Euphorbia fuentesii V.W. Steinm., 2013

Steinmann, Victor W., 2013, Three new species of Euphorbia subg. Chamaesyce (Euphorbiaceae) from Bolivia, Phytotaxa 114 (1), pp. 23-32 : 26-28

Euphorbia fuentesii V.W. Steinm. , sp. nov. ( Fig. 2 )

Similar to Euphorbia catamarcensis but differs by possessing seeds that are rugulose or with irregular transverse ridges.

Type:— BOLIVIA. Santa Cruz, Prov. Cordillera, Parque Nacional Kaa-Iya del Gran Chaco, Palmar de las Islas , 270 m, 19°25'56"S, 60°32'24"W, 11 February 1998 (fl, fr), A . Fuentes & G . Navarro 2243 (holotype NY!, isotype USZ) .

Annual or perhaps short-lived perennial herb from a slender to slightly thickened taproot. Stems erect to ascending, few arising from the base, dichotomously branched above, 10–15 cm tall, glabrous or with a sparse puberulent pubescence of trichomes 0.1 mm long or less restricted to the base of the plant and uniformly distributed around the circumference, terete, smooth, internodes 0.6–1.8 cm long. Leaves opposite; stipules interpetiolar, 0.4–1.0 mm long, united at the base and divided distally into 3–5 subulate-filiform segments, glabrous; petiole 0.2–0.7 mm long; blade ovate, narrowly ovate, oblong or narrowly oblong, 0.8–1.7 × 0.3–1.0 cm, membranaceous, margin entire with the exception of 3–5 inconspicuous teeth at the apex, apex rounded but with a fine apiculate tip, base inequilateral-oblique, one side hemicordate the other side rounded, usually only the mid-vein conspicuous but sometimes visible 2 or 4 faint secondary veins diverging from the base, glabrous. Cyathia solitary in the upper nodes of the stems; peduncle 0.8–1.2 mm long. Involucre obconical, 0.8–1.2 × 0.6–0.7 mm; glands 4 per involucre, transversely oblong, 0.1 × 0.2–0.3 mm, concave; appendages present, equal, represented by a semilunate, linear to shortly flabellate rim, 0.1–0.2 × 0.25–0.6 mm, entire to shallowly and irregularly lobed, whitish, lobes subulate, 0.2 mm, pilose-ciliate, sinus shallow, less than 1/4 the length of the involucre. Staminate flowers 8–10, bracteoles few, filiform, pilose. Ovary ovoid, 3-lobed, glabrous; styles 3, free to the base, bifid 1/4–1/2 their length, 0.4–0.6 mm long, glabrous, terete, not dilated at the apex. Capsule on an exserted gynophore 1.0– 1.3 mm long, broadly ovoid, strongly 3-lobed, 2.0–2.5 × 2.0– 2.4 mm, distally with a shallow depression between the carpels; columella 1.8–2.2 mm. Seeds narrowly ovoid, quadrangular in cross-section, 1.6–1.9 × 0.9–1.1 mm, base truncate, apex bluntly pointed, dorsal keel well developed, rugulose and with irregular transverse ridges that pass through the dorsal keel, tan, ecarunculate.

Distribution, habitat and phenology:— Euphorbia fuentesii is known from a single collection made at an elevation of 270 m in the Kaa-Iya del Gran Chaco National Park, Cordillera province, department of Santa Cruz. Due to the close proximity of this locality to the international border (less than 2 km), the species likely also occurs in adjacent northern Paraguay. It was encountered along roadsides, and the vegetation is described as "bosque chaqueño seco," an extensive area of tropical deciduous forest and thornscrub frequent in southeastern Bolivia, Paraguay, southwestern Brazil and northern Argentina. Associate genera include Browningia Britton & Rose (Cactaceae) and Aspidosperma Mart. & Zucc. (Apocynaceae) . Reproductive plants have been encountered in February, but flowering and fruiting broadly overlap and likely occur throughout the summer rainy season.

Etymology:—This species is dedicated to the biologist Alfredo F. Fuentes, prolific plant collector and researcher at the Herbario Nacional de Bolivia. He extensively studied the habitat of this species ( Fuentes & Navarro 2000) and together with Dr. Gonzalo Navarro made the only known collection in 1998.

Discussion:—The morphological characters exhibited by Euphorbia fuentesii agree with those of Euphorbia subg. Chamaesyce sect. Anisophyllum Roeper (in Duby 1828: 412), as circumscribed by Yang et al. (2012): sympodial growth; interpetiolar stipules; opposite, inequilateral leaves; involucres with four glands; and ecarunculate seeds. This section corresponds to Euphorbia subg. Chamaesyce sensu Wheeler (1943) and Johnston (1975) or the segregate genus Chamaesyce Gray (1821: 260) sensu Radcliffe-Smith (2001) and Webster (1994). The novelty is a rather ordinary member of the section and no single feature is outstanding. It is distinguished by the combination of the following traits: essentially glabrous herbage, leaves entire with the exception of a few inconspicuous teeth at the apex, solitary cyathia, involucre appendages present and equal, divided styles, and relatively large seeds. Of these characteristics, seed size is the most distinctive, and the seeds are bigger than in any other Bolivian representative of the section. Although its affinities are not certain, Euphorbia fuentesii resembles the Argentinian endemic E. catamarcensis ( Croizat 1943: 187) Subils (1977: 150) , with which it shares nearly all of its distinctive features, including the large size of the seeds. However, seed sculpturing of these two species is strikingly distinct, and whereas the seeds of Euphorbia fuentesii are rugulose or with irregular transverse ridges, those of E. catamarcensis possess well-defined transverse sulci separated by sharp, horizontal costae.

Figures

FIGURE 2. Euphorbia fuentesii. A) Habit; B) Detail of stipules; C) Flowering branchlet; D) Young cyathium with emerging pistillate flower; E) Mature cyathium after fruit dehiscense; F) Apical portion of the ovary with styles; G) Capsule; H) Seed, ventral (left) and dorsal (right) views. Based on Fuentes & Navarro 2243 (MO), and drawn by José Manuel Ramírez-Amezcua.