Clethraceae Klotzsch

Milliken, W., Klitgard, B. and Baracat, A. (2009 onwards), Neotropikey - Interactive key and information resources for flowering plants of the Neotropics. http://creativecommons.org/licenses/by/3.0

Morphology General Habit

Shrubs or trees, usually evergreen; indumentum well developed on the branchlets, leaves, and inflorescences, the trichomes acicular (simple), fasciculate or stellate

Morphology Leaves

Leaves alternate simple, entire or serrate, pubescent or glabrous, petiolate, exstipulate

Morphology Reproductive morphology Inflorescences

Inflorescence terminal, consisting of a single raceme or mostly several racemes arranged in fascicles or paniculate clusters, dense or lax -flowered; bracts soon caducous; pedicels articulated, thin to thick

Morphology Reproductive morphology Flowers

Flowers bisexual, actinomorphic, usually small, fragrant; sepals pubescent , lobes 5, the outer somewhat wider than the inner, becoming lignified during fruit development, persistent; petals 5, usually distinct or connate at base, often abaxially convex (spoon-shaped), glabrous or sometimes pilose toward base, white to cream,  apex obtuse to rounded, usually erose - fimbriate

Morphology Reproductive morphology Flowers Androecium Stamens

Stamens 10, obdiplostemonous, hypogynous, filaments slightly fused at base of petals and somewhat enlarged at base, flattened laterally, glabrous , anthers extrorsely inflexed in bud, later introrse, bilocular, dorsifixed, dehiscence poricidal; disk absent but  basal part of  ovary often nectariferous; ovary superior, sericeous , carpels 3,  locules 3,  style glabrous or rarely pilose ,  stigma 3- lobed; placentation axile , 3 pendent placentas arising from upper portion of central column, ovules numerous per carpel, unitegmic, tenuinucellate, anatropous

Morphology Reproductive morphology Fruits

Fruit a 3 loculicidal subglobose capsule; seeds numerous, flattened and winged; embryo short, straight, cylindrical,  endosperm fleshy .

Distribution

Clethra ranges from Mexico, Central America, the Windward Islands and into South America as far as Argentina. The tropical species are predominantly found in humid montane habitats, reaching their upper altitudinal limit at about 3,800 m in the Andes. The species reported in tropical lowlands, commonly in secondary vegetation, are found as low as 20 m above sea level.

Diagnostic

Key differences from similar families: The following families differ from Clethraceae in having the following features: Cyrillaceae (Purdiaea Planch.): Sepals uniqual, the outer much larger than the inner. Petals pink to violet. Style unbranched. Nectary disk present. Ovary (3-) 5-locular. Ovule one per locule. Fruitindehiscent. Seed coat absent. Ericaceae: Corolla urceolate, campanulate or tubular, often brightly coloured (red to orangish). Stamens in 2 whorls. Stigmasimple. Anthers with 2 distal tubules or 2-4 awns. Ovarysuperior or inferior. Fruits berries, drupes or capsules. Glandular trichomes often present. Actinidiaceae (Saurauia Willd.): Young organs covered with variously branched trichomes, sometimes glandular. Raphides contained in elongated idioblasts occurring in most tissues. Leaves with strigose, scale and stellate trichomes on the abaxial surface. Inflorescences thyrses with few to numerous flowers. Stamens numerous, often yellow-coloured. Fruits usually berries, 5-sulcate. Most species are evergreen. Bark smooth or corky. Branches generally with large scars from the fallen leaves. Leaf coriaceous or subcoriaceous, margins entire, serrate or dentate. Abaxial leaf surface glabrous, pubescent or tomentose. Inflorescences can be terminal and axillary, racemose or paniculate, upright or slightly curved. Flowers white or cream, mostly fragrant. Style often persistent. Seeds subovoid or irregular angular to subtrigonous and showing a foveolate-reticulateseed coat. Distinguishing characters (always present): Trees or shrubs. Leaves crowded apically on the branchlets, petiolate, alternate, simple. Bracts present, soon caducous. Indumentum on the branchlets, leaves, and inflorescences present as acicular, fasciculate or stellate trichomes; no glandular trichomes present. Flowers 5-merous, small, bisexual. Sepals persistent then becoming lignified Petals saucer-shaped, the margin erose-fimbriate. Anthers extrorsely inflexed in bud but becoming antrorse during anthesis. Stigma deeply trifid. Ovarysuperior, sericeous. Fruit three-valved loculicidal capsule. Seeds numerous, minute, flattened, winged. The genus is most diverse in the tropical montane habitats. South eastern Mexico is an important center of diversity for the genus Clethra. Species of Clethra are mostly found growing in acid soils (as Ericaceae). Most members are pioneers or primary invaders, adapted to the colonization of disturbed areas. They are very tolerant of disturbance and sprout readily after destruction of original forest by timber cutting, agriculture, or fire. In general, they are light-demanding, remaining sterile in dense forest and flowering best in forest borders with open vegetation, as well as in secondary plant communities where they often bloom throughout the year. Seedlings and very young shrubs are very common in open areas. Hybridization events of  are very common, with local hybridization believed to occur among many of the species. Nothing is known about the pollinators or the dispersal biology of tropical species. Flowers are often visited by ants.

Note

The timber of some species is used for furniture and construction, mostly in Central America and South America. Number of genera: Clethraceae has been considered as a monogeneric family with ca. 120 species.  In Tropical America the genus Clethra section Cuellaria contains approximately half of the total number of species in the world. The genus was recently expanded to contain Purdiaea Planch., a genus recently removed from its former position in the Cyrillaceae, well supported by morphological and molecular data by Anderberg and Zhang (2002). For ample information on this aspect see Anderberg and Zhang (2002), and Fior et al. (2002). Notes on delimitation: Clethraceae are a monophyletic family within the reclassified order Ericales s.l. (APG 1998), belonging to the large ericoid clade and forming the sister group of Cyrillaceae and Ericaceae (Anderberg et al. 2002).  Clethra L. has an eastern Asian-American distribution with the single exception of C. arborea Aiton that occurs in Madeira. Section Clethra includes the two North American species and all Asian species, whereas section Cuellaria Ruiz & Pav. comprises all South American, Central American, and Mexican species (subsection Cuellaria) as well as C. arborea (subsection Pseudocuellaria). In additon, the subsection Cuellaria was further divided by Sleumer (1967) into four series according to the leaf indumentum: Glabrae, Tomentellae, Tomentosae and Ferrugineae.