Mimosa rastrera Atahuachi & C.E. Hughes, 2016

Atahuachi, Margoth, Bent, M. Leontien Van Der, Wood, John R. I., Lewis, Gwilym P. & Hughes, Colin E., 2016, Bolivian Mimosa (Leguminosae, Mimosoideae): three new species and a species checklist, Phytotaxa 260 (3), pp. 201-222 : 209-211

Mimosa rastrera Atahuachi & C.E. Hughes , sp. nov. ( Figs. 4 & 5)

M. rastrera is closely allied to ser. Neptunioideae and ser. Auriculatae, but can be clearly distinguished from the four species of ser. Neptunioideae by lack of interpinnal spicules and presence of a pair of suborbicular to ovate paraphyllidia resembling diminutive leaflets, and from M. auriculata , the only member of ser. Auriculatae, by its prostrate as opposed to erect woody shrub habit, multijugate as opposed to 1-jugate leaves, and much smaller leaflets.

Type:— BOLIVIA. Santa Cruz: Velasco, Parque Nacional Noel Kempff Mercado, on trail from Camp. Huanchaca 2 to ‘ la Piscina’, meseta de Caparuch, midway beyond stream, 729 m, 14°30´36´´ S, 60°44´58´´ W, 14 May 2010, (fl, fr), J. R. I. Wood et al. 26736 (holotype K!; isotypes LPB, UB, USZ!) .

Unarmed procumbent or diffuse, almost prostrate, functionally herbaceous subshrub, arising from an extensive xylopodium, branching from the base, the slender pliant stems trailing 40–60 cm in length, shoots angled, tinged reddish maroon, the whole plant more or less glabrous apart from a few very scattered short white hairs, capitula 1 (–2) at nodes on short slender un-branched efoliate panicles weakly exserted 5–7 cm beyond the foliage, or in axils of leaves. Stipules persistent, weakly foliaceous, deltate triangular, cordate at base, 3.5 × 1.8–2 mm, glabrous, striately venulose, with 12–15 longitudinal, prominent and somewhat anastomosing nerves on dorsal side. Leaves bipinnate, the whole leaf up to 6 cm long, leaf stalks including petiole to 4 cm long, deeply grooved and ending in a short pointed mucro to 1.5 mm, the petiole 1.3 cm long, with 3–4 pairs of pinnae, the interpinnal segments 8–9 mm; pinnae with 7–10 pairs of leaflets and a pair of suborbicular to ovate paraphyllidia inserted immediately above the pulvinules, these like diminutive leaflets, to 0.5 mm long, the pinna rachis 2–2.5 cm, the interfoliar segments 1.2–1.8 mm; larger leaflets 5.5 × 1.3 mm, linear, rounded at apex, asymmetric at base, glabrous throughout, venation prominent on lower leaflet surfaces, 3–4 primary veins from base, the midrib almost central, 5–6 (–8) pairs of almost perpendicular secondary veins. Flowers clustered in globose capitula to 5 mm diameter prior to anthesis, these on slender, glabrous, striate 1.2 cm long erect peduncles, the flowers sessile and subtended by linear-lanceolate, 2.8 × 0.5 mm glabrous bracts, striate with 6–7 longitudinal ribs, the bracts exserted and the capitula cone-like in bud; calyx campanulate to 1 mm including shallowly truncate blunt lobes; corolla 4-merous, the petals 1-nerved and 2.5 × 1 mm, the corolla lobes to 1.2 mm; 8- androus, diplostemonous, filaments pale whitish to striking bright pink; ovary linear-oblong, densely hairy to 1.2 mm. Fruits (few seen) one per capitulum, short stipitate, linear-oblong, apex long-acuminate, base acute, strongly planocompressed, markedly constricted between seeds such that almost lomentiform, 20–25 × 4.5–5 mm, the pods breaking up into one-seeded articles to leave a persistent replum, the replum prominent, 0.4 mm wide, whole pod glabrous, the valves pale yellow-brown, chartaceous, colliculate over seeds; seeds 2–3 per pod.

Distribution and Ecology: — Mimosa rastrera is known only from a handful of localities in the Serranía Huanchaca in the Noel Kempff National Park and the adjacent Serra de Ricardo Franco in Brazil. It occurs locally in sandy places in the very extensive open dry stony campo limpo that dominates this vast meseta, interspersed with rock outcrops on ridges and streams lined with adjacent campo húmedo and sometimes gallery forest.

Additional specimens examined: — BOLIVIA. Santa Cruz: Velasco, Parque Nacional Noel Kempff Merc ado, Las Gamas, sobre rocas areniscas, 900 m, 14° 49´S, 60° 23´W, 27 March 1993, (fl buds), T.J. Killeen et al. 4805 (MO, USZ). BRAZIL. Mato Grosso, Vila Bela da Santissima Trinidade, Fazenda Pelicano, caminho para a serra de Ricardo Franco, próxima ao rio Paraíso, 560 m, 14° 51´S, 60° 14´W, 20 March 2014, M.F. Simon et al. 2128 (UB).

Etymology: —The species epithet derives from the Spanish ‘ rastrera ’ meaning creeping or low in reference to the prostrate habit of this plant.

Phenology: —Flowering in April and May.

Conservation Status: — Mimosa rastrera is currently known from three localities all of them on the Serrranía de Huanchaca in the Noel Kempff National Park in Bolivia and the adjacent Serra de Ricardo Franco in Brazil, and must therefore be considered globally rare pending further exploration and collections. However, the Noel Kempff National Park remains largely inaccessible and under-collected botanically and this species is probably more widespread on the Meseta de Caparuch, at least in the southern part. It may well be inconspicuous except amongst new growth following burning.

Notes: —Ovate to suborbicular paraphyllidia that simulate diminutive l eaflets, as found in M. rastrera , are restricted within sect. Batocaulon to ser. Stipellares and the monotypic ser. Auriculatae ( Barneby 1991). Previously these two series were thought to be closely allied ( Bentham 1875; Barneby 1991), but recent molecular phylogenetic analysis of Mimosa including representatives of both series shows that this is not so ( Simon et al. 2011). This tell-tale character of dilated paraphyllidia along with lack of interpinnal spicules together suggest placement of M. rastrera in ser. Stipellares. An alternative would be placement of M. rastrera in ser. Auriculatae, or ser. Neptunioideae, which together form a group (Clade M in Simon et al. 2011) characterized by striately 12–50-nerved stipules, which are also found in M. rastrera , but which are generally lacking in ser. Stipellares. Phylogenetic analysis of a trnD-trnT sequence from M. rastrera alongside data from Simon et al. (2011) provides robust support (PP=1.0) for placement of M. rastrera within the striate-stipuled clade that includes ser. Neptunioideae and Auriculatae ( Fig. 1). Based on this analysis, and pending a new infrageneric classification, we assign M. rastrera to ser. Neptunioideae of sect. Habbasia . Placement here implies that dilated paraphyllidia are independently derived in ser. Stipellares and within clade M in M. rastrera and M. auriculata Bentham (1875: 430) , both of which also lack interpinnal spicules which are a feature of ser. Neptunioideae. Independent derivations of these traits perhaps come as no surprise given the high levels of morphological homoplasy in Mimosa more generally ( Simon et al. 2011).

Within Clade M of Simon et al. (2011), M. rastrera is readily distinguished from all four members of ser. Neptunioideae by lack of interpinnal spicules and presence of a pair of suborbicular to ovate paraphyllidia resembling diminutive leaflets, and from M. auriculata , the only member of ser. Auriculatae, which is an erect woody shrub and has a radically different leaf formula with a single pair of pinnae and much larger leaflets than M. rastrera .