Luz María González-Villarreal
Universidad de Guadalajara, México & University of Wisconsin-Madison, U.S.A.
Shrubs or trees, usually evergreen; indumentum well developed on the branchlets, leaves, and inflorescences, the trichomes acicular (simple), fasciculate or stellate. Leaves alternate, simple, entire or serrate, pubescent or glabrous, petiolate, exstipulate. 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. 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. 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. Fruit a 3 loculicidal subglobose capsule; seeds numerous, flattened and winged; embryo short, straight, cylindrical, endosperm fleshy.
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.
Distribution in the Neotropics
- 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.
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.
- Ovary superior, sericeous.
- Fruit three-valved loculicidal capsule.
- Seeds numerous, minute, flattened, winged.
Other important characters
- 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-reticulate seed coat.
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.
- Fruit indehiscent.
- Seed coat absent.
- Corolla urceolate, campanulate or tubular, often brightly coloured (red to orangish).
- Stamens in 2 whorls.
- Stigma simple.
- Anthers with 2 distal tubules or 2-4 awns.
- Ovary superior 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.
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).
Useful tips for generic identification
- 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.
The timber of some species is used for furniture and construction, mostly in Central America and South America.
Anderberg, A. A., C. Rydin, and M. Källersjö. 2002. Phylogenetic relationships in the order Ericales s.l.: analyses of molecular data from five genes from the plastid and mitochondrial genomes. Am. J. Bot. 89: 677-687.
Anderberg, A.A., and X. Zhang. 2002. Phylogenetic relationships of Cyrillaceae and Clethraceae (Ericales) with special emphasis on the genus Purdiaea Planch. Organisms Divers. Evol. 2: 127-137.APG Angiosperm Phylogeny Group. 1998. An ordinal classification for the families of flowering plants. Ann. Missouri Bot. Gar. 85: 531-553.Cronquist, A. 1993. An intergrated system of classification of flowering plants. 469-471. Columbia University Press, New York.
Carlquist, S. and E. K. Schneider. 2004. Perforation plate pit membrane remnants and other vessel details of Clethraceae: primitive features in wood of Ericales. Int. J. Plant Sci. 165(3): 369-375.
Fior, S., P. O. Karis, and A. A. Anderberg. 2003. Phylogeny, taxonomy, and systematic position of Clethra (Clethraceae, Ericales) with notes on biogeography: evidence from plastid and nuclear DNA sequences. Int. J. Plant Sci. 164(6): 997-1006.
González-Villarreal, L. M. 1996b. La familia Clethraceae en el estado de Jalisco, México. Flora de Jalisco, México 5:1-34. Inst. Bot. Universidad de Guadalajara, México.
González-Villarreal, L. M. 1996c. Clethraceae. Flora del Bajío y de Regiones Adyacentes. Instituto de Ecología A. C. Fasciculo 47: 1-19.
González-Villarreal, L. M. 1998. Three new species of Clethra (Clethraceae) from México. Boletín del Instituto de Botánica, Universidad de Guadalajara. Vol 5(1-3): 137-155.
González-Villarreal, L. M. 2005. Novelties in Clethra (Clethraceae) from México. Ibugana 13(1): 11-25.
González-Villarreal, L. M. 2007. Foliar trichome variation in Clethra subsect. Cuellaria (Clethraceae) from México. Ibugana 13(2): 17-65.
Gustafsson, C. 1992. Clethraceae. In: G. Harling & L. Andersson (eds), Flora of Ecuador. Nordic Journal of Botany 45: 1-26, Copenhagen.
Gustafsson, C. 2004. Clethraceae. In: N. Smith, S. A. Mori, A. Henderson, D. Wm. Stevenson, and S. V. Heald (eds). Flowering Plants of the Neotropics. 104-105 p. Princeton University Press.
Hamilton,C. W. 1985. Notes and description of seven new species of Mesoamerican Clethraceae. Ann. Missouri Bot. Gard. 72: 539-543.
Hamilton, C. W. 2001. Clethraceae. Flora de Nicaragua. In: W. D. Stvens, C. Ulloa Ulloa, A. Pool y O. M. Montiel (eds). Monographs Syst. Bot. Missouri Bot. Gard. 85(1): 615-616.
Morales, J. F. & E. Alfaro. 2006. Clethra formosa (Clethraceae), una nueva especie de Costa Rica. Annales del Jardin Botánico de Madrid 63(1): 35-39.
Robertson, K. 1967 . Clethraceae. Flora of Panama Part VIII. In: Woodson, R. E., R. W. Schery and collaborators (eds.). Ann. Missouri Bot. Gard. 54(3): 389-392.
Schneider, J. V. and C. Bayer. 2004. Clethraceae. In: K. Kubitzki (ed.). The families and genera of vascular plants VI. 69-73 p. Springer-Verlag, Berlin, Heidelberg, New York.
Sleumer, H. 1967. Monographia Clethracearum. Bot. Jahrb. Syst. 87: 36-175.
Standley, P. C. 1938. Clethraceae. Flora of Costa Rica. Publications of the Field Museum of Natural History. Botanical series 18(3): 867-868.
Standley, P. C. & L. O. Williams. 1966. Clethraceae. Flora of Guatemala. Fieldiana, Bot. 24(8): 74-81.Takhtajan, A. 2009. Flowering Plants. Clethraceae. Pp. 188-191. Second Edition. Springer.
Vickery, A. R. 2004. Clethraceae. Flora Mesoamericana on-line. [http://www.mobot.org/mobot/TROPICOS/Meso/people/VICKERY.html].
How to cite
González-Villarreal, L.M. (2009). Neotropical Clethraceae. In: Milliken, W., Klitgård, B. & Baracat, A. (2009 onwards), Neotropikey - Interactive key and information resources for flowering plants of the Neotropics. http://www.kew.org/science/tropamerica/neotropikey/families/Clethraceae.htm.
Click images to enlarge
Racemose inflorescence, Clethra fragans © L. M.González-Villarreal
Racemose inflorescence, Clethra fragans © L.M. González-Villarreal
Paniculate inflorescence, Clethra lanata © L.M. González-Villarreal
Series Tomentosae. Long-stipitate fasciculate and stellate trichomes on abaxial leaf surface, Clethra lanata © L. M. González-Villarreal
Articulate pedicels and pubescent sepals, Clethra luzmariae © L.M. González-Villarreal
Series Tomentellae. Appressed stellate trichomes with peltate center, Clethra luzmariae © L.M. González-Villarreal
Series Tomentosae. Appressed stellate and stipitate-fasciculate trichomes on abaxial leaf surface, Clethra mexicana © L.M. González-Villarreal
Small domatia in the abaxial leaf surface and covered by appressed stellate and short-stipitate fasciculate trichomes of Clethra nicaraguensis © L.M. González-Villarreal.
Inmature fruits showing the trifid stigma of Clethra oleoides © L.M. González-Villarreal.
Series Tomentellae. Entire leaves covered by appressed stellate trichomes with peltate center of Clethra oleoides © L.M. González-Villarreal.
Racemes with long and thin pedicels of Clethra pyrogena © L.M. González-Villarreal.
Densely flowered inflorescence of Clethra rosei © L.M. González-Villarreal
Bracts densely tomentose, longer than the pedicels of Clethra rosei © L. M. González-Villarreal.
Three-valved loculicidal capsule of Clethra rosei © L.M. González-Villarreal.
Habit of Clethra schlechtendalii © L.M. González-Villarreal.
Abaxial leaf surface covered by appressed, stellate trichomes of Clethra schlechtendalli © L.M. González-Villarreal.
Flowering branchlet with glabrous and shiny leaves of Clethra suaveolens © L.M. González-Villarreal.
Series Glabrae. Scattered appressed stellate trichomes of Clethra suaveolens © L.M. González-Villarreal.
Flattened, winged seeds showing a foveolate-reticulate seed coat of Clethra seeds © L.M. González-Villarreal.