n = ploidy missing =12 voucher missing = (Moscone 1992)
Native to southeastern Brazil from the states of Minas Gerais to Rio Grande do Sul to the mouth of the Río de la Plata in Argentina and Uruguay, in Atlantic rainforest, Araucaria forests, gallery forests and open forest margins, from nearly sea level to above 500 m elevation; widely cultivated worldwide in both temperate and subtropical zones, often escaped and naturalized.
Solanum laxum is a member of the Dulcamaroid clade (sensu Weese & Bohs, 2007), with a vining habit, polymorphic leaves with twining petioles and the pedicels inserted into a small sleeve. In a molecular analysis based on chloroplast ndhF sequences (as S. jasminoides), it emerged as sister to S. calileguae (Bohs, 2005). However, relationships within the Dulcamaroid clade are poorly resolved and supported and should be clarified using additional sequence data.
Solanum laxum was long known in cultivation as S. jasminoides, but the former name has priority and is slowly becoming accepted in botanical (see Mentz & Oliveira, 2004) and horticultural circles. The original source of material cultivated in Great Britain was southern Brazil, and in England the plant overwinters well even in very cold winters (hardy to USDA zone 8), where it dies back from frost, but quickly resprouts. In cultivation vines of S. laxum grow very large; a vine growing up the south-facing wall of the Chelsea Physic Garden in London is over 10 cm in diameter at the base.
Solanum laxum is similar, and probably closely related, to the sympatric S. viscosissimum with which it shares rotate-stellate corollas, pubescent styles and purplish black berries. It differs from that species in its almost always simple leaves and in the tufts of trichomes in the vein axils of the leaf undersurfaces; S. viscosissimum usually has at least some deeply pinnatifid leaves and has long, glandular trichomes evenly spread over the leaf surfaces. Flowers of S. laxum are usually white, but can have a purplish tinge, especially when growing in strong light. Solanum laxum could also potentially be confused with S. flaccidum, also sympatric, which differs in its larger purple or violet flowers with anthers borne on unequal filaments and in its more evenly distributed pubescence on leaf surfaces and stems. A few collections (e.g., Stuckert 11592, G) have deeply pinnatifid leaves, but can be distinguished from other similar species such as S. seaforthianum (also common in cultivation) by the conspicuous tufts of trichomes in the vein axils on the leaf undersides. These piinatifid leaves are probably juvenile, and are very occasionally found on stems mixed with simple leaves (e.g., Osten 8265) as is common in other species of the Dulcamaroid clade.
Although the tufts of trichomes in the vein axils of the leaf undersides are a good diagnostic character for S. laxum, some populations and individuals appear to lack pubescence of any sort. These plants have been called var. calvum, and they differ from other populations only in pubescence density; there are usually a few trichomes, but they can be difficult to see without a microscope. This variation is common in the Dulcamaroid clade, and in many other non-spiny solanums.
Many of the synonyms of S. laxum, as well as the accepted name itself, were described from types destroyed in Berlin, and will need to be lectotypified or neotypified to stabilise usage for this commonly cultivated plant.
The origin of the plants described as Solanum jasminoides is discussed by Harley (1970), who speculates that the plants grown in the Young Nursery were originally from Sir William Hooker, who received material from Brazil collected by Tweedie. A specimen at Kew collected in 1832 in Rio Grande do Sul by Tweedie is thought by Harley (1970) to probably be the origin of both the Kew and the Epsom material; Tweedie collected both specimens and seeds for Hooker.
The epithet boerhaviifolium was spelled “boerhaviaefolium” with a single ‘a’ by Sendtner (1846) and although the name honours Hermann Boerhaave, the original spelling is retained, following Article 60.1 of the Code (McNeill et al., 2006).
Sendtner, O. 1846. Solanaceae.
Pp. 1-228 in C. P. F. von Martius, Flora Brasiliensis. 10: 5-338. C. Wolf, Munich, Germany.
Dunal, M.F. 1852. Solanaceae.
Pp. 1-690 in A. P. DeCandolle (ed.), Prodromus Systematis Naturalis Regni Vegetabilis 13(1). Victoris Masson, Paris, France.
Harley, R.M. 1970. Solanum jasminoides (Solanaceae).
Bot. Mag. 177: tab. 568.
Morton, C.V. 1976. A revision of the Argentine species of Solanum.
Pp. 1-260. Academia Nacional de Ciencias, Córdoba, Argentina.
Cabrera, A. 1983. Solanaceae.
In Flora de la provincia de Jujuy, ed. A. Cabrera, 292-493. Buenos Aires, Argentina: Instituto Nacional de Tecnología Agropecuaria.
Moscone, E.A. 1992. Estudios de cromosomas meióticos en Solanaceae de Argentina.
Darwiniana 31: 261-297.
Mentz, L.A. & P.L. de Oliveira 2004. Solanum (Solanaceae) na região sul do Brasil.
Pesquisas, Bot. 54: 1-327.
McNeill, J., F.R. Barrie, H.M. Burdet, V. Demoulin, D.L. Hawksworth, K. Marhold, D.H. Nicolson, J. Prado, P.C. Silva, J.E. Skog, J. Wiersema, & N.J. Turland 2006. International code of botanical nomenclature (Vienna Code).
Regnum Veg. 146: 1-586.
Weese, T.L. & L. Bohs 2007. A Three-Gene Phylogeny of the Genus Solanum (Solanaceae)
Syst. Bot. 32(2): 445-463.