Isoetes rwandensis Eb. Fisch. & Lobin, 2022

Fischer, Eberhard & Lobin, Wolfram, 2022, The genus Isoetes (Isoetaceae) in Central Africa (Democratic Republic of the Congo, Rwanda, Burundi) with the description of three new species, Willdenowia 52 (3), pp. 315-334 : 332

Isoetes rwandensis Eb. Fisch. & Lobin , sp. nov. – Fig. 12–13, 14B.

Holotype: Rwanda, Eastern Province , Bugesera, Nyamata, 02°07'00.91''S, 30°04'07.67''E, 1415 m, seasonal shallow ponds on ferricretes, 9 Jan 2012, Fischer 66/2012 ( BR0000015253576 V).

Diagnosis — The new species differs from Isoetes abyssinica and I. welwitschii in the shorter leaves (55–62 mm vs 114–185 mm in I. abyssinica and 50–200 mm in I. welwitschii ). It differs in the colour of the megasporangium wall (dark brownish to blackish vs pale in I. abyssinica and I. welwitschii ). The megaspores are smaller (c. 400 × 400 µm vs c. 480 × 460–480 µm in I. abyssinica ). The distal face is densely pustulate-tuberculate with larger, rounded pustules intermixed with smaller tubercles, sometimes anastomosing, whereas in I. abyssinica the distal face is clavate with distant projections and small tubercles in between, and I. welwitschii has a distal face with large pustules, partly anastomosing and with small tubercles intermixed.

Description — Corm 3-lobed, up to 5 mm wide, bud scales 2–2.5 × c. 1.5 mm, cuspidate, pale green. Leaves 15–21, 55–62 mm long, 1.2–1.6 mm wide at base, 0.4– 0.6 mm wide at apex, tapering gradually to an acute apex. Ligule not seen. Velum absent. Megasporangia 2.2–2.6 × 1.2–1.4 mm, sporangium wall dark brownish to blackish. Megaspores c. 400 × 400 µm; proximal face trilete, evenly tuberculate with small and dense tubercles 28–30 µm in diam., ridges c. 50 µm wide and c. 33 µm high, areoles and ridges with dense threads; distal face densely pustulate-tuberculate with larger, rounded pustules 39.9–60 µm in diam. intermixed with smaller tubercles, sometimes anastomosing, up to 26 µm in diam., larger pustules sometimes anastomosing, dense threads between pustules, only apex of pustules ± free of waxes. Microspores not seen.

Distribution — Known only from the type locality and one additional locality in Eastern Province of Rwanda (Fig. 14 B).

Habitat — In small, temporary pools on ferricretes ( Fig. 12A) along with Callitriche oreophila Schotsman , Cyperus species, Lindernia parviflora (Roxb.) Haines , Marsilea minuta L. and Rotala tenella (Guill. & Perr.) Hiern (see Fischer & Hinkel 1992).

IUCN conservation assessment (preliminary) — Critically Endangered: CR B2 ab(iii). Isoetes rwandensis is now known only from one locality. The type locality near Nyamata was destroyed during road work in 2021 by covering the ferricrete with more than 1 m of soil. The estimated AOO is therefore 4 km ² (assuming a 4 km ² grid cell size). At the second location, there is a potential threat of house construction and road work .

Etymology — Named after the country of Rwanda, where the new species is probably endemic.

Additional specimens studied — RWANDA: EASTERN PROVINCE: Mpanga , entrance of Ranch Mpanga between Lac Cyambwe and Lac Mpanga, 02°04'56.65''S, 30°46'39.96''E, 1310 m, seasonal shallow ponds on ferricretes, 30 Mar 1987, Fischer 110 ( KOBL) .