| # The genera of Leguminosae-Caesalpinioideae and Swartzieae |

L. Watson and M.J. Dallwitz

### Arcoa Urb.

Type species: A. gonavensis Urb.

Habit and leaf form. Trees, or shrubs; leaves and inflorescences crowded on ‘short shoots’; armed, or unarmed.

Phyllotaxy distichous. The leaves compound; bipinnate; with opposite or sub-opposite leaflets. The leaflets small, many per leaf. Stipels absent.

Inflorescence and floral morphology. The inflorescences branched; of racemose units; fascicles (of spikes). The flowers sessile, not distichous. Bracteoles present; small, not enclosing the flower buds; persistent beyond anthesis.

The flowers small; hermaphrodite and unisexual (andromonoecious or dioecious?); not pentamerous throughout; departing from pentamery in the androecium, or in the corolla and in the androecium; coloured. Floral tube length relative to total hypanthium + calyx length 0.25. Hypanthium present; very short, saucer-shaped, or cupular. Calyx 5; not covering the rest of the flower in bud; polysepalous. Corolla present; regular; 5–6; without greatly reduced members; polypetalous. Petals semi-orbicular, imbricate; imbricate-ascending; yellow. Disk absent. The androecium comprising 12 members; members all more or less equal in length; including staminodia (the pistillate flowers), or comprising only fertile stamens (? - male flowers). Fertile stamens in male flowers 12. Anthers attached at the base of the connective (but sagittate below). Stigma dilated. Ovules numerous.

Fruit, seed and seedling. Fruit fleshy indehiscent; not internally septate; not becoming woody. Seeds endospermic; with a straight or slightly oblique radicle.

Transverse section of lamina. Leaves without conspicuous phloem transfer cells in the minor veins. Druses absent from the mesophyll. Mesophyll secretory cavities absent. Adaxial hypodermis absent. Leaf girders absent. Laminae dorsiventral. Mesophyll without unaligned fibres or sclereids. Minor veins mainly with abundant accompanying fibres.

Adaxial: Adaxial interveinal epidermal cell walls thick. Abaxial: Abaxial stomata not predominantly paracytic (actinocytic). Abaxial interveinal epidermal cell walls of medium thickness.

Pollen ultrastructure. Tectum reticulate; strongly irregularly coarse-reticulate. Length of colpi inapplicable — grain porate, no margocolpus. Foot layer of pollen wall smooth.

Species number and distribution. 1 species (A. gonavensis). Santo Domingo.

Tribe. Caesalpinieae.

Miscellaneous. Illustrations: • Arcoa gonariensis, 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: 4th August 2019. delta-intkey.com’.

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