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The families of flowering plants

L. Watson and M.J. Dallwitz

Clethraceae Klotzsch.

Habit and leaf form. Small trees, or shrubs; leptocaul. Leaves persistent, or deciduous; alternate; spiral; ‘herbaceous’, or leathery; petiolate; non-sheathing; simple. Lamina entire; pinnately veined; cross-venulate. Leaves exstipulate. Lamina margins serrate, or dentate, or entire. Domatia occurring in the family; manifested as hair tufts.

Leaf anatomy. The leaf lamina dorsiventral. Stomata present; mainly confined to one surface (abaxial); anomocytic, anisocytic, and paracytic (mixed, but mostly paracytic). Hairs present. Complex hairs usually present; stellate. Lamina without secretory cavities. Cystoliths present, or absent. The mesophyll containing crystals. The crystals mostly druses. Minor leaf veins without phloem transfer cells.

Axial (stem, wood) anatomy. Pith very heterogeneous. Secretory cavities absent. Cork cambium present; initially deep-seated. Nodes unilacunar. Primary vascular tissues in a cylinder, without separate bundles; collateral. Internal phloem absent. Cortical bundles absent. Medullary bundles absent. Secondary thickening developing from a conventional cambial ring. Primary medullary rays narrow, or mixed wide and narrow.

The wood diffuse porous. The vessels moderately small; exclusively solitary. The vessel end-walls oblique; scalariform. The vessels without vestured pits; with spiral thickening, or without spiral thickening. The axial xylem with tracheids; without vasicentric tracheids; with fibre tracheids; without libriform fibres; without septate fibres. The fibres without spiral thickening. The parenchyma apotracheal. ‘Included’ phloem absent. The wood not storied.

Reproductive type, pollination. Plants hermaphrodite.

Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’; in racemes, or in panicles. The ultimate inflorescence units cymose, or racemose. Inflorescences terminal; racemes or panicles. Flowers ebracteolate; small; sometimes fragrant; regular; 5(–6) merous; cyclic; pentacyclic. Free hypanthium absent. Hypogynous disk absent.

Perianth with distinct calyx and corolla; 10(–12); 2 whorled; isomerous. Calyx 5(–6); 1 whorled; gamosepalous; deeply blunt-lobed. Calyx lobes about the same length as the tube, or markedly longer than the tube. Calyx regular; persistent; non-accrescent; imbricate. Corolla 5(–6); 1 whorled; polypetalous (or nearly so); imbricate; regular; white, or pink; deciduous. Petals sessile.

Androecium 10, or 2. Androecial members free of the perianth (or no more than barely adnate to the petal bases); free of one another; 2 whorled. Androecium exclusively of fertile stamens. Stamens 10, or 2; diplostemonous; alternisepalous; both alternating with and opposite the corolla members. Anthers dorsifixed; becoming inverted during development, their morphological bases ostensibly apical in the mature stamens; versatile; dehiscing via pores (the anthers becoming inverted during ontogeny and the pores ostensibly apical, cf. Ericaceae); bilocular (sagittate); tetrasporangiate; appendaged (tailed), or unappendaged (pointed). The anther appendages ostensibly apical. Endothecium developing fibrous thickenings. Anther epidermis persistent. Microsporogenesis simultaneous. The initial microspore tetrads tetrahedral. Anther wall initially with more than one middle layer. Tapetum glandular. Pollen shed as single grains. Pollen grains aperturate; 3 aperturate; colporate; 2-celled.

Gynoecium 3 carpelled. Carpels reduced in number relative to the perianth. The pistil 3 celled. Gynoecium syncarpous; synstylovarious, or eu-syncarpous; superior. Ovary 3 locular; sessile. Gynoecium stylate. Styles 1; attenuate from the ovary; apical. Stylar canal present. Stigmas 1 (trilobed), or 3; dry type; papillate; Group II type. Placentation axile. Ovules 2–50 per locule (‘many’); funicled; anatropous; unitegmic; tenuinucellate. Endothelium differentiated. Embryo-sac development Polygonum-type. Polar nuclei fusing prior to fertilization. Antipodal cells formed; 3; not proliferating. Synergids pear-shaped. Endosperm formation cellular. Endosperm haustoria present; chalazal and micropylar (weak). Embryogeny asterad.

Fruit non-fleshy; dehiscent; a capsule. Capsules loculicidal. Seeds endospermic. Endosperm oily. Seeds with a testa (this very thin); winged, or wingless. Embryo well differentiated. Cotyledons 2. Embryo straight.

Seedling. Germination phanerocotylar.

Physiology, phytochemistry. Sugars transported as oligosaccharides + sucrose. Not cyanogenic. Alkaloids absent (4 species). Iridoids not detected. Proanthocyanidins present; cyanidin, or cyanidin and delphinidin. Flavonols present; quercetin. Ellagic acid absent (2 species). Aluminium accumulation not found (but sometimes accumulating cobalt).

Geography, cytology. Holarctic, Paleotropical, and Neotropical. Temperate to tropical. Eastern Asia, Malaysia, Central America, southeastern U.S.A., one species in Madeira. X = 8.

Taxonomy. Subclass Dicotyledonae; Tenuinucelli. Dahlgren’s Superorder Corniflorae; Ericales. Cronquist’s Subclass Dilleniidae; Ericales. APG III core angiosperms; core eudicot; Superorder Asteranae. APG IV Order Ericales.

Species 120. Genera 1; Clethra.

Economic uses, etc. A few cultivated ornamentals.

Illustrations. • Clethra quercifolia: Bot. Reg. 23, 1842. • Clethra arborea: Hutchinson.

We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, genera included in each family, and classifications (Dahlgren; Dahlgren, Clifford, and Yeo; Cronquist; APG). See also Guidelines for using data taken from Web publications.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 1992 onwards. The families of flowering plants: descriptions, illustrations, identification, and information retrieval. Version: 5th March 2018.’.