Isolona Engl., Nat. Pflanzenfam. Nachtr. I: 161, 1897

Type species.

Isolona madagascariensis (A.DC.) Engl. & Diels (a species from Madagascar).

Description.

Trees, 3-30 m tall, d.b.h. 5-60 cm; stilt roots or buttresses absent. Indumentum of simple hairs or glabrous. Leaves: petiole 1-15 mm long, 1-4 mm in diameter, blade 8.5-29 cm long, 3-15 cm wide, elliptic or obovate or oblong, apex acuminate, base decurrent to rounded or acute, concolorous; midrib raised on upper surface; secondary veins 7 to 20 pairs; tertiary venation reticulate. Inflorescences ramiflorous on old leafless or young foliate branches, axillary. Flowers bisexual with 9 perianth parts in 2 whorls, 1 to 3 per inflorescence; pedicel 1-25 mm long, 1-2 mm in diameter; in fruit 2-29 mm long, 2-5 mm in diameter; bracts 2 to 7, several basal and one upper, lower half of pedicel; sepals 3, valvate, free, 1-9 mm long, 2-5 mm wide, ovate or elliptic, apex acute or acuminate or rounded, base truncate; petals 6, basally fused, tube 3-11 mm long, inner and outer whorl not differentiated, equal; lobes 6-31 mm long, 2-12 mm wide; stamens numerous, in 3 to 4 rows, 1-2 mm long, broad; connective discoid; staminodes absent; carpels fused - syncarpous, forming a single visible gynoecium, 1-3 mm long, stigma bilobed, slightly capitate or capitate. Fruit syncarpous, forming a single visible fruit, 30-90 mm long, 15-50 mm in diameter, ovoid or ellipsoid, apex apiculate or rounded or cuspidate; seeds numerous not seriate, 8-25 mm long, 5-15 mm in diameter, ellipsoid or flattened ellipsoid; aril absent.

A genus of trees with 20 known species, 15 in Africa and 5 in Madagascar. In Cameroon nine species are known, none endemic.

Isolona, together with its sister genus Monodora, are unique in Annonaceae in having truly syncarpous flowers (fused carpels) and fruits. This translates into single fruits with unordered seeds, in contrast to other genera which have either uni- or biseriate placentation. Petals in Isolona are basally fused forming a clearly visible tube, with six equal lobes of equal length in a single whorl. In the vegetative state, Isolona and Monodora (together with Polyceratocarpus pellegrinii) are characterized by a raised midrib, in contrast to a sunken or flat midrib in all other genera found in Cameroon.

Taxonomy.

Couvreur (2009).