Isoetes hallei 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 : 330

Isoetes hallei Eb. Fisch. & Lobin , sp. nov. – Fig. 8–9, 14A.

Holotype: D. R. Congo, Bas-Congo, Zongo, Kasangulu, chutes, 8 Apr 1971, Hallé 1922 ( BR0000015460486 ).

Diagnosis — The new species resembles Isoetes alstonii and I. schweinfurthii but differs from both in the stouter habit and much shorter leaves (20–25 mm vs 150–400[–600] mm). The megaspores are slightly larger (c. 560 × 480 µm vs 400–520 × 480–520 µm in I. alstonii and 360–520 × 360–520 µm in I. schweinfurthii ). The proximal face is densely pustulate with larger and smaller pustules, sometimes anastomosing, whereas in I. alstonii the proximal face is densely tuberculate-pustulate, with larger and smaller projections, never anastomosing, and in I. schweinfurthii the proximal face is laevigate or bears 3 tubercles at most.

Description — Corm 3-lobed, 5–13 mm wide, bud scales absent. Leaves 12–38, 20–25 mm long, c. 3 mm wide at base, 0.9–1 mm wide at apex, tapering gradually to a slender point. Ligule not seen. Velum rudimentary to absent. Megasporangia c. 5 × 2.5 mm, ovate, sporangial wall pale. Megaspores c. 560 × 480 µm; proximal face trilete, densely pustulate, with densely large and small pustules, sometimes anastomosing, 48–60 µm in diam., wax-like structures sometimes rodlet-like and filiform threads on pustules and dense threads between pustules, larger pustules partly anastomosing, ridges also partly pustulate, up to 60 µm wide and 60 µm high; distal face densely pustulate with large and smaller intermixed pustules, large pustules 45–57 µm in diam., partly anastomosing, small pustules 30–38 µm in diam., pustules almost completely covered with dense threads, sometimes rodlet-like. Microspores not seen.

Distribution — Known only from the type locality in western D. R. Congo (Fig. 14 A).

Habitat — On wet rocks in waterfalls, probably perennial, c. 320 m.

IUCN conservation assessment (preliminary) — Critically Endangered: CR B2 ab(iii). Isoetes hallei is known only from the type locality. The last two collections were made c. 50 years ago. The estimated AOO is 4 km ² (assuming a 4 km ² grid cell size). The habitat, a waterfall, is actually not protected, but there seem to be no immediate threats .

Etymology — Named after Francis Hallé (born 1938), who first collected the new species.

Additional specimens studied — D. R. CONGO: BAS-CONGO: chutes de Zongo, dans une pente rocheuse mouillée d’embruns, 21 Feb 1971 , F. Hallé 1900 ( P01590086 ) .