stipitate- or sessile-glandular; bracts absent, rarely subtending basal branches, absent distally, usually subtending peduncles, sometimes absent; bracteoles 0.5–2(–3) mm. Flowers 3–4 mm diam.; hypanthium 1.5–2.5 mm diam.; sepals triangular to deltate, 1.3–2.5 mm, apex acute to acuminate, abaxially glabrate, sparsely sessile-glandular or densely villous, glandular; petals white, ovate to elliptic, 1.5–2.5 mm; stamens longer than petals, 2.5–3 mm; carpels 1.3–2 mm. Achenes beige to gray, appearing white from hairs, densely hirsute on edges, sometimes on faces, glands 20–40 per face, sessile.
Varieties 3 (3 in the flora): w United States, nw Mexico.
The distribution of Holodiscus microphyllus is centered in the Great Basin of the western United States, with expansions into arid rocky habitat of adjacent mountain ranges; it is adapted to xeric conditions on rocky, infertile sites. Short shoots are predominant in this species, and as the percentage of short shoots increases, the architecture of the plant becomes increasingly tangled and dense. In populations with a wider spreading habit, long shoots develop more frequently. Depending upon the moisture conditions of the growth season, the long-shoot leaves often differ strongly from the short-shoot ones.
Populations of Holodiscus microphyllus are usually small (1–20 plants), and in a number of locations they appear to consist of clones, and the identity of individuals cannot be determined with certainty. Individual populations often are on mountaintops, isolated rock outcrops, or rimrock areas that often are distant from the nearest adjacent population. The potential for genetic exchange between populations may be very limited.
The varieties of Holodiscus microphyllus are quite distinct and have strong geographic correlations. Fieldwork on intra-populational variation indicated no clinal variation in the indumentum features that separate varieties. No hybrid was found between varieties of H. discolor and H. microphyllus.