Isolona linearis
Tree to 15 m tall; d.b.h. to 60 cm; bark dark brown to black, smooth; young branches drying black, glabrous, smooth; old branches dark brown to grey, glabrous, striate. Petioles 4-5 mm long, 1.0-1.5 mm in diameter, glabrous, leaf lamina inserted on side, narrowly grooved adaxially. Leaf lamina 14-18(-25) cm long, 5-7(-9) cm wide, length:width ratio 2.3-3.3, narrowly oblong to oblong or narrowly elliptic to elliptic, coriaceous, glabrous, base rounded to narrowly cuneate, apex acuminate, acumen 5-17 mm long; midrib glabrous on both sides; secondary veins 11-13 pairs, uniformly curved upwards, glabrous on both sides. Rhipidia 1-2, on leafy branches. Flowering pedicels (3-)5-10 mm long, 1-1.5 mm in diameter, glabrous. Lower bracts 2-6, ca. 0.5 mm long, ca. 0.5 mm wide, glabrous, margins sparsely covered with short erect hairs. Upper bract inserted basally or subbasally on the pedicel, 1-2 mm long, ca. 1 mm wide, glabrous; margins sparsely covered with short erect hairs. Sepals 2-3 mm long, ca. 3 mm wide, length:width ratio 0.7-1, transversely elliptic to broadly elliptic, base truncate, apex acuminate, green, persistent in fruit; margins sparsely covered with short erect hairs. Corolla green when immature, red at anthesis; tube 4-5 mm long, lobe:tube ratio 2.5-7.5, glabrous, verrucose when dried; lobes 10-30 mm long, 3-4(-8) mm wide, length:width ratio 2.5-7, linear to narrowly elliptic or rarely elliptic, narrowed at base, apex rounded, sparsely covered with short appressed hairs when very young, glabrous at anthesis, papyraceous. Receptacle 3-5 mm in diameter. Stamens ca. 1.1 mm long; connective shield ca. 0.1 mm long, glabrous, those of innermost whorl slightly extended over ovary wall. Ovary ca. 2 mm, ca. 1 mm wide; stigma ca. 2.5 mm in diameter, glabrous. Fruiting pedicels 5-15 mm long, ca. 3 mm in diameter, glabrous. Fruits ca. 5 cm long, 2-3 cm in diameter, length:width ratio 1.5-2.5, irregularly elliptic, apex apiculate, apicule ca. 1 cm long, furrowed, green tinged with white; pericarp 2-3 mm thick. Seeds 10-15 mm long, ca. 5 mm in diameter, transversely ellipsoid; testa smooth, light brown; raphe thickened, dark brown; hilum not seen.
Isolona linearis would appear closely related to I. heinsenii, as indicated by the molecular phylogeny (Fig. 1), by the shape and disposition of the corolla lobes (curving uniformly over the receptacle). Moreover, both species are restricted to the montane rain forests of the Eastern Arc. However, I. linearis is always completely glabrous, and the sepals are generally smaller than those of I. heinsenii.
The shape and size of the corolla lobes in I. linearis vary, with the predominant shape being linear. One other striking feature is that some flowers of various specimens of I. linearis have five corolla lobes instead of six, which could indicate a tendency towards pentamery (Luke 7732 (associated photograph, EA), Frimodt-Møller TZ59 (C), and Frimodt-Møller TZ5 (C) all indicates “5-merous flowers”). This tendency is also observed in I. campanulata Engl. & Diels (Adam 1971).
EN B2ab(iv). Isolona linearis is moderately represented in herbaria, but has numerous recent collections. Most of these originate from protected areas: one natural reserve (Amani), one national park (Udzungwas) and two forest reserves (Lulanda and Luhega). However, the area of occupancy is quite small (ca. 100 km2) and there are only three main locations rendering it vulnerable to extinction. The “endangered” category is therefore recommended.
Tanzania (Eastern Arc Mountains); in moist montane forest; at 1100-1700 m altitude.