Isoetes dubsii is distinguished from other species from the Brazilian Pantanal wetlands by a set of characters that include leaves that are long (90‒100 cm), flexuous and trigonal in the transverse section, tri-lobated stems, rudimentary velum, pustulate and small megaspores of 310‒390 µm in diameter (average 350 µm) and laesurae of the megaspore at least four times wider than high.

BRAZIL. Mato Grosso do Sul: Fazenda Salina, Pantanal do Rio Negro, 19°30'S, 56°10'W, 3 Jun 1988, Dubs 829 (holotype: Z!).

Figure 1. 

Type of Isoetes dubsii (image courtesy of the herbarium Z/ZT). Note that the name Isoetes flexuosa J.B.S. Pereira on the label sheet is a provisional, never published, name. Photographer: Franziska Schmid

Figure 2. 

SEM images of the mega- and microspores of Isoetes dubsii (Dubs 829, Z/ZT), I. santacruzensis (Killeen et al. 7131, MO), I. pedersenii (Troels 8105, L) and I. panamensis (Balansa 3294, P).

Plants aquatic submerged, growing in fresh water ponds. Roots conspicuous, dichotomous. Stems globose, 3–lobate, ca. 2 cm wide. Leaves 90‒100 cm long, 0.2–0.3 cm wide at mid length, 50–60 per individual, linear, flexuous, erect, apex attenuate; alae 12–13 cm long, extending from the base ca. 1/10 of total leaf length, red-brown, membranaceous, apex attenuate; subula olive-green, trigonal. Labium present, persistent, 1.5–2.0 × 2.0–2.5 mm, cordate. Ligula not observed in herbarium material. Velum rudimentary. Sclerified phyllopodia absent. Sporangium at the base of the leaf, 8–12 × 3–4 mm, oblong, light brown, concolorous. Megaspores 310–390 µm in diameter (average = 350 µm, n = 20), trilete, white, not lustrous; proximal and distal surfaces pustulate, macrosculpture 3–8 × 22–45 µm, wider than high; laesurae 8–10 × 45–55 µm, at least four times wider than high. Microspores 31–36 µm long (average = 34 µm, n = 20), light brown, monolete, proximal surface smooth, distal surface sparsely echinate.

This species is only known from its type locality (state of Mato Grosso do Sul, Brazil; Fig. 3), where it grows in a fresh water pond at ca. 100 m a.s.l. Although we tried to re-collect Isoetes dubsii in the area indicated by Balthasar Dubs, no additional collections have been made.

Figure 3. 

Map showing the type locations of Isoetes dubsii and I. santacruzensis.

The specific epithet honours the Swiss ornithologist and botanist B. Dubs, for his valuable contributions to the knowledge of the flora and fauna of the Pantanal wetlands of Brazil. He was also the first naturalist to collect Isoetes dubsii.

Amongst the species found in Brazilian Pantanal wetlands, Isoetes panamensis Maxon C.V. Morton sensu lato is similar to I. dubsii due to its long leaves up to 80 cm. However, the megaspores in I. panamensis are 380–600 µm (vs. 310–390 µm) in diameter and baculate (vs. pustulate) (Fig. 2).

Since I. dubsii is currently known from a single locality, it may deserve special attention concerning its conservation status. However, based on our current knowledge on this species and according to IUCN Red List criteria (IUCN 2012), it is assessed here as data deficient (DD).

Isoetes santacruzensis is characterised by having flexuous, filiform leaves ranging from 0.4–0.8 mm wide at mid length and reaching up to 15 cm long, 15‒30 leaves per individual, black or reddish-black sporangia, sclerified phyllopodia present, sparsely verrucate megaspores of 320‒390 µm (average of 350 µm) in diameter.

BOLIVIA. Santa Cruz: Nuflo de Chaves, 15°32'40"S, 61°59'28"W, 450 m a.s.l., 11 Nov 1994, Killeen et al. 7131 (holotype: MO!).

Plant aquatic partially submerged or ephemeral terrestrial in rocky granite outcrops. Roots conspicuous, dichotomous. Stems globose, 3–lobate, 0.8–1.2 cm wide. Leaves 6–15 cm long, 0.4–0.8 mm wide at mid length, 15–30 per individual, filiform, flexuous, laxly ascending, apex attenuate; alae 0.3–3.5 cm long, extending from the base ca. 1/5 of total leaf length, hyaline or light brown, membranaceous, apex attenuate; subula olive green, trigonal. Labium present, persistent, cordate, 0.2–0.5 × 0.6–0.9 mm. Ligule not observed in herbarium material. Velum rudimentary to > 0.2 mm wide along the lateral edges of the sporangium. Sclerified phyllopodia present. Sporangium at the base of the leaf 2.5–3.5 × 2.0–2.5 mm, oblong, black or reddish-black, concolorous. Megaspores 320‒390 µm in diameter (average = 350 µm, n = 20), trilete, white, not lustrous; proximal and distal surfaces sparsely verrucate, macrosculpture 10‒25 × 19‒31 µm, slightly wider than high; laesurae 26‒30 × 16‒21 µm, slightly higher than wide. Microspores 23‒27 µm long (average = 25 µm, n = 20), light brown, monolete, proximal surface smooth, distal surface sparsely echinate.

Isoetes santacruzensis is only known from its type locality, where it grows as aquatic to ephemeral terrestrial in rocky granite outcrops, at elevations of about 450 m.

The specific epithet refers to the type region, the Department of Santa Cruz in Bolivia (Fig. 3).

Until now, six species of Isoetes were known from Bolivia, although the presence of unpublished species has already been mentioned (Kessler and Smith 2018). Most of the known Bolivian species are from Andean habitats (Kessler and Smith 2018) and have rugulate, laevigate (Fig. 5) or tuberculate megaspores (see Hickey 1986b, Fig. 2) and laevigate, echinate or tuberculate microspores (Fig. 5). Besides habitat, the macrosculpture of at least one of the spore types, megaspore or microspore, helps to differentiate I. santacruzensis from the Andean Isoetes species (Figs 2, 4). Additionally, I. santacruzensis is similar to I. pedersenii by its small and verrucate megaspores. However, I. santacruzensis can be readily distinguished by its erect and flexuous leaves (vs. ascending, linear and straight; Fig. 6), as well as by the characters present in the taxonomic key.

Figure 4. 

Type of Isoetes santacruzensis (image courtesy of the herbarium MO). Photographer: Mike Blomberg.

Figure 5. 

SEM images of the mega- and microspores of Isoetes andicola (Fuentes 13907, MO), I. boliviensis (Casas 6619, MO), I. herzogii (Ritter 2209, MO) and I. lechleri (Solomon 15517, MO).

Isoetes santacruzensis is currently known from a single locality. The expansion of agricultural activities and cattle farming in this area show that this species may be prone to the effects of human activities within a very short time. However, given its potential occurrence in other areas and the lack of current knowledge about its distribution range, I. santacruzensis should be assessed as data deficient (DD), according to IUCN criteria (IUCN 2012).

Figure 6. 

Type of Isoetes pedersenii (image courtesy of the herbarium L). Photographer: Christel Schollaardt.