Clethra

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Clethra

Description

Trees or shrubs, evergreen (Mal. spp.); Leaves crowded towards the end of the shoots, spiral, simple, exstipulate, serrate with glandular teeth, often with an apical gland, more rarely entire; Inflorescences sometimes simple solitary terminal racemes, but mostly consisting of a terminal raceme and several lower approximate racemes, each of the latter from the axil of a ± reduced or caducous leaf, thus forming together a panicle-, fascicle- or umbel-like inflorescence; Flowers bisexual, regular, 5(-6)-merous. Petals 5(-6), generally free, sometimes cohering to some degree, alternate with the calyx lobes, rather early caducous, generally sweet-scented. Stamens 10(-12) in 2 whorls of 5(-6), the outer whorl opposite the petals, the inner one opposite the calyx lobes; Ovary superior, 3-celled, with axile placentation; Fruit a 3-valved, loculicidal capsule, the septae of which become loose from the persistent central axis, subtended or ± enclosed at maturity by the persistent calyx. Seeds ∞, small, subovoid to irregularly angular or subtrigonous, with a foveolate-reticulate testa (all Mal. spp.).

Distribution

(sub)tropical America: present (sub)tropical Asiatic-Malesian, and temperate and tropical American distribution: present Andes: present Asia-Tropical:, Borneopresent; Jawa (Jawapresent); Lesser Sunda Is.present; Malaya (Peninsular Malaysiapresent); Maluku (Malukupresent); New Guineapresent; Philippines (Philippinespresent); Sulawesi (Sulawesipresent); Sumatera (Sumaterapresent) Cuba-Jamaica-Central America: present Japan: present Mt Roraima area: present NW. Argentina: present North America: present S. America: present SE. Asia: present SE. Bolivia: present SW., Central and SE. China: present Southern America:, Venezuela (Venezuelapresent) Trinidad: present Upper Burma: present small separate area in SE. Brazil: present
A small, monogeneric family in the Ericales, of (sub)tropical Asiatic-Malesian, and temperate and tropical American distribution, and with 1 sp. in Macaronesia (Madeira, and formerly in Teneriffe).
Of the total of 64 spp., 2 temperate spp. are found in North America (C. alnifolia L. and its var. pu-bescens AIT., and C. acuminata MICHX), 38 spp. inhabit (sub)tropical America (Cuba-Jamaica-Central America, in S. America in the Mt Roraima area and in Trinidad, in the Andes from Venezuela to SE. Bolivia and NW. Argentina, and a small separate area in SE. Brazil), 10 spp. are found in SE. Asia (Upper Burma, SW., Central and SE. China, Japan, Indo-China).
In Malesia 13 spp., of which 2 in the Malay Peninsula, 1 in Sumatra, 1 in Java, 3 in the Lesser Sunda Islands, 4 in the Philippines, 3 in Borneo, 2 in Celebes, 1 in the Moluccas, and 4 in New Guinea. .

Pollination

The flowers are protandrous. The pollen is shed at least partly before the flowers are open and the stigmas are receptive.

Dispersal

Nothing is known about the dispersal of Clethras. The rather small, deeply impressed-reticulate and light seeds of the Malesian spp. point to dispersal by wind.

Anatomy

. For general surveys of older literature see .
The wood shows primitive characters such as narrow solitary vessels with scalariform vessel perforations, fibre-tracheids, apotracheal parenchyma which is diffuse or arranged in short uniseriate bands and heterogeneous rays of two sizes. The petiole is supplied with a strongly incurved to closed vascular strand. Hairs occur as multicellular stellate and uniseriate types. Stomata mainly paracytic. Crystals solitary and clustered. Secretory tanniferous cells are present in the parenchymatous tissues of petiole and stem. METCALFE & CHALK (l.c.) state that Clethra has many anatomical features in common with Ericaceae. — P. BAAS.

Taxonomy

A monograph of the genus has been published by the author (). In this work it was shown, that the North American and the Asiatic-Malesian species of Clethra belong to sect. Clethra by their subovoid to subtrigonous and impressedly reticulate seeds, whilst the Central and South American species and C. arborea in Madeira form a second section, Cuel-laria, distinguished by flat and variously winged seeds.
For the determination of Clethras close observation of the indumentum with a lens is necessary. There are (i) simple, mostly rather long, appressed or patent hairs, (ii) fascicled, ± obliquely erect, generally elongated hairs, (iii) stellate hairs of generally small size and ± flattish. On most Clethras a combination of 2 or all 3 types of hairs is found, but their density may differ greatly on the various organs, resulting in a fine tomentellous, a thicker tomentulose or tomentose to villous tomentum of white to greyish, pale ferrugineous to dark rusty and even rufous colour. Besides the various kinds of hairs, the discrimination of species is based on floral characters. Collectors should try to collect flowering material and (sterile) branchlets with young, not yet glabrate leaves from the same specimen, and abstain from collecting sterile or fruiting material.

Uses

Temperate species of Clethra, mainly from North America, Madeira and Japan, much less from China, are cultivated as ornamentals in many parts of the world. Not a single species from Malesia so far has been introduced in Botanic Gardens.

Phytochemo

Leucoanthocyanins, caffeic acid, flavonols, taraxeron and ursolic acid are known to occur in species of Clethra (). This spectrum of phytoconstituents is compatible with the generally accepted relationship between Clethraceae and Ericaceae. — R. HEGNAUER.

Citation

LINNÉ, Sp. Pl. 1753: 396
SLEUM. - in Bot. Jahrb. 1967: 55