The genera of Leguminosae-Caesalpinioideae & Swartzieae
Oligostemon pictus Benth.
Type species: D. orchidacea Baill.
Habit and leaf form. Climbers or scramblers (forming bush ropes); without tendrils; unarmed.
Phyllotaxy spiral. The leaves compound; pinnate; imparipinnate. The leaflets large, oblong-ovate few per leaf; opposite or sub-opposite; quite long petiolulate; without noticeably twisted petiolules; symmetrical or nearly so; pinnately veined, with a predominant midrib. Stipules ovate, absent or early caducous or very inconspicuous in mature leaves (caducous); not connate. Stipels absent.
Inflorescence and floral morphology. The inflorescences terminal; branched; of racemose units; panicles. The flowers not distichous. Bracts minute and scale-like. Bracteoles present (minute, scale-like); small, not enclosing the flower buds (the pedicels being articulated in the middle, above them); absent at anthesis; not valvate.
The flowers remarkably orchid-like; hermaphrodite; not papilionaceous; not pentamerous throughout; departing from pentamery in the calyx, or in the calyx and in the androecium; coloured. Hypanthium absent, the androecium hypogynous. The perianth comprising distinct calyx and corolla. Calyx 4; covering the rest of the flower in bud; not Swartzieae type; ostensibly polysepalous; markedly irregular (heteromorphic, with the two exterior members dorsally opposed, the dorsal one of them probably representing a joined pair, and two lobed laterals); members imbricate (the abaxial member of the dorsoventrally opposed pair on the outside, and the lateral pair imbricate abaxially to the synandrium). Corolla present; very irregular (the very heteromorphic petals comprising 3 larger adaxial members, and two very small abaxials easily misinterpreted as staminodes); 5; including greatly reduced members (two of them); polypetalous. Petals imbricate-descending with the posterior petal (vexillum) outside; all red striped and gland-fimbriate. Disk absent. The androecium comprising 4–5 members; with united members (these cohering by their filaments, anthers or both into an incurved, hood-like synandrium, with the anther thecae prolonged beyond the thickened connective into a subulate horn and dehiscing subapically via a short slit); members all more or less equal in length; comprising only fertile stamens (discounting the staminodal petals). Fertile stamens 4–5. Anthers attached at the base of the connective. Ovary shortly stipitate; free. Stigma terminal, small, dilated (capitate). Ovules few (2–3).
Fruit, seed and seedling. Fruit plano-compressed, a two-valved pod; valves twisting and enrolling during dehiscence; winged longitudinally; 4 winged (i.e., with four lengthwise wings); not becoming woody. Seeds with a straight or slightly oblique radicle; amyloid-negative; with starch.
Transverse section of lamina. Leaves without conspicuous phloem transfer cells in the minor veins. Druses common in the mesophyll. Mesophyll secretory cavities absent. Adaxial hypodermis absent. Leaf girders common (the veins transcurrent). Laminae dorsiventral. Mesophyll without unaligned fibres or sclereids. Minor veins mainly with abundant accompanying fibres.
Leaf lamina epidermes. Epidermal crystals not seen either adaxially or abaxially. Simple unbranched hairs common. No compound or branched eglandular hairs seen. Capitate glands not seen. Hooked hairs not seen. Cassieae-type leaf pseudo-glands present. Expanded and embedded hair-feet absent. Adaxial: Adaxial interveinal epidermal cell walls markedly sinuous in high-focus optical section; thin. Stomata adaxially very rare. Abaxial: Abaxial stomata predominantly paracytic. Abaxial epidermis not papillate. Abaxial interveinal epidermal cell walls markedly sinuous in high-focus optical section; conspicuously pitted in optical section; scarcely staining with safranin; of medium thickness.
Pollen ultrastructure. Length of colpi inapplicable pollen diporate, unique in legumes.
Species number and distribution. 1 species (D. orchidacea). Tropical West Africa.
Comments. Very unusual among Leguminosae in having wood lacking vestured pits and diporate pollen grains, as well as in the very peculiar floral morphology.
Miscellaneous. Illustrations: • D. orchidacea: Baillon, Adansonia VI (1865). • D. orchidacea: Aubréville, Flore du Gabon (1968). • D. orchidacea, e.m. scanned pollen (Graham & Barker, 1981).
We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, and classification. See also Guidelines for using data taken from Web publications.
Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 1993 onwards. The genera of Leguminosae-Caesalpinioideae and Swartzieae: descriptions, illustrations, identification, and information retrieval. In English and French. Version: 22nd March 2017. delta-intkey.com/caes’.