| # The genera of Leguminosae-Caesalpinioideae and Swartzieae |

L. Watson and M.J. Dallwitz

### Mezoneuron Desf.

~ Caesalpinia.

Auct.: Mezoneurum., Mezonevron

Type species: M. glabrum Desf.

Habit and leaf form. Trees (rarely), or shrubs, or climbers or scramblers; without tendrils; armed (the branchlets and petioles often very prickly), or unarmed.

Phyllotaxy spiral. The leaves compound; bipinnate; with opposite or sub-opposite pinnae; with opposite or sub-opposite leaflets; with rachides adaxially ridged. The leaflets many per leaf; petiolulate; symmetrical or nearly so; pinnately veined, with a predominant ‘midrib’. Stipules small, absent or early caducous or very inconspicuous in mature leaves; membranous. Stipels absent.

Inflorescence and floral morphology. The inflorescences axillary, or terminal; unbranched and branched; of racemose units; simple racemes, or panicles (of racemes). Bracts narrow, absent at anthesis, or persistent beyond anthesis (rarely). Bracteoles absent; absent at anthesis.

The flowers hermaphrodite; pentamerous; coloured. Floral tube length relative to total hypanthium + calyx length about 0.25–0.75. Hypanthium present; variously saucer-shaped to cupular, or tubular (?). Calyx 5; covering the rest of the flower in bud; polysepalous; markedly irregular (the lowermost segment larger than the rest, concave and naviculate); members imbricate, or not imbricate (rarely, the members then all connate into a tube). Corolla present; regular to slightly irregular (the petals all similar, or the adaxial, innermost one somewhat dissimilar); 5; without greatly reduced members; polypetalous. Petals clawed, or sessile; spreading, orbicular, imbricate; imbricate-ascending; yellow, or red. Disk absent. The androecium comprising 10 members; declinate; members all free of one another; members all more or less equal in length; comprising only fertile stamens. Fertile stamens 10. Anthers uniform, attached well above the base of the connective; dehiscing introrsely; dehiscing longitudinally. Ovary sessile or subsessile to stipitate; free. Stigma somewhat dilated (and truncate on the often obliquely clavate apex of the subulate style). Ovules few, or numerous, or solitary.

Fruit, seed and seedling. Fruit compressed, with two or more ovules, a two-valved pod, or indehiscent; winged longitudinally; 1 winged (along the upper suture); membranous or leathery, not becoming woody. The mature valves with conspicuous, prominent, raised venation; conspicuous venation not predominantly longitudinal. Seeds non-endospermic; with a straight or slightly oblique radicle; amyloid-negative. Cotyledons flat.

Transverse section of lamina. Leaves without conspicuous phloem transfer cells in the minor veins. Druses common in the mesophyll. Mesophyll secretory cavities (gland-dots) common, or absent; without a lining of epithelium. Adaxial hypodermis absent. Leaf girders absent. Laminae dorsiventral. Mesophyll without unaligned fibres or sclereids. Minor veins lacking accompanying fibrous tissue.

Leaf lamina epidermes. Epidermal crystals not seen either adaxially or abaxially. Simple unbranched hairs common; scabrid. No compound or branched eglandular hairs seen. Capitate glands not seen. Hooked hairs not seen. Cassieae-type leaf pseudo-glands not seen. Expanded and embedded hair-feet absent. Basally bent hairs absent. Adaxial: Adaxial interveinal epidermal cell walls straight in optical section, or markedly sinuous in high-focus optical section; conspicuously pitted. Stomata adaxially very rare. Abaxial: Abaxial stomata not predominantly paracytic. Abaxial epidermis papillate interveinally, or not papillate; with papillae over-arching the stomata. Abaxial interveinal epidermal cell walls straight, or gently undulating, or markedly sinuous in high-focus optical section; not conspicuously pitted in optical section; scarcely staining with safranin; thin.

Pollen ultrastructure. Tectum punctate; smooth punctate, or puncticulate. Length of colpi greater than one half pole to pole distance (with a margocolpus).

Cytology. Basic chromosome number, x = 11.

Species number and distribution. About 35 species. Africa and Madagascar to Australia and Pacific.

Tribe. Caesalpinieae.

Comments. Widely cultivated.

Miscellaneous. Illustrations: • Mezoneuron angolense: Brenan, Fl. Tropical East Africa (1967).

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: 4th August 2019. delta-intkey.com’.

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