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

Couvreur, Thomas L. P., Dagallier, Leo-Paul M. J., Crozier, Francoise, Ghogue, Jean-Paul, Hoekstra, Paul H., Kamdem, Narcisse G., Johnson, David M., Murray, Nancy A. & Sonke, Bonaventure, 2022, Flora of Cameroon - Annonaceae Vol 45, PhytoKeys 207, pp. 1-532 : 129-130

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).