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(Last Modified On 11/13/2012)
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(Last Modified On 11/13/2012)
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Genus
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HOLODISCUS (Koch) Maxim.
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PlaceOfPublication
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Acta Hort. Petrop. 6:253. 1879
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Reference
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Focke in Engl. & Prantl, Nat. Pflanzenfam. 33:18. 1894; Ley in Bull. Torrey Bot. Club 70:275-288. 1943, nom. conserv.
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Synonym
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Sericotheca Raf. Sylva Tellur. 152. 1838; Rydb. in N. Am. Fl. 22:261-266. 1908. Spiraea sect. Holodiscus Koch, Dendr. 1:309. 1 869.
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Description
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Deciduous shrubs or small trees up to 7 m. tall, with simple, short-petiolate, estipulate, dentate leaves, the blades mostly with a long cuneate entire base, the veins ascending and parallel to the cuneate base, impressed on the upper surface, the lower surface often tomentose. Flowers white, 5-merous, in large terminal panicle-like clusters; pistils distinct, 2-ovulate, alternate with the sepals, inserted on center of disk, each forming at maturity an indehiscent 1-seeded dry fruit.
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Note
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This genus has experienced several systematic revisions, and those who have worked on it have usually recognized two principal species-groups, one ranging from British Columbia to northern Mexico, the other from southern Mexico to Colombia. The southern group has been distinguished by having long-mucronate teeth on the leaf-blades, by having the stamens not longer than the sepals, and by the rather strongly pubescent outer surface of the petals. The two most recent monographers of Holodiscus, Rydberg (in the North American Flora) and Ley (in the Bulletin of the Torrey Botanical Club) each recognized three species in this "southern group" of the genus, although the two authors differed in their ideas of how the species were to be delimited.
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Distribution
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British Columbia to northern Mexico, the other from southern Mexico to Colombia
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Note
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Examination of large series of specimens from western United States indicates that neither author is conservative in delimitation of species, each upholding as species several groups which are morphologically scarcely distinguishable, and in addition are connected by numerous intermediates. The differences between the supposed species are chiefly those in the leaves, which vary in size, shape and vesti- ture. Study of the meager amount of Central American and South American material which is available indicates that all the supposed species of the above ttsouthern group" are in actuality but representatives of a single species. The leaves are usually oblanceolate or obovate, with the lower surface tomentose and bearing in addition few or many straight silky hairs. Rydberg and Ley have at- tempted to separate species here on the basis of the pubescence of the upper and lower surfaces of the leaves, respectively, and Ley has also used the shape of the leaf-tip as a criterion. These characters do not seem to be correlated with other morphological ones, however, nor with any geographical area, and in any case both characters are notably untrustworthy in the Rosaceae. The single Panamanian specimen which has been available for study has some leaves rounded at tips and others acute (the usual condition in this genus and other rosaceous ones, the rounded ones being produced first at the initiation of the growing season), so that according to Ley's key it could be either Holodiscus argenteus or H. fissus; many of the leaves agree precisely in shape with her illustration (Bull. Torrey Bot. Club 70:287, fig. 3. 1943) of H. argenteus, but the pubescence of the lower surface is less abundantly provided with silky hairs than most specimens she has called H. argenteus, and agrees well with those she has named H. fissus. The latter, how- ever, she does not report from south of Guatemala, and it is moreover excluded by having the upper leaf-surface glabrous, not strongly puberulent as in the Pana- manian material. The jumble of characters used to separate H. argenteus and H. fissus is so inconclusive that the latter is here reduced to synonymy, following Standley and Steyermark in the 'Flora of Guatemala' (Fieldiana: Bot. 24:453. 1946).
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