20. Begonia willemii Ardi, Girm. & D.C.Thomas [§ Petermannia], Taiwania 66(3): 374 (2021).

– Type: Indonesia, Sulawesi, Central Sulawesi, Luwuk Banggai District, Hanga-hanga waterfall, 4 vi 2016, D. Girmansyah 2470 (holotype BO; isotypes CEB, E) .

Distribution. Indonesia: endemic to Sulawesi, Gorontalo and Central Sulawesi Provinces (see Figure 2).

Habitat. Lowland forest on limestone karst, on limestone rock walls or terrestrially at the base of limestone cliffs, in partial to dense shade, at 10–50 m elevation.

Proposed IUCN conservation category. Near Threatened (NT) (Ardi et al., 2021).

Additional specimens examined. INDONESIA. Sulawesi. Northern arm of Sulawesi. Western North Sulawesi: Toli-toli regency, 16 i 2019, Zulfadli ZF 42 (BO, SING) . Central North Sulawesi: Gorontalo, Gunung Boliohutu, 22 iv 2002, M. Mendum et al. 155 (BO, E) ; 14 iii 2008, K. Amstrong 364 (E).

Central Sulawesi Province: Buol, 15 viii 1894, Sarasin 683 (K) ; 15 viii 1894, Sarasin 689 (B); Luwuk Banggai regency: Hanga-hanga Waterfall, 8 ii 2019, W.H. Ardi et al. WI413 (BO, KRB, SING) ; Batu tikar waterfall, 9 ii 2019, W.H. Ardi et al. WI415 (BO, KRB, SING) ; 9 ii 2019, W.H. Ardi et al. WI416 (BO, KRB, SING); Salodik, 11 ii 2019, W.H. Ardi et al. WI422 (BO, SING) ; Bunta, 14 ii 2019, W.H. Ardi et al. WI428 (BO, KRB, SING) .