Clethraceae Klotzsch.
Habit and leaf form. Small trees, or shrubs; leptocaul. Leaves evergreen, 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 recorded; represented by hair tufts.
Leaf anatomy. Stomata present; anomocytic, or anisocytic, or paracytic. Hairs present. Complex hairs usually present; stellate.
Minor leaf veins without phloem transfer cells.
Stem anatomy. Cork cambium present; initially deep-seated. Nodes unilacunar. Internal phloem absent. Secondary thickening developing from a conventional cambial ring. ‘Included’ phloem absent. Xylem with tracheids; with vessels. Vessel end-walls oblique; scalariform. Wood parenchyma apotracheal.
Reproductive type, pollination. Plants hermaphrodite.
Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’; in racemes, or in panicles. The terminal inflorescence unit 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, biochemistry. 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). Sugars transported as oligosaccharides + sucrose.
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 (1998) Eudicot; core Eudicot; Asterid; unassigned to Euasterid I or Euasterid II; Ericales. Species 120. Genera 1; Clethra.
Economic uses, etc. A few cultivated ornamentals.
Cite this publication as: ‘L. Watson and M. J. Dallwitz (1992 onwards). The Families of Flowering Plants: Descriptions, Illustrations, Identification, and Information Retrieval. Version: 14th December 2000. http://biodiversity.uno.edu/delta/’. Dallwitz (1980), Dallwitz, Paine and Zurcher (1993, 1995, 2000), and Watson and Dallwitz (1991) should also be cited (see References).