Isoetes eludens J.P.Roux, Hopper & Rhian J.Sm.

Roux, J.P., Hopper, S.D. & Smith, R.J. 2009. Isoetes eludens (Isoetaceae), a new endemic species from the Kamiesberg, Northern Cape, South Africa. Kew Bulletin 64: 123. DOI: https://doi.org/10.1007/s12225-008-9092-0

Type

South Africa, Northern Province, S. D. Hopper & R. J. Smith RJS114 (holotypus K; isotypus NBG).

Morphology General Habit

Plants aquatic

Morphology Stem

Stem strongly 3-lobed, the lobes to 6 mm long, the central part to 7 mm in diam., to 4 mm tall, to 5 mm below the soil surface, scales or phyllopodia absent, with a large number of simple roots borne along the lobes and stem grooves, roots long and simple, with few inconspicuous, widely spaced, pluricellular, uniseriate root-hairs

Morphology Reproductive morphology Sporophylls

Sporophylls up to 12 per plant, crowded into a rosette, erect, subulate, to 52 mm long, transversely broadly ovate at the base and up to 10 mm wide at the point of attachment; vagina abruptly narrowing to 4 mm in diam. above the sporangium, to 18 mm long, the adaxial shallow V-shaped sulcus extends along the entire length of the vagina and proximal part of the subula, with a low hump above the ligule pit, rounded abaxially, alae dorso-lateral, generally <0.5 mm broad and hardly distinguishable; subula pale green, to 32 mm long, 3 mm in diam. for most of its length tapering gradually towards the apex, proximally shallowly sulcate, adaxially flattened to terete distally, with a narrow single central vascular strand, with four intermittently septate lacunae extending from above the sporangium to the apex; epidermal cells oblong, (64 –) 107 (– 180) µm long (n = 50; sd = 25.1), with nearly straight anticlinal walls; estomate; ligule not observed; labium obsolete; velum complete, but rarely forming a small window <0.5 mm in diam. at the sporophyll base near the point of attachment, cells oblong to isodiametric in outline, with gently curved anticlinal walls, unpigmented

Morphology Reproductive morphology Sporangia

Sporangium embedded into the sporophyll base; megasporangia borne on the outer sporophylls, microsporangia on the inner sporophylls, almost square in frontal view, but with a shallow notch apically, 3 – 3.5 mm long, to 3 mm wide, the proximal face margins rounded, reniform in planar view, to 2 mm deep, with several thick trabeculae, sporangium enveloped by a single cell-layered membrane or sack, sack cells oblong to isodiametric, cell walls not thickened, gently curved or shallowly sinuate anticlinal walls, sporangium wall pale brown when mature, the cells oblong to rectangular in outline, (96 –) 138 (– 164) (n = 21; sd = 16.9) × (18 –) 27 (– 48) µm (n = 21; sd = 7.3), the anticlinal walls almost straight, without secondary thickenings

Morphology Reproductive morphology Megaspores

Megaspores chalk-white when immature, drying graphite-black at maturity, tetrahederal globose, (258 –) 270 (– 280) µm in equatorial diameter, with a prominent equatorial flange and narrow, but tall laesura, the proximal faces laevigate, distal face laevigate or with very low, almost inconspicuous verrucae

Morphology Reproductive morphology Microspores

Microspores dark brown appearing black en masse, elliptic, to 80 × 18 µm, monolete, the proximal faces appearing laevigate, but minutely aculeate, the distal face papillate

Distribution

Africa: South Africa

Ecology

Vegetation at the type locality consists of woody scrub that has been classified as Kamiesberg Granite Fynbos (Rebeloet al. 2006). The plants of Isoeteseludens occur in a single undisturbed !gau in Kamiesberg gneiss at an altitude of 1,284 m. The !gau is approximately 2 m in diameter, forming a shallow pan-gnamma. It is situated on the spine of a ridge that does not allow drainage into the pool from higher terrain. Aeoline detritus forms a thin layer at the pool bottom, which also supports a single Limosella africana Glück (Scrophulariaceae) plant and a large population of a Nostoc (Cyanobacteria: Nostocaceae) species. The I. eludens population in the pool consists of approximately 250 to 300 plants and grows in clear water 100 – 150 mm deep.

Conservation

The current IUCN (2001) classification recommended is Vulnerable (VU D1 & 2). The plants, although known from a single locality only, are not threatened by farming activities as vegetation in the vicinity of the site largely consists of unpalatable woody Kamiesberg granite fynbos and lithophyticherbfields not suitable for sheep farming. The site is also far from any other human activities that may impact on the population. However, clearly, the species would be critically endangered if the sole !gau in which it is known was its only location. It would take just one eutrophication event through deposition of animal dung, for example, to alter hydrological conditions sufficiently to threaten this population. Urgent further surveys are needed to establish the geographical extent and conservation status of this remarkable local endemic.

Note

The specific epithet is derived from eludo = avoid, evade, frustrate or baffle, referring to the species eluding searches over several years for Isoetes on flat-topped inselbergs and granite mountains in Namaqualand by the junior authors and others. The location of Isoeteseludens in the Kamiesberg not only extends the known distribution of the genus in South Africa significantly, but it also underpins the Kamiesberg as a centre of endemism. I. toximontana Musselman & J. P. Roux was also recorded from a seepage area near the type locality of I. eludens. No scales (arrested sporophylls) or phyllopodia have been observed in Isoeteseludens. This is contrary to the observations of Duthie (1929) and Hickey (1986) who found these structures to be present in virtually all the species subject to periods of drought-associated dormancy. Scales may, however, be formed during seasons when rains are more sporadic and when the plants are subject to fluctuating wet and dry spells. The root-hair cells are generally very long and variously thickened at the point of attachment between two cells. The root hairs appear not to arise on idioblasts as in some Isoetes species. The dorsolateral alae are generally less than 0.5 mm wide at the base, but gradually taper higher up and mostly disappear 12 mm or less from the sporophyll base. In cross section the sporophylls show no peripheral fibrous bundles, a feature typical of some obligate submerged aquatic Isoetes species (Hickey 1986).