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glabrous inside, lobes valvate in bud, spreading in open flowers. Stamens included, inserted at the base of the tube, filaments short. Ovary 2-celled, each locule with a multiovulate placenta; common style very short, the 2 short stigma lobes filiform, hairy, included. Fruits indehiscent (?), crowned by persistent calyx lobes; each locule with numerous minute seeds.

A monotypic, disjunct genus of uncertain tribal position, known from the W. Himalayas (India: Uttar Pradesh), Upper Myanmar, Thailand and China (Yunnan, Guizhou). See further notes below.

Clarkella nana (Edgew.) Hook.f., Fl. Brit. India 3: 46. 1880; Lauener, Notes R.B.G. Edinb. 32: 104.1972.— Ophiorrhiza nana Edgew.,Trans. Linn. Soc. London 20: 60. 1846.— Ophiorrhiza pellucida H.Le´v., Repert. Spec. Nov. RegniVeg.13:176.1914.— Clarkella siamensis Craib, Bull. Misc. Inform. Kew 1931: 216. 1931; Craib in Fl. Siam. 2: 36. 1932.— Clarkella nana (Edgew.) Hook.f. var. siamensis (Craib) Fukuoka & Kurosaki, Tonan Ajia Kenkyu 8: 178

(1970). Fig. 1.

Distribution.— NORTHERN: Mae Hong Son, Chiang Mai, Nan, Lampang, Tak; SOUTHWESTERN: Kanchanaburi (Sangkhla Buri); PENINSULAR: Nakhon Si Thammarat [Tung Song, Kao Chem, Rabil 139: type of C. siamensis).

Notes.— Fukuoka (1978), studying the species’ floral anatomy and morphology, uncritically associated Clarkella and Argostemma (actually tribe Argostemmateae!) with tribe Hedyotideae. In any case, floral features alone are inconclusive and believed to be insufficient to determine the plant’s tribal position. Fruit and seed morphological and anatomical, palynological, as well as molecular data are needed to evaluate the tribal alliance.

Flowers of Clarkella superficially resemble those of certain Ophiorrhiza species, so that it does not come as a total surprise that Chinese material of Clarkella was originally described as Ophiorrhiza pelludica (see Lauener & Ferguson, 1972; apparently overlooked by Fukuoka, 1978). Vegetatively, Clarkella resembles certain Argostemma species (tuber- like rhizome; suppression of one leaf of a pair, i.e., extreme anisophylly). And, together with certain Argostemma species, it is among the few true rubiaceous geophytes whose aerial parts disappear in the dry season; in the following rainy season new flowering shoots originate from the underground parts (i.e., tuber-like rhizomes).

Clarkella fruits, too, look very similar to those of Argostemma (Fig. 1D–E). A direct comparison is not possible at this stage as their detailed morphology and anatomy is unknown because of lack of preserved, fully mature material. It is assumed that its fruits (described as “indehiscent” from not fully mature material) will, very much as in Argostemma, eventually develop into lid capsules from which seeds are ejected by drops of rain water (ombrophily; see Puff et al., 2005: 189). This does not necessarily imply a relationship between the two genera but is more likely to be a comparable ecological adaptation (also found in genera of other families, notably Gesneriaceae or Melastomataceae).

The species occurs at altitudes ranging from 450 to 1100 m and grow in cracks of rocks of moist to wet limestone outcrops or cliffs, usually in shade or semi-shade, and are often associated with other characteristic limestone rock plants such as various Gesneriaceae. As is typical for plants of such habitats, they exhibit rather extreme

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