47. Holodiscus (K. Koch) Maximowicz, Trudy Imp. S.-Peterburgsk. Bot. Sada. 6: 253. 1879.
[Greek holos, whole, and diskos, disc, alluding to entire floral disc]
Spiraea Linnaeus [unranked] Holodiscus K. Koch, Dendrologie 1: 309. 1869
Shrubs, spreading to densely compact, (2–)3–30(–60) dm. Stems 1–10+, erect to arching; bark gray, ˂exfoliating, periderm reddish to nearly black˃; shoots 3 types: long ˂strongly angled or pleated 1st year˃, short, and water-sprout; glabrous, glabrate, downy, canescent, or pilose-tomentose. Leaves deciduous, cauline, alternate, ˂dimorphic with shoot type, those of short shoots in fascicles of 3–8˃, simple; petiole absent or present, ˂distinct, sometimes obscured by decurrent leaf base˃; blade ± ovate, rhombic, trullate to broadly trullate, broadly obovate, or obtrullate, 1–10 cm, membranaceous to chartaceous, sometimes coriaceous, margins flat, usually serrate, sometimes doubly serrate, venation pinnate, simple craspedodromous, abaxial surface glabrous, glabrate, pilose to villous, tomentose, or sericeous, ˂sometimes sessile- or stipitate-glandular, granular deposits present or absent˃, adaxial glabrous, glabrate, puberulent, pubescent, short-hirsute, velutinous, villous, ˂sessile- or stipitate-glandular or eglandular˃. Inflorescences terminal ˂on long shoots, erect or pendulous˃, 10–100+-flowered, panicles, glabrate, puberulent, villous, or tomentose ˂hairs golden, tan, or white, stipitate- or sessile-glandular or absent˃; bracts absent or subtending inflorescence branches; bracteoles present. Pedicels present. Flowers appearing after leaves, 2–6 mm diam.; epicalyx bractlets 0; hypanthium patelliform to shallowly crateriform, 1.5–5 mm diam.; sepals 5, erect to spreading or slightly reflexed, triangular- to elliptic-ovate or deltate; petals 5, usually white, sometimes pink-tinged, rarely pink, ovate to elliptic; stamens 15–20, equal to or longer than petals, ˂with prominent, raised nectar disc proximal to inner rim˃, exterior puberulent to tomentose, ˂sometimes sessile-glandular˃; torus inconspicuous; carpels 5, adnate to hypanthium base, hirsute and hispid, styles terminal, ˂stigmas 2-lobed˃; ovules 2 (1 aborted). Fruits aggregated achenes, 5, laterally flattened, inverted dolabriform, with abaxial edge strongly convex, adaxial straight, 1–1.5 mm, hispid, hirsute, and often sessile- or stipitate-glandular; hypanthium persistent; sepals persistent, erect; styles deciduous or persistent. x = 9.
Species 5 (2 in the flora): North America, Mexico, Central America (Costa Rica, Guatemala, Honduras), South America (Colombia, Venezuela).
Holodiscus contains two species complexes: the argenteus and the discolor complexes (R. A. Lis 1990). The discolor complex comprises H. discolor and H. microphyllus and ranges from British Columbia and Montana through the western United States and south through Mexico along the Sierra Madre Occidental and Oriental, with the southern terminus near the Valley of Mexico. The argenteus complex comprises H. argenteus (Linnaeus f.) Maximowicz, H. fissus (Lindley) C. K. Schneider, and H. orizabae F. A. Ley and ranges from the Trans-Mexican Volcanic Belt south to Colombia and Venezuela.
A number of species and varieties have been described from the western United States that appear locally unique, and many names have been applied at the species and varietal levels. The leaves have provided the primary characters for circumscribing taxa in the genus, as the flowers and inflorescences show a narrow range of variation. Holodiscus can exhibit phenotypic plasticity in leaf morphology, particularly in response to changes in relative amounts of moisture and light.
Each plant has three shoot types that bear leaves of morphologies slightly to widely divergent among types: short shoots, long shoots, and water-sprouts. The short shoots bear leaves in fascicles of 3–8, the most diagnostic leaves for the taxa. Multiple seasons of leaf development on a short shoot result in a short woody stem with leaf scars in close succession in a phyllotactic spiral.
The long shoots develop in the spring, grow rapidly, and may attain a length of 1–2 m in a month or two; they often have a strongly angled or pleated appearance; leaves are alternate along the stem. The long-shoot leaves have rapid growth and expansion and differ from short-shoot leaves in characteristics such as petiole condition, blade size, shape, serration, and base and apex shapes. Long-shoot leaves are quite responsive to local environmental conditions and the season’s water regime under which they developed; consequently, they are more variable morphologically than short-shoot leaves. Long shoots are often terminated by an inflorescence and are more frequently observed and collected. Long shoots may arise from anywhere on the plant, and in some plants, populations, or taxa, they may be relatively short (sometimes as short as 2–6 cm), but maintain the particular morphology of the long shoot.
Water-sprout shoots typically arise from the crown of the plant and appear to occur for two reasons: trauma to the plant by severe clipping, mowing, or intense grazing; or unusually high moisture during the spring and early summer growth phase. Water-sprout leaves are the least diagnostic because they have a similar morphology across taxa of the discolor complex. They are quite striking when found and can be dramatically different from short-shoot leaves. Water-sprout leaves are typically thin and delicate, strongly serrate with primary and secondary teeth, and glabrous or glabrescent to very sparsely pubescent, hence appearing very green.
Holodiscus has been segregated from the Spiraeeae, with a follicle fruit type, and placed in its own tribe (Holodisceae Focke) based upon the presence of achenes (W. O. Focke 1888–1891; P. A. Rydberg 1908–1918; C. Kalkman 1988; A. L. Takhtajan 1997). Recent molecular work by D. R. Morgan et al. (1994) and D. A. Potter et al. (2007, 2007b) confirms that Holodiscus belongs in Spiraeeae. The investigation into floral ontogeny and morphology in Amygdaloideae by R. C. Evans and T. A. Dickinson (1999b) found that Aruncus, Holodiscus, and Spiraea share three character states of ovule morphology, further supporting the inclusion of Holodiscus in Spiraeeae.
Two other generic names have been used for taxa in Holodiscus: Schizonotus Rafinesque, an illegitimate name, and Sericotheca Rafinesque, a rejected name.
Holodiscus was of particular horticultural interest in the nineteenth century, which led to the description of many species and varieties. The panicle of flowers can be attractive, and a shrub arched and sagging with relatively many panicles is eye-catching.
SELECTED REFERENCES Ley, F. A. 1943. A taxonomic revision of the genus Holodiscus (Rosaceae). Bull. Torrey Bot. Club 70: 275–288. Lis, R. A. 1990. A Taxonomic Revision of Holodiscus Maxim. (Spiraeoideae: Rosaceae) Based upon Numerical Analyses of Leaf Morphometric Variation. Ph.D. dissertation. University of California, Berkeley.