Tree 2-3 m tall, DBH 3-5 cm. Twigs slender, longitudinally striate, densely covered with erect hairs 1.0-1.3 mm long, soon glabrate, brown. Leaf blades 7-14 cm long, 3-6 cm wide, length:width ratio 1.9-2.6, obovate or rarely elliptic, obliquely broadly cuneate to subcordate at base, acuminate to cuspidate at the apex, the acumen 5-22 mm long; midrib slightly impressed and glabrous above, raised and sparsely covered with appressed hairs to glabrous below; secondary veins 8-10 per side, diverging at 45-60° from midrib, slightly impressed to slightly raised above, slightly raised below, higher-order veins irregular, plane to slightly raised above, slightly raised below; petiole 1-2.5 mm long, 0.8-1 mm in diameter, covered with short erect hairs to glabrous, leaf lamina inserted on top. Inflorescences axillary, sometimes cauliflorous, a 1-flowered rhipidium. Buds not enclosed in sepals, ovoid to conic, densely covered with short appressed hairs, white to white-cream. Sympodial rachis to 7 mm long, densely covered with short appressed hairs, brown, with 0-6 minute lower bracts which are densely covered with short appressed hairs and brown. Flowering pedicels 10-22 mm long, densely covered with short appressed hairs, green, upper bract inserted at 1/3 from the base, minute, appressed against pedicel, covered with short appressed hairs, green. Flowers bisexual, with two whorls of 3 free petals. Sepals 3, free, slightly imbricate at base, 4-3 mm long, 4-5 mm wide, length:width ratio 0.75-0.8, broadly ovate, base subcordate, apex acute, densely covered with short appressed hairs outside, glabrous inside, appressed against petals, persistent in fruit. Petals white to white-cream in vivo, drying dark black; outer petals 12-18 mm long, 10-7.5 mm wide, length:width ratio 1.8-2.1, elliptic, base truncate, apex obtuse; sericeous outside, glabrous inside except for fine puberulence towards apex and along margins, flat, spreading horizontally at anthesis; inner petals 13-18 mm long, 5-6 mm wide, length:width ratio 2.6-3, narrowly oblong, base truncate, apex acute, glabrous on both sides, the margins with short appressed hairs, concave and erect at anthesis. Stamens in 6-8 whorls, 2-2.7 mm long, filament < 0.2 mm long, narrowed, apical prolongation of connective flat, c. 2 mm long, glabrous, white cream. Torus c. 1.8-2.2 mm long, 2-2.8 mm in diameter at apex, short-cylindric. Carpels 4(3?), 2-3 mm long, c. 1 mm wide, oblong, densely covered with appressed hairs; ovules uniseriate, 5-8 in number, placentation lateral; stigmas bilobed, glabrous, bright yellow to white cream. Fruiting pedicels c. 10-15 mm long, covered with short appressed hairs. Monocarps 18-25 mm long, 10-15 mm in diameter, ellipsoid, sessile, densely covered with short dark brown erect hairs in combination with additional sparsely spaced longer light brown hairs. Seeds c. 2 per monocarp, c. 11 mm long, c. 10 mm wide, 4-5 mm thick, spherical and flattened; testa rugose, brown; raphe depressed; hilum c. 4 mm long, 2 mm wide, elliptic.

VU D2. Mwasumbia has been collected seven times, always from the same locality in the Kimboza Forest Reserve. During the 1980s the reserve was severely logged, depriving the forest of its tall canopy trees, even though human activities had been prohibited since 1982 (Madoffe et al. 2006). There appears to be no decline in collections during the past half century. Although no precise population studies have been conducted, the first author encountered it a few times near the road. In addition, it has been shown that even though there has been a significant loss of forest in lowland and submontane areas in the Ulugurus, most endemic plant species still survive, having been collected again recently (Burgess et al. 2002). However, there is no doubt that the distribution area of Mwasumbia is very restricted in size being concentrated in less that 20 km2, and we therefore apply the category of “Vulnerable” under criterion D2. This is regarded as a special criterion. Because this species is restricted to a small area and occurs in just one locality it is “prone to the effects of human activities or stochastic events within a very short time period in an uncertain future, and is thus capable of becoming Critically Endangered or even Extinct in a very short time period” (IUCN 2001).

The species occurs in a single known locality namely
the Kimboza Forest Reserve located on both sides of the Ruvu
River in the eastern foothills of the Uluguru Mountains in the Morogoro Region, Tanzania. It
lies at c. 7°00'S and 37°48'E with an altitudinal range of 200--500 m above sea level and covers a total area of 386
ha (c. 4 km²).

The species occurs in a single known locality namely the Kimboza Forest Reserve located on both sides of the Ruvu River in the eastern foothills of the Uluguru Mountains in the Morogoro Region. It lies at c. 7°00'S and 37°48'E with an altitudinal range of 200--500 m above sea level and covers a total area of 386 ha (c. 4 km2). The soil is characterized by Tropical Rendzina on Precambrian dolomitic marble base rocks, which creates a distinctive tropical karstic landscape with large isolated blocks and pinnacles of marble scattered throughout the forest (Rodgers et al. 1983). The mean annual rainfall is approximately 1700 mm with the main rainy season extending from November to April. Only 3 months (June-August) present less than 50 mm of rain. Humidity is high all year round. The forest has two main vegetation types: swamp forest dominated by dense stands of Pandanus engleri and lowland rain forest with a closed canopy at c. 20 m and a few emergents at 40 m (e.g. Sterculia appendiculata) (Rodgers et al. 1983). Mwasumbia alba occurs frequently in the latter habitat, where collectors have noted Breonadia salicina (Rubiaceae), Ricinodendron heudelotii (Euphorbiaceae), and Zenkerella sp. (Fabaceae) as other dominants. The lowland rain forests adjacent to the Uluguru Mountains (which includes the Kimboza forest) have been identified as one of the sub-centers of endemism within the coastal forests (Burgess et al. 1998). The Kimboza forest now counts three endemic genera and c. 17 endemic species of angiosperms (Burgess et al. 1998; Clarke et al. 2000). The restricted distribution of Mwasumbia is a common pattern in the coastal East African belt vegetation, with about 40% of all coastal forest plant endemics known from a single forest (Clarke et al. 2000). Within Annonaceae, another example can be found in the monotypic genus Sanrafaelia, restricted to the lowland rain forests adjacent to the East Usambara Mountains (Verdcourt 1996; Couvreur et al. 2006). Compared to other families found in the East African rain forests, Annonaceae appear particularly rich in endemic genera, now totaling six (c. 21% of total number of endemic genera, Clarke et al. 2000). Other highly diverse families such as Rubiaceae have, for example, just four endemic genera (c. 14% of total number of endemic genera, Clarke et al. 2000).