Type:
Based on Solanum incanum Ruiz & Pav., non L.
Habit:
Large spreading shrubs or small to moderate sized trees, 2–8 m high; trunks reaching 5–20 cm in diameter, glabrate, usually armed with large conical prickles 1–5 cm long; flowering stems glabrate and dark except on very young developing portions; prickles often absent on flowering stems, if present very sparse, 2–5 mm long, broad-based, laterally compressed.
Sympodial structure:
Sympodial units difoliate, geminate.
Leaves:
Leaves simple, those on flowering stems with blades 10–35 × (3–) 8–21 cm, ca. 2 times as long as wide, ovate to broadly ovate, more rarely lanceolate, chartaceous, strongly bicolorous, glabrate green adaxially, the stellae deciduous during leaf expansion, these sessile or very short stalked, 8–14 rayed, 0.5–0.7 mm in diameter, without recognizable midpoints, densely white pubescent abaxially, the stellae densely overlapping, persistent, short stalked, 10–16 rayed, 0.6–0.8 mm in diameter, lacking midpoints; major veins impressed adaxially, the midvein and the 5–6 principal lateral veins persistently tomentose or soon glabrate abaxially; blades usually unarmed; base asymmetrically obtuse or rounded; margin sinuate–repand to entire; apex acuminate; petioles of major leaves 3–8 cm, soon glabrate and dark like the stems, unarmed or occasionally with a few small prickles; juvenile leaves very large and more prickly than those on flowering stems, usually pinnatifid.
Inflorescences:
Inflorescences 3–13 cm in flower, larger in fruit, extra-axillary, usually unequally bifurcate at or near base or at times short pedunculate, usually with one or both branches further branched, the ultimate racemose branches 2–6.5 cm long, with 5–20 flowers,<strong> </strong>the axes with dense and velvety stellate pubescence much like that of leaf undersides, variously persistent or deciduous, unarmed; peduncle 0–1.6 cm; rachis 3.2–8 cm; pedicels 4–7 mm in flower, to 10 mm in fruit, stout, persistently stellate woolly, spaced ca. 1 mm apart, articulated at the base.
Flowers:
Flowers apparently all fertile although fewer fruits forming than flowers, 5-merous. Calyx 2–5 mm long, the tube 2–3 mm, 4–6 mm in diameter, broadly campanulate, the lobes low, 1–3 × 1.8–3 mm, broadly deltoid or rounded and apiculate, sometimes oblong in age, densely stellate woolly abaxially, glabrous adaxially; fruiting calyx not or only weakly accrescent. Corolla 1.5–2.5 cm in diameter, 12–16 mm long, stellate, white, pale violet or occasionally deeper purple, the central portion linear-lanceolate, densely and minutely white-stellate abaxially, with a small amount of glabrous interpetalar tissue in the sinus, the tube 3–6 mm, the lobes 6–13 × 3.5–6 mm, ovate lanceolate, acute at apices, adaxially with a single row of minute stellae along the central nerve, occasionally glabrous. Stamens free; filaments 2–2.9 mm, glabrous; anthers 4–5.5 × 1-1.6 mm, slender, tapering, yellow, the pores directed distally. Ovary glabrous; style 7–9 mm, filiform, slightly expanded distally, well exceeding the anthers, glabrous; stigma capitate.
Fruits:
Fruit a berry 0.8–1.5 cm in diameter, globose, dull yellow when ripe, eventually turning black, glabrous.
Seeds:
Seeds numerous, 1.8–2.3 × 1.7-2.1 mm, lenticular, broadly asymmetric ovate, yellow or yellow brown, the surface minutely pitted.
Solanum albidum is closely related to S. whalenii, an endemic species from Bolivia. Solanum whalenii differs mainly by being more robust in all aspects, and more restricted altitudinally and ecologically at 1800–3000 m and more confined to humid forests, whereas S. albidum is often found in more open, weedier, and drier habitats over a much greater altitudinal range.
The only other species of sect. Torva in the New World with such strongly bicolorous leaves are S. hayesii of Colombia to Nicaragua, and S. paniculatum of eastern Brazil; both differ in a number of characters. Solanum hayesii has much smaller flowers. Solanum paniculatum is a less robust plant in all respects and highly disjunct to eastern Brazil, northeastern Argentina and Paraguay.
Solanum albidum is sympatric with a number of other species of sect. Torva, but the only putative hybrid that has been noted was from near Montero, Prov. Santiesteban, Depto. Santa Cruz, at an altitude of 295 m, where a plant intermediate between S. albidum and the common S. acutilobum Dunal was found in an area where both species are present (Nee 33441). Solanum acutilobum is distributed in the plains from Santa Cruz city through northern Bolivia and sparingly into adjacent Brazil and Peru; it differs in many characters, significantly by the small bright orange berries.
At one time Nee thought that the variety Solanum paniculatum var. chulumani should be recognized and transferred to S. albidum, but although the difference in pubescence of the veins of the underside of the leaf is very visible and the distribution of these specimens is geographically quite distinct (southernmost Peru and Bolivia), these differences now seem too trivial to recognize taxonomically.
Field observations in Bolivia seem to indicate that this species is clonal, forming colonies from root sprouts. This character needs to be investigated in this and other species of Solanum sect. Torva.