A revision of the Solanum elaeagnifolium clade (Elaeagnifolium clade; subgenus Leptostemonum, Solanaceae)

Abstract The Solanum elaeagnifolium clade (Elaeagnifolium clade) contains five species of small, often rhizomatous, shrubs from deserts and dry forests in North and South America. Members of the clade were previously classified in sections Leprophora, Nycterium and Lathyrocarpum, and were not thought to be closely related. The group is sister to the species-rich monophyletic Old World clade of spiny solanums. The species of the group have an amphitropical distribution, with three species in Mexico and the southwestern United States and three species in Argentina. Solanum elaeagnifolium occurs in both North and South America, and is a noxious invasive weed in dry areas worldwide. Members of the group are highly variable morphologically, and this variability has led to much synonymy, particularly in the widespread S. elaeagnifolium. We here review the taxonomic history, morphology, relationships and ecology of these species and provide keys for their identification, descriptions, full synonymy (including designations of lectotypes) and nomenclatural notes. Illustrations, distribution maps and preliminary conservation assessments are provided for all species.


Introduction
Solanum L. is one of the ten most species-rich genera of flowering plants (Frodin 2004) and has approximately 1,400 species occurring on all temperate and tropical continents. The highest diversity of both groups and species is in tropical South America, concentrated in a circle around the Amazon Basin (see Knapp 2002b), but significant diversity occurs in various parts of the Old World. Solanum was one of Linneaus's (1753) larger genera, with 23 species mostly described from European or African collections. The last time Solanum was monographed in its entirety was in De Candolle's Prodromus (Dunal 1852), which included 901 species (with an additional 19 recorded as incompletely known by him at the time). Until the 21 st century, the taxonomy of Solanum was largely limited to rearrangements of infrageneric taxa, species-level treatments of smaller groups within the genus, and floristic works.
The large size of Solanum and its poorly understood infrageneric structure has meant that Solanum taxonomy proceeded in a piecemeal fashion until relatively recently and the genus acquired a reputation of being intractable. A project funded by the United States National Science Foundation's Planetary Biodiversity Inventory (PBI) program begun in 2004 sought to accelerate species-level taxonomic work across the genus and resulted in a series of monographic and phylogenetic treatments from both Old and New Worlds (e.g., Tepe and Bohs 2011;Stern et al. 2013;Knapp 2013a;Knapp and Vorontsova 2016;Clark et al. 2015;Wahlert et al. 2014Wahlert et al. , 2015Aubriot et al. 2016;Vorontsova and Knapp 2016;Spooner et al. 2016). An electronic monographic treatment of the entire genus, with species and species groups added as they are completed, is available online in the web resource Solanaceae Source (http://www.solanaceaesource.org). This treatment is part of that collaborative effort.

Taxonomy and relationships
Solanum has been divided into 13 major clades (Bohs 2005;Särkinen et al. 2013), of which the spiny solanums (subgenus Leptostemonum Bitter, or the Leptostemonum clade) is the largest, with some 550 currently accepted species. The Solanum elaeagnifolium species group is part of this large group, and within that, is sister to the monophyletic Old World clade (see Vorontsova et al. 2013).
Plants collected by William Houstoun on the Caribbean coast of Mexico were cultivated by Philip Miller of the Chelsea Physic Garden in London and were described as S. carolinense Mill. (= S. houstonii Martyn;Miller 1768). Solanum elaeagnifolium Cav. was described from material grown in the Real Jardín Botánico de Madrid collected on voyages made by Spanish explorers in the early 19 th century (Cavanilles 1800); S. leprosum Ortega was probably described from the same living material (see Knapp 2013b). Dunal (1813) treated both species as members of his section Leprophora Dunal, along with S. sericeum Ruiz & Pav. (now recognised as a member of the Potato clade, Särkinen et al. 2015), based on their whitish grey pubescence and leaf morphology. A year later, Dunal (1814) described S. tridynamum Dunal (=S. houstonii) based upon the drawings of José Sessé and Mariano Mociño he had seen either at the herbarium in Geneva or in Montpellier ), but did not associate his new species with S. elaeagnifolium. Based on the collections made by Alexander von Humboldt and Aimé Bonpland during their brief stay in Mexico, Dunal (1816) described S. obtusifolium Dunal (= S. elaeagnifolium) as an additional species in his Leprophora. Bentham (1844) recognised the similarity of his S. hindsianum from Baja California with Mexican and South American populations of S. elaeagnifolium, and with populations from Texas (that he suggested represented an additional undescribed taxon).
Exploration of the western United States along with the trade in seeds between European botanic gardens led to the description of many synonyms of the extremely variable species S. elaeagnifolium (e.g., S. flavidum Torr. described as growing "from Mississippi River to Rocky Mtns") and S. houstonii (e.g., S. herbertianum Paxt., described as of unknown origin but with detailed descriptions of methods of cultivation).
In his treatment of the Solanaceae for Candolle's Prodromus Dunal (1852) placed his S. tridynamum, plus all of the names we consider synonymous with it, as a member of his group "Nycterium" that was characterized by having unequal anthers (see discussion of S. houstonii). Other species placed by him in this group were S. dubium Fresen. (= S. coagulans Forssk.) from northern Africa, S. vespertilio Aiton of the Canary Islands and S. wightii Nees of India. Convergent evolution of heterandry has been well-documented in Solanum, and has led to the break-up of many of Dunal's groups based on this character (Lester et al. 1999;Anderson et al. 2006;Bohs et al. 2007). The Old World species with heteromorphic anthers do not form a monophyletic group either (Vorontsova et al. 2013;Aubriot et al. 2016). Dunal placed S. elaeagnifolium, S. hindsianum and many species of Australian spiny solanums (e.g., S. pungetium R.Br.) in a heterogenous un-named group characterized by having plicate corollas.
In floristic works S. elaeagnifolium has in general been treated as being native to Mexico (e.g., Morton 1976), based on Cavanilles' locality of "America calidiore" (Cavanilles 1795), and the name S. leprosum had been used for South American material, either as a species or at infraspecific rank (Morton 1976). The material used by Cavanilles (and most likely also by Ortega, see Knapp 2013b) is said to be from "el viaje de los españoles alrededor del Mundo" (the voyage of the Spanish around the world), and could therefore be from either South or North America.
All of the members of this small species group were only recognised as being related as the result of molecular phylogenetics (Levin et al. 2006;Stern et al. 2011;Wahlert et al. 2014). Whalen (1984) suggested that S. elaeagnifolium was related to Australian species and placed it in his S. ellipticum group, and placed S. houstonii (as S. tridynamum) in his S. vespertilio group, based on its zygomorphic flowers; he included S. hindsianum in his unplaced species. In his treatment of groups of New World solanums Nee (1999) placed S. elaeagnifolium, S. hindsianum and S. houstonii (as S. tridynamum) with S. vespertilio Aiton and S. lidii Sunding (both Canary Island endemics with strongly zygomorphic flowers, see Anderson et al. 2006) in an un-named series (Series 4) of subsection Lathyrocarpum G.Don. He placed S. mortonii in "Series 2" along with other species such as S. comptum C.V. Morton and S. pumilum Dunal, most of which are now currently recognised as members of the Carolinense clade (section Lathyrocarpum; Wahlert et al. 2015), but others are species of uncertain affinity (e.g., S. euacanthum Phil. is weakly sister to the Elaeagnifolium clade, see Wahlert et al. 2014;S. multispinum N.E.Br. is a species of uncertain affinities; see Stern et al. 2011, Wahlert et al. 2014. In combined analyses using plastid and nuclear markers S. elaeagnifolium accessions from North and South America group together as sister taxa (Stern et al. 2011;Wahlert et al. 2014). Stern et al. (2011) recovered S. mortonii as sister to S. elaeagnifolium (both North and South American accessions). Solanum homalospermum was recovered as a member of the Elaeagnifolium clade by Wahlert et al. (2014), and as sister to the grouped S. elaeagnifolium accessions used in that study, with S. mortonii. Wahlert et al. (2014, see their Fig. 1), in the most recent phylogenetic treatment of these taxa, recovered two groups within the Elaeagnifolium clade -one consisting of the South American species plus all accessions of S. elaeagnifolium, and the other with S. hindsianum + S. houstonii, both from North America.

Morphology
Habit and stems. Members of the S. elaeagnifolium group are all small to mediumsized shrubs with rhizomatous underground stems. The plants are usually less than 1 m and rarely exceed 2 m tall, although label data indicate S. hindsianum can grow to reach 3 m in height. Due to their underground stems, plants are often found in dense colonies, often in disturbed areas (see below). The roots of S. elaeagnifolium have been characterised as tuberised ("tuberizada"; Cosa et al. 1998) with up to 20 cortical layers. Only the vertical underground parts are thus thickened, the horizontal spreading stems (rhizomes) are not corky and thickened.
The stems of all members of the group are variably prickly; this variability is most pronounced in S. elaeagnifolium, but occurs in all the species of the clade (see discussion of S. houstonii). Prickles and their morphology are described in detail below. Prickly and non-prickly morphs of the same species have often been described as different species or forms, leading to much synonymy.
Sympodial growth is characteristic of Solanaceae giving the stems a typical "zigzag" appearance; details of sympodial structure have proved useful for infrageneric classification within Solanum (Child and Lester 1991;Knapp 2002b). Vegetative growth is initially monopodial, but with the onset of flowering, becomes sympodial. The inflorescence is developmentally terminal, and stem continuation is initiated in the axil of the leaf below each inflorescence. Each lateral shoot with alternate leaves arranged in a 1/3 phyllotaxic spiral and a terminal inflorescence is termed a sympodial unit. In some cases, when the axes of sympodial units are fused, the inflorescences appear to originate laterally from the middle of an internode; and when growth of the axes is suppressed, the leaves appear paired (geminate) at a node (Danert 1958). All of the members of the S. elaeagnifolium group have difoliate sympodial units with leaves not strongly paired at the nodes, but occasionally sympodial units are tri-or plurifoliate.
Leaves. Leaf morphology in subgenus Leptostemonum is very diverse, not only among groups of species, but also within groups and even within individuals of a single species. The highly variable size, shape, and lobing of leaves are often the first characters to be noticed by herbarium taxonomists, and attributing undue importance to this variability is one of the causes of excessive synonymy. All members of the S. elaeagnifolium group have simple leaves with entire or lobed margins. Leaves on pre-reproductive shoots are usually more lobed than those of reproductive shoots; this is common in many spiny solanums (see Vorontsova and Knapp 2016). Some populations of S. houstonii from the western coast of Mexico (Sinaloa and Sonora) have deeply lobed leaves (see description of S. houstonii).
Leaf size also varies with season and environmental conditions; plants collected in the wet season or from wetter areas always have larger leaves than those from drier microhabitats or that were collected in the dry season.
Pubescence. In common with the rest of the Leptostemonum clade, species in the S. elaeagnifolium group have stellate pubescence. Minute simple trichomes (papillae) are also present, usually on the new growth. Trichomes are generally similar throughout the plant in these species, but pubescence density is usually greater on leaf undersides. Stellate trichomes in the group can be characterized as one of three types: 1) porrect, with straight unicellular rays arranged horizontally in a single plane, and a unicellular midpoint perpendicular to the rays (Fig. 1A, B); 2) multangulate, i.e., with the numerous rays arranged in more than one plane, (Fig. 1C); or 3) lepidote, with the acicular rays fused near the centre (Fig. 1D) to form a scutellate or shield-like structure around a midpoint of variable length (Carvalho et al. 1991;D'Arcy 1992;Roe 1971).
Solanum elaeagnifolium is characterised by its lepidote pubescence, but some individuals of S. hindsianum can have porrect stellate trichomes with some fusion of the ray bases; these trichomes, however, never develop into the typical shield-like structure found in S. elaeagnifolium. The basal cells of some of the lepidote trichomes in S. elaeagnifolium are inserted into the mesoderm (Cosa et al. 1998;Bruno et al. 1999);Bruno et al. (1999) suggest this is related to water conservation in the xeric environments in which S. elaeagnifolium grows. Solanum homalospermum has midpoints equal to or longer than the rays, while other taxa usually have reduced midpoints. Solanum mortonii has multangulate trichomes on all parts.
In herbarium specimens, the trichomes of all these species are pale grey, giving the plants a silvery cast. On live plants, the trichomes of S. elaeagnifolium are similarly silvery (hence the specific epithet referring to Elaeagnus L. (Elaeagnaceae, the Russian olive) but trichomes of S. houstonii can be golden or reddish yellow (see Fig. 2E, 3E). Solanum mortonii has whitish grey trichomes that are much denser on the leaf undersurfaces, making the leaves strongly discolorous (see Fig. 3C, 4C, D).
Prickles. Prickles in Solanum are epidermal in origin and are thought to be modified multicellular stellate trichomes with layers of elongate and lignified cells (Whalen 1984). The common origin of trichomes and prickles can be observed on young stems of where some trichomes develop longer lignified stalks that become prickles with an apical stellate trichome that is later deciduous (see for example Fig. 124 in Vorontsova and Knapp 2016). Often prickles can themselves bear trichomes, reflecting their epidermal nature. The development of prickles in Solanum has not been studied in detail in any species.
Prickles can occur on all above-ground parts of a plant except the corolla and the fruit. Density and distribution of prickles vary with the age of the plant and environmental conditions and, thus, are not particularly useful characters; all species of the group can have branches (see Figs 2, 3 and species illustrations) or entire individuals with no prickles. This plasticity has led to much taxonomic confusion (Jaeger 1985). Where prickles are present they are usually straight and acicular, but the stem prickles of S. hindsianum and S. houstonii are broader at the base and occasionally somewhat curved. On an individual plant prickles are usually uniform throughout the plant. Solanum hindsianum and S. mortonii usually lack prickles on the leaves.
Inflorescences. As with all species of Solanum, the inflorescence in members of the S. elaeagnifolium clade is developmentally terminal, and is later overtopped by the leading axillary shoot making it appear lateral. The basic inflorescence, as in all other species of Solanum, is a scorpioid cyme that is branched or unbranched. In Solanum the inflorescence expands from the tip with each apical meristem producing multiple flowers in a proliferating manner (Lippmann et al. 2008 has forked inflorescences (see Fig. 2 and individual species illustrations). In strongly heteromorphic species (S. homalospermum, S. houstonii) the solitary basal (hermaphroditic) flower is borne very near the base, and the more distal staminate flowers are borne at some distance from it.
Calyces. Members of the Elaeagnifolium clade have 5-merous flowers like most other species of Solanum, but occasional tetramerous individuals do occur. The calyx in all members of the group is composed of deltate lobes with prominent keels on the abaxial surface, and with narrow, elongate acumens usually as long as or longer than the deltate portion of the lobes (Figs 2 and 3). Staminate flowers of S. houstonii have non-prickly, shorter calyces than those of hermaphroditic flowers which are ca. 1.5 times larger and usually densely prickly (see Fig. 2D, 3E). The degree to which the calyx is accrescent in fruit varies in the species of the group, and can be a useful identification character (see Fig. 3). The calyx is not markedly accrescent in S. elaeagnifolium, where the berry is clearly visible, partly accrescent and covering about half of the berry in S. hindsianum, and markedly accrescent in S. houstonii, S. homalospermum and S. mortonii, where it completely encloses the mature berry. We have seen some specimens of S. elaeagnifolium, however, with accrescent calyces (e.g., Barkley 14-A539 from Cochise County, Arizona USA) and some collections of S. houstonii with somewhat exposed berries (e.g., Dorantes et al. 1033 from Veracruz, Mexico), suggesting this character might be quite variable in individual species or that in S. houstonii the calyx splits with fruit maturity and drying.
Corollas. Like the calyces, corollas of members of the Elaeagnifolium clade are most often 5-merous. Corollas are stellate, and usually divided about halfway to the base, the lobes are deltate to triangular with copious to sparse interpetalar tissue, and are usually spreading at anthesis (see Fig. 2). The corollas of S. elaeagnifolium, S. hindsianum, S. homalospermum and S. mortonii are actinomorphic, with all lobes the same size and shape. Solanum houstonii, however, has markedly zygomorphic corollas, with the two lower lobes enlarged relative to the upper ones (see Fig. 2D,E;Fig. 10), and with corolla shape differing between hermaphroditic and staminate flowers. Staminate flowers (S) normally have slightly larger corollas than hermaphrodites (H)(length: H: 37.8 ± 1.3, S: 40.0 ± 1.2; width: H: 38.8 ± 1.4, S: 41.8 ± 1.2, A.Z.K. Carbonell, measurements from field individuals), and are more zygomorphic.
The abaxial surfaces of the corolla lobes are densely pubescent with stellate trichomes where they are exposed in the bud; the interpetalar tissue in the sinuses is glabrous on both surfaces in all species. In exceptionally prickly individuals of S. houstonii a few minute prickles are borne on the abaxial midvein of each corolla lobe.
Androecium. The anthers of all members of the Eleagnifolium clade, like those of most of the spiny solanums, are long-tapering, with distally directed pores that do not lengthen to slits with age. In all five species the anthers are heteromorphic, with three of the five slightly longer than the rest, and are usually slightly curved (see Fig. 2 and individual species illustrations). The anthers of S. elaeagnifolium are the least heteromorphic, and those of S. houstonii the most. In S. houstonii staminate flowers have three long and curved and two short and straight anthers, while those hermaphroditic flowers are similar but are not as unequal or curved as the anthers of staminate flowers. These anthers in hermaphroditic flowers of S. houstonii are very similar to those of S. hindsianum.
Petanatti and Del Vitto (1991) record structures they called "bridas" (flanges) on the anthers of S. elaeagnifolium (see Fig. 5E). These small folds in the anther surface are similar to the papillate (papillose) anther surfaces of S. mortonii and some species of non-spiny solanum from Madagascar (Chiarini 2007;Knapp and Vorontsova 2016). These papillate anther surfaces occur in some, but not all, herbarium specimens of all members of the Elaeagnifolium clade (see species descriptions). Structures similar to the papillae seen on the abaxial anther surfaces in this group are also found in the Tomato clade (Carrizo-García 2003;Peralta et al. 2008), where the anthers are held tightly together with elongate and hair-like papillae. In tomato, the development of these structures is controlled by a transcription factor similar to that which controls the conical cells of petals (see Glover et al. 2004); ongoing investigations across Solanum are being undertaken to assess its presence and function in other taxa (G. Davies and B.J. Glover, pers. comm.).
The filaments in all members of the group are composed of a very short filament tube, and a glabrous free portion that is usually ca. 1/5 the length of the anthers themselves (see Fig. 2). The free portion of the filaments is often somewhat curved in live plants.
Gynoecium. All of the species of the Elaeagnifolium clade except S. elaeagnifolium exhibit heterostyly, with long-and short-styled flowers borne in the same inflorescence. The ovary in short-styled flowers in strongly andromonoecious species such as S. houstonii is vestigial and the style is very short. The long style in hermaphroditic flowers (and in all flowers of S. elaeagnifolium) is slightly curved and usually white (see Fig. 2); styles are usually pubescent in the basal part with porrect-stellate trichomes. The style in S. houstonii curves in a similar way to the largest anthers in the staminate flowers and the flowers are somewhat enantiostylous. The stigma is bright green and clavate.
Fruit. As with all species of Solanum, the fruit is a bicarpellate berry. All of the species in the S. elaeagnifolium group have globose berries in which the pericarp dries at maturity. Unripe berries of all species are mottled green (see Fig. 3A), but at maturity they become either yellow or orange (S. elaeagnifolium) or dark brownish black (e.g., S. houstonii) and the fruit wall becomes brittle and breaks (see Fig. 3D, F). In S. elaeagnifolium the berries remain on the plant for months, eventually falling as a unit -the seeds are often fused together with a sticky glue-like substance (Fig. 3B). Other species, especially those in which the calyx is accrescent in fruit and completely covers the berry (S. houstonii, S. mortonii) break near the apex of the berry and the seeds are released a few at a time over a long period (Lester and Symon 1989). This type of "censer" or spray-cup mechanism is also found in species of Solanum section Androceras (Nutt.) Marzell (e.g., S. rostratum Dunal; Whalen 1979) and in some Australian species (e.g., S. tununduggae Symon, S. vansittartensis C.A.Gardner;Symon 1981). The berry wall in S. elaeagnifolium, S. homalospermum and S. mortonii includes fibres and sclerids of calcium oxalate in an unusual arrangement with the fibres enclosing a crystalline sclerid or group of sclerids (see Fig. 3.7 in Chiarini 2007;Chiarini and Barboza 2007a).
These inclusions are not known elsewhere in spiny solanums, except in taxa whose berries are dry and dehiscent (e.g., S. euacanthum), and may be related to dehiscence and the irregular rupture of the berry wall mediated by changes in temperature and humidity (Chiarini 2007;Chiarini and Barboza 2007a) like that occurring during anther dehiscence (D'Arcy et al. 1996). Berries of S. hindisanum and S. houstonii have not yet been examined anatomically.
Protein extracts from ripe berries of S. elaeagnifolium contain compounds similar to the aspartic proteinases such as rennin and chrymosin used in cheese manufacture (Gutiérrez-Méndez et al. 2012). Tests with the berries suggested that these fruits could be useful in the production of soft cheeses such as cream cheese due to their lower activity than traditional coagulants (Gutiérrez-Méndez et al. 2012); berries are used in local cheese manufacture in indigenous communities in both Mexico and the United States (see description of S. elaeagnifolium).
Seeds. Seed morphology has been suggested to be a useful character for species-level taxonomy in Solanum (Souèges 1907;Lester and Durands 1984) and has been used to define morphological groups in other clades (e.g., Geminata, see Knapp 2002b). Seeds of members of the Elaeagnifolium clade are quite large relative to most spiny solanums, flattened reniform and have pitted surfaces. Seeds of S. elaeagnifolium are pale tan colored (Fig. 3B); all other species have dark brown or black seeds (Fig. 3F). In all species the seeds are often "glued" together in mature fruits with a shiny, sticky substance. Seed characteristics are not useful in distinguishing species in the group, although the thickened margin and warty surface of S. homalospermum are distinctive (Chiarini and Barboza 2007b).
Chromosomes. Of the five species in the Elaeagnifolium clade, only S. mortonii lacks data for chromosome number (see Table 1). The other four species of the group have chromosome numbers based on 2n = 12 or multiples of 12, like most other Solanum species (Chiarini et al. in press). Only a few metaphasic preparations of S. homalospermum have been obtained, all with ca. 48 chromosomes, but none of them with exactly the same chromosome number. Karyotypes are available for S. houstonii (Chiarini et al. in press) and S. elaeagnifolium (Acosta et al. 2012); chromosomes are symmetric, small to medium-sized (1.15 to 2.64 mm), and are similar to those found elsewhere in Solanum. Further details of chromosome morphology, such as fluorescent banding patterns and in situ hybridization, are only available for S. elaeagnifolium (Acosta et al. 2012;Chiarini 2014).
In the southern part of its range S. elaeagnifolium has three ploidy races; diploid, tetraploid and hexaploid (Scaldaferro et al. 2012). Solanum homalospermum also appears to be polyploid (Chiarini et al. in press). A relationship between polyploidy and clonality has been recorded in other groups of angiosperms (Baldwin and Husband 2013;James and McDougall 2014;Wahlert et al. 2015), but in this group diploid populations (S. elaeagnifolium in both North and South America) and species (possibly S. mortonii, but ploidy status is as yet not known) also appear to reproduce vegetatively. In Argentina, polyploid populations of S. elaeagnifolium are generally found in wetter parts of the species range (Scaldaferro et al. 2012). Chiarini et al. (2016) have shown that tetraploid populations have arisen repeatedly from both diploid and hexaploid progenitors, but also show that using plastid markers, all populations at particular ploidy levels cluster together (see discussion of S. elaeagnifolium).

Biology and natural history
Habitats and distribution. Members of the S. elaeagnifolium group are generally rather weedy, and grow in open habitats in dry forests and scrublands (Fig. 4). This habit has resulted in S. elaeagnifolium becoming an invasive weed on several continents (see discussion of S. elaeagnifolium); other species in the group with rhizomatous underground stems (e.g., S. homalospermum, S. mortonii) also have this potential. Solanum elaeagnifolium grows in a wide variety of habitats, from deserts to montane Chaco habitats in Argentina. Scaldaferro et al. (2012) found that polyploid populations were more likely to be growing in wetter habitats in Argentina. Solanum houstonii similarly grows across a wide variety of habitats and elevations; it occurs in most of the arid habitats of Mexico (Rzedowski 1978) from the Sonoran Desert zones in western Mexico, across the volcanic belt up to 2000 m to the limestone pans of the Caribbean coast. The members of the group (excluding the invasive populations of S. elaeagnifolium) exhibit a typical New World amphitropical distribution, with species occurring in the arid regions in both hemispheres; this pattern has long been of interest to botanists (see Gray and Hooker 1880;Bray 1900;Raven 1963). Various hypotheses for these patterns have been extinction of intervening tropical ancestral populations (Johnston 1940), island hopping via stepping stones of arid habitat (Raven 1963) or long-distance dispersal (Raven 1963;Simpson and Neff 1985). Analysis of the phylogenetic relationships in groups with amphitropical distributions suggests that, as might be expected, these patterns result from a mixture of causes, even within a given group (Simpson et al. 2005). Raven (1963) hypothesized that desert (arid zone) disjuncts were primarily the result of migration or dispersal from south to north; this is the case in some groups (e.g., Simpson et al. 2005, 2006, Hoffmannseggia Cav. andPomaria Cav., Leguminosae) while in others long-distance dispersal appears to have occurred from north to south (e.g., Moore et al. 2006aMoore et al. , 2006b, Tiquilia Pers. Boraginaceae).  Barboza et al. 3437). Photographs by S. Knapp Simpson et al. (2005) showed that the current distribution of Hoffmannseggia species was the result of numerous dispersal events from South to North America, rather than a single event followed by diversification. In all studies to date the dispersal events leading to these amphitropical disjunctions are relatively recent. Biogeographic history in the Solanaceae as a whole has involved multiple dispersal events from South to North America (Dupin et al. 2017) and vice versa.
Unravelling dispersal history of members of the Elaeagnifolium clade will require more detailed phylogenetic sampling in the spiny solanums; the amphitropical pattern is found in other small groups in the larger Leptostemonum clade, and so is likely to have multiple origins. Wahlert et al. (2015) suggested that because the North American taxa were nested among a much larger clade of Neotropical species, mostly South American, that North American taxa originated from a South American progenitor. In the Elaeagnifolium clade (see Fig. 1 in Wahlert et al. 2014) it is clear that two distinct dispersal events have taken place, one to account for the two species groups (S. hindsianum+S. houstonii and S. mortonii + [S. elaeagnifolium+S. homalospermum]) and the other for the disjunct distribution of S. elaeagnifolium itself. This latter event may be more recent, such as has been observed for some species of Tiquilia (Moore et al. 2006b). Studies of chloroplast haplotypes suggest S. elaeagnifolium has a long history in southern South America (Chiarini et al. 2016) and is not a recent introduction.
Pollination and breeding systems. Like all Solanum species with poricidal anthers, pollen of members of the Elaeagnifolium clade can only be extracted by insects that can sonicate the anthers (Michener 1962;Linsley and Cazier 1963;Buchmann et al. 1977). In the majority of cases these are bees from a variety of families; they use their indirect flight muscles to set up vibrations in the anther cone that causes pollen to squirt out and be deposited on the ventral surface of the bee (Buchmann et al. 1977). Bee visitation to S. elaeagnifolium has been studied in its native North American range (Linsley and Cazier 1963) where a number of species of solitary bees (see Table 2) visit and buzz the flowers. In Arizona (Linsley 1962;Cazier 1963, 1970;Buchmann and Cane 1989) bees of the genera Ptiloglossa and Psaenythia are Solanum specialists, and Ptiloglossa did most of its foraging in the hours just prior to sunrise (matinal behaviour), and forced flowers open to access the anthers (Linsley and Cazier 1963); visits to S. elaeagnifolium stopped when the sun reached flowers. Bumblebees of the genus Bombus, more generalist pollinators, visited throughout the day (Linsley and Cazier 1963). Jensen-Haarup (1908) and Jörgensen (1909) recorded a large number of bee species (see Table 2) visiting flowers of S. elaeagnifolium, but did not record times of day. The most specialist ("apparently preferring Solanum elaeagnifolium") of the taxa observed were Augochloropsis argentina, Psaenythia philanthoides and females of the carpenter bee Xylocopa brasilianorum (Jensen-Haarup 1908). In Greece, where it is not native, S. elaeagnifolium competes for bee visits with native plants, significantly impacting their reproduction (Tscheulin and Petanidou 2013); here flowers are visited throughout the day. Carpenter (Xylocopa spp.) and bumble bees (Bombus spp.) have been reported as visiting S. hindsianum (https://www.desertmuseum.org/visit/sheets/Solhin.pdf ). The anthers of members of the Elaeagnifolium clade are not tightly connivent (pepper pot configuration of Glover et al. 2004), and so only large bees who can contact both style and anthers are likely to effect pollination; smaller bees such as halictids who visit single anthers do not contact stigmas and so are more correctly viewed as pollen thieves, as are honeybees, who glean pollen from petals and anthers (Buchmann and Cane 1989).
Andromonoecy, possession of hermaphroditic and functionally staminate flowers in a single inflorescence, has evolved multiple times in Solanum (Symon 1970;Symon 1979;Whalen and Costich 1989;Vorontsova et al. 2013;Aubriot et al. 2016). Members of the Elaeagnifolium clade range from weakly (e.g., S. elaeagnifolium) to strongly (e.g., S. houstonii) andromonoecious. In S. elaeagnifolium, most flowers are perfect with a few short-styled flowers distally in some plants, while S. homalospermum, S. houstonii, and S. mortonii have a single hermaphroditic basal flower with all the rest of the flowers in an inflorescence short-styled and functionally staminate. Solanum hindsianum appears to be somewhat intermediate between these states, with a few hermaphroditic flowers in each inflorescence. To date, the strength and plasticity of andromonoecy in the species of the group has not been assessed, but ongoing studies by one of us (A.K.Z. Carbonell) are aimed at elucidating breeding systems in the species of the Elaeagnifolium clade.
Conservation status. With the exception of S. homalospermum and S. mortonii, the members of the S. elaeagnifolium group are relatively widespread and not of immediate conservation concern. Their propensity to be common where they occur through vegetative reproduction, however, may mean that some taxa (e.g., S. mortonii) have limited genetic diversity. Solanum houstonii, while widespread, has considerable morphological diversity within its populations, suggesting a more populational rather species level approach to conservation would be beneficial. For preliminary conservation assessments of each species, see Table 3 and the individual species treatments below.

Species concepts
Our goal for the treatment of the species of this small clade has been to provide circumscriptions for the members of this morphologically variable group of species, while clearly highlighting those taxa and populations where further in-depth research would be useful. Delimitation of species here basically follows what is known as the "morphological cluster" species concept (Mallet 1995): i.e., "assemblages of individuals with morphological features in common and separate from other such assemblages by correlated morphological discontinuities in a number of features" (Davis and Heywood 1963). Biological (Mayr 1982), phylogenetic (Cracraft 1989) and the host of other finely defined species concepts (see Mallet 1995) are almost impossible to apply in practice and are therefore of little utility in a practical sense. It is important, however, to clearly state the criteria for the delimitation of species, rather than dogmatically follow particular ideological lines (see Luckow 1995;Davis 1997). Our decisions relied on clear morphological discontinuities to define the easily distinguished species. Specific characters used for recognition are detailed with each species description and in the key. Some potential reasons for variability and intergradation are recent divergence, hybridization and environmental influences on morphology. In this revision we have tried to emphasise similarities between populations instead of differences, which so often reflect incomplete collecting or local variation. We have not recognised subspecies or varieties, but have rather described and documented variation where present, rather than formalised such variability with a name which then encumbers the literature. We have been conservative in our approach, recognising as distinct entities those population systems (sets of specimens) that differ in several morphological characteristics. All of the species in the clade are extremely widespread and variable; variation exists in certain characters, but the pattern of variation is such that no reliable units can be consistently extracted, nor is geography a completely reliable predictor of character states. Here variability within and between populations seems more important than the variations of the extremes other taxonomists have recognised as distinct. We describe this variation realising that others may wish to interpret it differently.

Materials and methods
This monograph is based on examination of herbarium specimens supplemented with field observations in North and South America. We examined approximately 2,100 collections (ca. 2,700 specimens) from the following herbaria (herbarium acronyms follow Index Herbariorum, found on-line at http://sweetgum.nybg.org/science/ih/): Measurements were made from dried herbarium material supplemented by measurements from living material. Colours of corollas, fruits, etc., are described from living material or from herbarium label data. Specimens with latitude and longitude data on the labels were mapped directly. Some species had few or no georeferenced collections; in these cases, we retrospectively georeferenced the collections using available locality data. The extremely widespread S. elaeagnifolium was mapped based on GBIF records (http://www.gbif.org/species/2929892; 3,032 georeferenced observations on 6 November 2016) and specimens we have seen. Maps were constructed with the points in the centres of degree squares in a 1° square grid. Conservation threat status was assessed following the IUCN Red List Categories and Criteria (IUCN 2014) using the GIS-based method of Moat (2007) as implemented in the online assessment tools in GeoCat (http://geocat.kew.org). The Extent of Occurrence (EOO) measures the range of the species, and the Area of Occupancy (AOO) represents the number of occupied points within that range based on the default grid size of 2 km 2 .
Where specific herbaria have not been cited in protologues we have followed Mc-Neill (2014) and designated lectotypes rather than assuming holotypes exist. We cite page numbers for all previous lectotypifications.
Type specimens with sheet numbers are cited with the herbarium acronym followed by a dash and the sheet number (i.e., MO-1781232); barcodes are written as a continuous string (i.e., G00104280). We have cited geographically representative specimens for taxa where more than 100 collections are known. Full specimen details are available on the Solanaceae Source website (www.solanaceaesource.org) and in the dataset for this study deposited in the Natural History Museum Data Portal (http:// dx.doi.org/10.5519/0007624). Specimens cited in the text are listed by country alphabetically, rather than geographically; within any country major political divisions are also listed in alphabetical order.
Citation of literature follows BPH-2 (Bridson 2004) with alterations implemented in IPNI (International Plant Names Index, http://www.ipni.org) and Harvard University Index of Botanical Publications (http://kiki.huh.harvard.edu/databases/publica-tion_index.html). Following Knapp (2013a) we have used the square bracket convention for publications in which a species is described by one author in a publication edited or compiled by another. These citations are the traditional "in" attributions such as Dunal in DC. Leaves simple to shallowly lobed to occasionally somewhat pinnatifid, concolorous or discolorous, densely pubescent with multangulate, stellate or lepidote trichomes; petioles well developed, sometimes channelled above. Inflorescences terminal to lateral, usually unbranched, occasionally furcate, not bracteate, with up to 10 flowers (exceptionally to 26 flowers in S. houstonii), in andromonoecious plants with a single (or two) hermaphroditic long-styled flowers at the base and all distal flowers short-styled and functionally staminate; peduncle robust, clearly distinct armed or unarmed; pedicels articulated at the base, armed or unarmed. Flowers 5-merous, actinomorphic to zygomorphic, perfect or strongly heteromorphic with long-and short-styled morphs and the plants andromonoecious. Calyx armed or unarmed, the lobes deltate and usually strongly keeled with an elongate acumen. Corolla stellate or rotate stellate, purple or occasionally white, usually with a green star at the base, the lobes spreading or slightly reflexed at anthesis. Stamens unequal, sometimes markedly so (S. houstonii), the filaments equal, the anthers strongly tapering with distally directed pores, usually somewhat spreading and not connivent. Ovary conical, vestigial in strongly andromonoecious species, glabrous or sparsely stellate-pubescent; style in long-styled flowers straight or curved, glabrous or sparsely stellate-pubescent near the base; stigma clavate in long-styled flowers, vestigial in short-styled flowers.
Distribution. An exclusively New World group occurring in North America (western United States and Mexico) and southern South America (Argentina, Paraguay, Brazil, Uruguay, Chile). One species, S. elaeagnifolium, is an invasive weed in dry areas worldwide.
Discussion. As discussed above under Phylogeny and as can be seen by the synonymy of the clade, members of this group were previously not thought to be closely related. Their resolution as sister to the diverse and diversifying Old World clade of spiny solanums makes them of particular interest in terms of character evolution. Nee (1999) placed all of the species treated here in his section Lathyrocarpum (whose type species is S. carolinense L., see Wahlert et al. 2015) in two different series; in this he followed the views of Hunziker (1979).
The clade, as is common with the groups of spiny solanums, has few unambiguous and unique synapomorphies. The andromonoecious habit (very weak in S. elaeagnifolium), unusual dry and often dehiscent berries, dark sticky seeds, silvery pubescence and propensity to grow in arid zones are all characters that are shared by the species in this group. None of these are unique to the S. elaeagnifolium group, however, either in Solanum or in Solanaceae more widely, although the dehiscent berries of all these species (save S. elaeagnifolium) are only rarely found in Solanum (e.g., S. tununduggae, S. vansittartensis of Australia, see Symon 1981;Knapp 2002a).
Distribution (Figure 6, showing American distribution only). Solanum elaeagnifolium has an amphitropical native distribution, occurring in the deserts and dry zones of the northern hemisphere in the southwestern United States of America and Mexico and in the southern hemisphere in Argentina, Paraguay, Uruguay and Chile, but this species is widespread and invasive in tropical and subtropical regions worldwide (see below). We have seen (non-cultivated) specimens of invasive S. elaeagnifolium from It is toxic to livestock and very hard to control, as root stocks less than 1 cm long can regenerate into plants (Cuthertson 1976;Fernández and Brevedan 1972;Stanton et al. 2011).
In areas where S. elaeagnifolium is invasive it is usually referred to as silverleaf nightshade (Boyd et al. 1984Hoffmann et al. 1998Wu et al. 2016), but in South Africa it is also known as silverleaf bitter apple (English), satansbos, bloubos and silwerblaarbitterappel (Afrikaans) (Henderson 2011;Invasive Species South Africa 2017).
The berries of S. elaeagnifolium were used in the southwestern United States by Pima, Navajo and Hopi people for making cheese and for tanning leather (Boyd et al. 1984;Foxx and Hoard 1984) and in local communities in the state of Chihuahua (Mexico) for making filata type asadero cheeses; more recently berries been investigated chemically for use in the food industry due to their milk clotting properties (Gutiérrez-Méndez et al. 2012). In Argentina, the berries are used as soap due to their high saponin content (Chiarini 2013).
Preliminary conservation status (IUCN 2016). LC (Least Concern). EOO = 298,812,730 km 2 (LC -Least Concern); AOO = 2,420 km 2 (NT -Near Threatened). Solanum elaeagnifolium is an invasive weed (see above) and is actively being eradicated in areas outside its native distribution. The low value for AOO is likely due to the low number of georeferenced collections.
Discussion. Solanum elaeagnifolium is the most widespread and variable of the species of this group. It can be easily distinguished from the rest of the species by its dense silvery lepidote pubescence, usually orange prickles (if present), mostly perfect flowers that are only slightly zygomorphic in the androecium and multiple berries per infructescence that are exposed at maturity. It is sympatric with S. hindsianum and S. houstonii in Mexico, and with S. homalospermum and S. mortonii in Argentina. Fruit type and pubescence are the most reliable characters for separating the taxa in both North and South America, but in addition, the leaf bases of S. elaeagnifolium are usually more decurrent onto the petiole than any of the other four species.
Solanum elaeagnifolium is extremely variable in its degree of prickliness; this extreme polymorphism also occurs in S. bahamense L. of the Caribbean (Strickland- Constable et al. 2000). Plants are densely prickly (type of S. elaeagnifolium) to completely unarmed (type of S. leprosum), and sometimes individual branches on a single plant differ in degree of prickliness (e.g., Bartholomew et al. 2430, MEXU).
Populations of S. elaeagnifolium in North America are diploid, with n=12 (Powell and Weedin 2005;Chiarini et al. 2016), while those in Argentina are diploid, tetraploid or hexaploid (Scaldaferro et al. 2012). Morphological differentiation between ploidy levels has not been found (Nilda Dottori, pers. comm.), but hexaploid populations appear to possess a number of differences in plastid haplotypes. Chiarini et al. (2016) suggest this might indicate presence of a cryptic species, but data from the entire genome is needed to test this hypothesis. Tetraploids appear to have arisen multiple times from either diploid or hexaploid populations (Chiarini et al. 2016). All plants tested outside of the native range (e.g., South Africa, Australia, the Mediterranean) are diploid, and in Australia, genetic variation as measured using AFLP markers suggests that populations in southern Australia are the result of several introductions (Zhu et al. 2013a).
As discussed above in the section on Habitats and distribution it was long assumed that S. elaeagnifolium was native to North America and was introduced, perhaps by the Spanish or Portuguese, to southern South America (Goeden 1971;Boyd et al. 1984;Wapshere 1988;OEPP/EPPO 2007). The clustering of all accessions of S. elaeagnifolium, regardless of origin, as sister to the South American S. mortonii+S. homalospermum (Wahlert et al. 2014) shows this is not the case. Haplotypes based on plastid sequences cluster all diploids in a group with tetraploids in a single cluster and hexaploids in another (Chiarini et al. 2016).
Introduction of S. elaeagnifolium outside of its native range has resulted in its becoming significant weed of cultivation and pasture (see summaries in Feuerhdert 2010; Uludag et al. 2016). In Australia it was introduced as a contaminant of grain and fodder in the early part of the 20 th century (Parsons and Cuthbertson 2001), but did not become a problem until the 1960s (Stanton et al. 2012). The species became a problem weed in South Africa in the 1970s (Olckers et al. 1999); our earliest record of it from that country is from 1919 (Carnegie 40). In the Mediterranean it has gone from a few accidental introductions, for example with cotton in Morocco (Bouhache 2010), to a weed necessitating control in many countries (Uludag et al. 2016). Plants from botanic gardens such as those used to describe S. elaeagnifolium and S. leprosum appear not to have escaped, and the weedy populations are all thought to be derived from accidental introductions associated with agriculture and subsequent mechanical disturbance of the soil. The Mediterranean populations have been shown to be most genetically similar to those from Texas (Suskiuw 2010). Our records from herbaria indicate that S. elaeagnifolium occurs in India, Iraq, Jordan, Kuwait and Saudi Arabia (see http:// dx.doi.org/10.5519/0007624 for complete specimen citations) in addition to in those areas such as Australia, South Africa and the Mediterranean where it is already a listed pest. A worldwide genetic similarity study will be necessary to document and track introduction and invasion of S. elaeagnifolium; studies on diversity of insect pests as indicators of origins (e.g., Goedin 1971) will need to be tested using new genetic tools. It will also be important to take into account new phylogenetic results (e.g., Wahlert et al. 2014) in determining where invasive populations have originated.
Measures for the control of S. elaeagnifolium have involved both chemical and biological agents. Herbicide control has been used in Australia and in the Mediterranean (Feuerherdt 2010;Uludag et al. 2016;Wu et al. 2016), while biological control using herbivorous insects from the native range in North America (Goeden 1971) has been successful in South Africa (Hoffmann et al. 1998;Olckers et al. 1999). In South Africa, the chrysomelid beetle Leptinotarsa texana Schaeffer, a relative of the Colorado potato beetle (Leptinotarsa decemlineata Say), strips S. elaeagnifolium plants and seriously compromises reproductive potential (Hoffmann et al. 1998); this species is being considered as a biological control agent in Australia (Wu et al. 2016). Other insects released to help control S. elaeagnifolium remained at low densities, with no impact; one of these biological control agents has been recorded to have impacted native Solanum species in South Africa (Olckers et al. 1998). Because S. elaeagnifolium reproduces both from seed and vegetatively, the importance of control of both seed and root stocks has been emphasized (Wu et al. 2016). The great morphological (Zhu et al. 2013a) and genetic (Zhu et al. 2013b, c) variability of S. elaeagnifolium surely contributes to its success as an invasive weed. Combining results from phylogenetics and biogeography with those from control measures will surely be important for future control and management of this species as it is introduced elsewhere.
Distribution ( Figure 8). Solanum hindsianum is endemic to the Sonoran Desert region of southern Arizona (Organ Pipe Cactus National Monument) and northern Mexico (States of Baja California, Baja California Sur, Sonora and Sinaloa); it occurs in matorral xerófilo, bosque tropical caducifolio and bosque espinoso (classification of Rzedowski 1978), from sea level to 400 m.
Ecology and habitat. In the Sonoran Desert biome (Pinkava et al. 1992), S. hindisanum is most often recorded as growing in rocky outcrops and scrubby areas, coastal scrub (matorral) in rocky areas and dunes near the shore.
Preliminary conservation status (IUCN 2016). LC (Least Concern). EOO 315,043 km 2 (LC -Least Concern); AOO 544 km 2 (VU-Vulnerable). Solanum hindsianum is widespread throughout Baja California and along the coast of the Gulf of California, and although it is habitat restricted, it appears to be common where it occurs.

Solanum homalospermum
Description. Erect rhizomatous shrub, 0.3-0.5 m tall. Stems sparsely armed or more commonly unarmed; young stems densely pubescent, the trichomes stellate, porrect, yellowish golden, sessile and short-stalked, the rays 6-8, ca. 1 mm long, the midpoints up to 1 mm long, equal in length to the rays; prickles if present 5-6 mm long, straight, slightly wider at the base, pale yellowish brown; bark of older stems smooth, brown or yellowish brown from persistent pubescence. Sympodial units difoliate or trifoliate, not markedly geminate. Leaves simple, 3-8 cm long, 1-3.1 cm wide, elliptic to narrowly elliptic, 4-5 times longer than wide, mostly concolorous, drying yellowish green to green; adaxial surfaces densely pubescent with translucent, mostly sessile porrect, stellate trichomes, the rays 8-14, 0.05-0.2(0.3) mm long, straight, the midpoint up to 0.1 mm, often reduced; abaxial surfaces densely pubescent with stalked porrect, stellate trichomes, the stalks to 0.5 mm long, the rays 8-10, 0.4-0.6 mm long, the midpoint shorter; principal veins 4-8 pairs, impressed adaxially, flat abaxially, spreading at ca. 45° to the midvein, the tertiary venation mostly not visible due to the dense pubescence; base acute; margins entire, rarely very shallowly lobed, the lobes 2-3 on each side, the sinus extending only 1/8 or less of the distance to the midvein, rounded; apex obtuse, rarely rounded; petiole 0.5-1.5 cm long, densely stellate-pubescent like the young stem, prickly, the prickles ca. 2 mm long, acicular. Inflorescences terminal or lateral, to 5 cm long, with up to 10 flowers, unbranched; peduncle less than 0.2 cm long, the lowest flower arising from very near the main axis; pedicels ca. 2 cm long, ca. 2 mm in diameter, robust, articulated near the base, densely stellate-pubescent like the leaf blades, the basal flower with the pedicel prickly, the prickles 2-3 mm long, orange-yellow; pedicel scars prominent, spaced 0.5-1.5 cm apart, space between the basal flower and the next much longer than that between the more distal staminate flowers. Buds ellipsoid, the calyx ca. 1/2 of the corolla length prior to anthesis. Flowers strongly heteromorphic and the plants andromonoecious, only the basal flower perfect and long-styled, the rest functionally staminate and short-styled. Calyx conical or cup-shaped, the tube 3-5 mm long, the lobes (3)6-8 mm long, 3-4 mm wide at base, lanceolate or long-deltate with acuminate apices, densely stellate-pubescent abaxially like the leaf blades, prickly in basal hermaphroditic flowers. Corolla 3-4 cm in diameter, white, drying yellowish-light brown, stellate, lobed for ca. 1/2 of the length, the lobes 0.8-1.5 cm long, ca. 1.2 cm wide, broadly deltate, densely stellate-pubescent abaxially. Stamens unequal, with the 2 adaxial anthers shorter than the 3 abaxial anthers; filament tube 1-1.5 mm, free portion of the filaments 3-5 mm; anthers 8-12 mm long, free, slightly unequal, poricidal at the tips, the pores about the same diameter as the anther apices, clearly delineated, the anther surface smooth to finely papillose. Ovary globose, minutely puberulent with simple glandular trichomes and some stellate trichomes; style of hermaphroditic flowers ca. 10 mm long, glabrous or minutely puberulent in the lower half; stigma capitate, papillose. Fruit a globose berry, 1 per infructescence, 1.2-3.5 cm in diameter when dry, the pericarp thin, smooth, glabrous, greyish brown and chartaceous when mature, when immature light green with dark stripes or marbled pattern, drying dark brown; fruiting pedicels 3-4 cm long, ca. 2 mm in diameter at base, 3-4 mm at apex, woody, strongly deflexed, sparsely armed with straight yellowish red prickles or unarmed, channelled in dry specimens; fruiting calyx accrescent, to 1.5 cm long, the lobes covering 2/3 or the entirety of the mature fruit, sparsely prickly. Seeds ca 10-30 per berry, 5-6 mm long, ca. 2.5 mm wide, flattened, reniform-rounded, brown, the surface minutely pitted and somewhat warty. Chromosome number: n=ca. 24 (Chiarini et al. in press;Chiarini 505).
Distribution (Figure 8). Solanum homalospermum is endemic to north central Argentina in the Provinces of Córdoba and adjacent Catamarca at ca. 700 m elevation.
Ecology and habitat. Solanum homalospermum grows in open areas in the dry Chaco forest region; the vegetation at the type locality is a highly disturbed palm (Trithrinax Mart., Arecaceae) woodland-grassland (Zak and Cabido 2002). This species has a complex root system that facilitates vegetative reproduction and spread like other members of the Elaeagnifolium clade.
Common names and uses. None known. Preliminary conservation status (IUCN 2016). Endangered/Critically Endangered (EN/CR). EOO = 273 km 2 (EN-Endangered); AOO = 12 km 2 (EN -Endangered). Solanum homalospermum might best be considered CR (Critically Endangered) because the EOO and AOO measures are at the low end of the EN scale (IUCN 2016). It is known only known from two localities (see Specimens examined) and despite repeated recent searches by F. Chiarini and colleagues, has not been found since its first description.
Discussion. Solanum homalospermum is partially sympatric with S. elaeagnifolium and S. mortonii. It is similar to S. elaeagnifolium in its narrowly elliptic or lanceolate leaves, but can be distinguished from that species by its porrect-stellate, rather than lepidote, pubescence and strongly andromonoecious breeding system with a single hermaphroditic flower at the base of the inflorescence. It differs from S. mortonii in its lanceolate, concolorous leaves, porrect-stellate rather than multangulate pubescence, and in its strongly deflexed fruiting pedicel. Solanum homalospermum occurs at (usually) lower elevations and to the east of the main distribution of S. mortonii (see Figs 8 and  Description. Shrub, 0.2-1.5 m tall. Stems erect, sparsely to densely armed, or unarmed; young stems densely stellate-pubescent; trichomes porrect, translucent and often reddish gold, sessile to subsessile, the rays 6 to 9, 0.1-0.3 mm long, the midpoints ca. 0.1 mm long, the prickles irregularly distributed throughout the plant, sometimes more dense on the calyx and pedicels, 1-10 mm long, 0.5-1.5 mm wide at base, straight, brown or reddish, sometimes yellow on the young stems, spaced 0.1-1 cm apart if dense, 1-10 cm apart if sparse; bark of older stems brown, glabrescent. Sympodial units usually difoliate, but not markedly geminate, sometimes plurifoliate. Leaves simple, variously lobed, 3-7(10) cm long, 1.5-3(5) cm wide, variable in shape, ovate to elliptic to broadly elliptic, 1.5-3 times longer than wide, mostly concolorous, drying yellowish green to green, densely stellate-pubescent adaxially and abaxially, the trichomes porrect, translucent, sessile to subsessile, the stalk up to 0.1 mm long, the rays 7 to 9, (0.1) 0.2-0.4 mm long, the midpoints 0.1-0.2 mm long; principal veins 3-6 pairs, raised abaxially, flat adaxially, spreading at ca. 45° to the midvein, the tertiary venation mostly not visible to the naked eye; base obtuse, rounded or truncate, sometimes oblique; margins variously lobed, rarely entire, the lobes 2-5 on each side, 0.1-1.5 cm long, usually rounded, rarely obtuse, the sinuses extending up to 1/3-1/2(2/3) of the distance to the midvein; apex rounded to acute; petiole 1-4 cm long, 1/2-1/10 of the leaf length, densely stellate-pubescent like the young stems. Inflorescence terminal or lateral, 4-7(15) cm long, usually unbranched but occasionally forked, with 4-9(26) flowers, usually with one hermaphrodite flower at the base, all distal flowers staminate; peduncle 0.5-2 cm long; rachis 1-6(9) cm long; pedicels 0.4-1 cm long, ca. 1 mm in diameter, filiform or apically dilated, densely or sparsely armed only in hermaphroditic flowers, unarmed in staminate flowers, articulated near the base; pedicel scars irregularly spaced 2-15 mm apart, prominent and brown. Buds strongly curved and zygomorphic, more so in staminate flowers; the corolla exserted ca. halfway from the calyx tube before anthesis. Flowers 5-merous, heterostylous, heterandrous and markedly dimorphic, the plants strongly andromonoecious, the basal flower long-styled and hermaphroditic, the distal flowers short-styled and staminate. Calyx tube 0.8-2 cm long, cup-shaped, the lobes 0.7-2.5 cm long, 1-2 mm wide, long-triangular with an elongate acumen, unarmed in staminate flowers, densely armed in hermaphroditic flowers, the prickles 0.5-1 cm long, 0.5-1 mm wide at base, yellow or reddish, straight, spaced 0.5-2 mm apart. Corolla 2-4 cm in diameter (slightly larger in staminate flowers), lilac or purple or occasionally white, the midvein paler and greenish yellow, drying yellow or brownish tan in herbarium specimens, stellate to broadly stellate, lobed ca. halfway to the base, the lobes 1-1.5 cm long, 0.8-1 cm wide, spreading or slightly reflexed at anthesis with abundant interpetalar tissue, the abaxial surfaces densely pubescent along the middle portions where exposed in bud, the adaxial surfaces glabrous or with a few trichomes along the margins, the tips somewhat cucullate. Stamens strongly unequal in both hermaphroditic and staminate flowers, with the 2 adaxial anthers shorter than the 3 abaxial anthers; filament tube minute; free portion of the filaments 1.5-2 mm long, glabrous; anthers tapering, yellow or occasionally purple (Sinaloa and Sonora), the surfaces smooth, poricidal at the tips, the pores about the same diameter as the anther apices, clearly delineated, directed distally, in hermaphroditic flowers three longer abaxial anthers ca. 9 mm long, two shorter adaxial ca. 6 mm long, in staminate flowers three curved abaxial anthers 15-20 mm long, two straight adaxial anthers ca. 10 mm long. Ovary in hermaphroditic flowers ca. 2 mm in diameter, conical. porrectstellate pubescent in the distal third, the trichomes sessile to subsessile, with 7-10 rays, the rays 0.1-0.2 mm long, the midpoints 0.1-0.15 mm long; ovary in staminate flowers vestigal; style 20-30 mm long in long-styled flowers, in staminate flowers ca. 1.5 mm long; stigma capitate and slightly bilobed in hermaphroditic flowers, indistinctly bilobed and much smaller in staminate flowers. Fruit a globose dehiscent berry, (1-)2-2.5 cm in diameter, pale green with darker green mottled areas when young, dark brown or orange-brownish when dry, dehiscing irregularly in several parts, usually breaking open irregularly in the upper half of the fruit, the pericarp thin, smooth, glabrous; fruiting pedicels 0.5-1.5 long, 2-3 mm in diameter at base, 2-5 mm in diameter at apex, erect, herbaceous to woody; fruiting calyx (hermaphroditic flowers only) strongly accrescent, the lobes covering up to the total length of the fruit, sparsely or densely armed with prickles up to 8 mm long. Seeds ca. 20-40 per berry, 3-4 mm long, 3-4 mm wide, flattened reniform, dark brown to black, the surface minutely pitted, the testal cells pentagonal. Chromosome number: n=12 (Averett and Powell 1972;Chiarini et al. in press).
Distribution (Figure 12). Solanum houstonii is endemic to Mexico, but widespread in south central Mexico on both coasts, from the Yucatán Peninsula and Veracruz to Sinaloa and Sonora where it is sympatric with S. hindsianum; from sea level to nearly 2000 m.
Ecology and habitat. Solanum houstonii grows in a wide variety of dry and semideciduous forests, from Sinaloan thorn scrub to the humid semi-deciduous forests of the Caribbean coast (selva mediana subcaducifolia) and in open situations such as coastal dunes. Like other members of the group, it can form large colonies, especially in disturbed ground.
The leaves of S. houstonii are used for cleansing after birth and as a dieting aid (Nee & Taylor 29629) and for skin diseases and problems (Rivera 90).
Preliminary conservation status (IUCN 2016). LC (Least Concern). EOO = 1,837,020 km 2 (LC -Least Concern); AOO = 1,052 km 2 (VU-Vulnerable). Based on its wide distribution S. houstonii is not of immediate conservation concern, but populations across Mexico exhibit significant heterogeneity in morphology that is presumably also reflected in genetic variability.
Discussion. Solanum houstonii has the most pronounced flower dimorphism in group, with differences in size and shape of both corolla and anthers. Staminate flowers are larger and exhibit stronger anther zygomorphy, while hermaphrodites tend to be smaller and with anthers of more equal size. The anther dimorphism in hermaphroditic flowers of S. houstonii is similar to that seen in both flower morphs of S. hind-sianum and S. homalospermum. On the coast of western Mexico S. houstonii grows in sympatry with S. hindsianum; it differs from that species in its larger fruit, more dimorphic flowers, more prickly calyx of long-styled flowers, and usually lobed leaves. Populations from around the bay of Topolobambo in Sinaloa are unusual in having deeply pinnatifid leaves, and individuals from northwestern Mexico are polymorphic for purple or yellow anther coloration.
This species was long known as either S. amazonium or S. tridynamum; Nee (1999) found that the name coined by Thomas Martyn (Miller 1807) as a replacement for the illegitimate S. carolinense Mill. had priority, thus S. houstonii becomes the correct name for this distinctive and relatively widespread Mexican species. Martyn coined his new name for this plant in honour of its collector, William Houstoun (with the original spelling 'houstoni'), who sent specimens and seeds of plants collected in western Mexico to Philip Miller at the Chelsea Physic Garden in London (Stearn 1974); many of Houstoun's plants were grown in the gardens and glasshouses of Chelsea. We select here as the lectotype the single specimen in BM (BM000514926) with labels in Houstoun's hand and that bears another in Miller's hand (the upper label); both of these feature verbatim in the polynomial description of S. carolinense Mill. (see image in Global Plants, http://plants.jstor.org/ -search for S. houstonii).
The protologue of S. tridynamum (Dunal 1814) cites no specimens, only two illustrations. One was an illustration in the collection of original drawings done for the Sessé and Mociño expedition to Mexico and Central America (see McVaugh 2000) and the other Dunal's own, unpublished illustration copied from this original, now held in the collections at MPU. Dunal is likely to have seen the former illustration when Mociño was in Montpellier between 1812 and 1817 (see McVaugh 2000: 12); this set of original illustrations was then copied for Candolle, but no images of Solanum species are held in the set of copies at G. Two illustrations in the Torner Collection of the Hunt Botanical Institute (accession numbers 6331.0674 and 6331.1883) both clearly show the distinctive flower morphology of S. houstonii. One is a sketch (6331.1883) upon which the other (6331.0674) showing the whole plant is based. The illustration made by Node-Veran and cited by Dunal in the protologue (MPU310713; Fig. 11) is apparently copied from these originals, but we select it as the lectotype of S. tridynanum because it is the only one of these elements we can be sure was seen by Dunal at the time he described the species.
Solanum amazonium was described from specimens cultivated at the Chelsea Physic Garden sent by Aylmer Lambert (Ker 1815); two sheets at Vienna (W-292940, W-292941) from the A.B. Lambert herbarium are possibly from the same introduction, but are not type material. The plate accompanying the description was prepared from live plants cultivated at Chelsea and is the only unambiguous authentic material that could be used as a lectotype. The protologue contains an excellent description of the anther dimorphism that differs in hermaphroditic and staminate flowers (Ker 1815). Solanum herbertianum was similarly described from cultivated material (Paxton 1838) of unknown origin that was grown in the Epsom Nursery in southern England. The plate is the only unambiguous material associated with the protologue and we select this here as  the lectotype. The plant depicted is completely devoid of prickles (like that in Ker 1815) and may have come from the same stock. In contemporaneous horticultural collections in W there appear to have been two distinct morphs of S. houstonii in cultivation in Vienna, one with (e.g., W-0001800) and the other without prickles (e.g., W-0001799).
The lectotype we have selected for S. obtusilobum is one of two sheets held in BR, where Martens and Galeotti worked. We have selected the sheet (BR00000523923) that was labelled (but not published) as lectotype by M. Nee in 1986. The other duplicate of Galeotti 1168 in BR (BR00000523971) is clearly labelled as "2 ème exemplaire" and is less complete. The protologue (Martens and Galeotti 1845) cites Galeotti 1168 from "montagnes cactifères et arides près Tehuacan de las Granadas" in today's state of Puebla. The sheet selected as the lectotype (BR00000523923) states only "Tehuacan"; other sheets labelled with the same number (Galeotti 1168) but with the locality "Cordillera de Oaxaca" are recognised here as isolectotypes, they are morphologically extremely similar to the sheets in BR.
Of the two varieties described in Dunal (1852) var. anoplocladum was based on plants cultivated in the gardens in Geneva and a specimen in the Candolle herbarium was cited, and var. stylosum was based on material from "Herb. Pavon" and is likely to be a collection made by Sessé and Mociño in Mexico (see  for a discussion of the distribution of Sessé and Mociño's collections by Pavón). This latter specimen (G00343357) could be related to the illustration used in the description of S. tridynamum, but does not exactly match it; it is probably from western populations with more deeply lobed leaves.

Solanum mortonii
Description. Erect rhizomatous shrub, to 1 m tall. Stems erect, woody, armed or unarmed; young stem densely stellate-pubescent, the trichomes multangulate, translucent, short-stalked, the rays 10-12, ca. 0.5 mm long; prickles if present 5-6 mm long, needle-like and straight, pale yellowish brown; bark smooth, brown or yellowish brown from persistent pubescence. Sympodial units difoliate, not markedly geminate. Leaves simple, (2-)4-9 cm long, (1-)2-4 cm wide, elliptic, ca. 3 times longer than wide, discolorous, drying yellowish green to greyish green; adaxial surfaces densely stellate-pubescent but the leaf blade tissue visible, the trichomes multangulate, translucent, sessile, the rays 10-12, ca. 0.5 mm long; abaxial surfaces more densely pubescent with similar multangulate trichomes; principal veins 5-7 pairs, impressed adaxially, flat abaxially, spreading at ca. 45° to the midvein, the tertiary venation usually visible in dry material; base truncate to slightly cordate, often somewhat oblique; margins shallowly lobed, the lobes 4-7 on each side, of varying sizes, becoming smaller towards the leaf apex, the sinuses extending only 1/4 or less of the distance to the midvein, triangular; apex acute to somewhat rounded, rarely obtuse; petiole 1-2 cm long, densely stellate-pubescent like the young stem, unarmed. Inflorescences terminal or lateral, to 6.5 cm long, with up to 10 flowers, unbranched; peduncle 1.5-3 cm long, densely pubescent with multangulate trichomes like those of the stems; pedicels 0.4-1 cm long, ca. 1.5 mm in diameter, robust, articulated less than 0.5 mm from the base, densely pubescent like the leaf blade; pedicel scars prominent, spaced ca. 0.5 cm apart. Buds turbinate, the corolla strongly exserted from the calyx tube prior the anthesis. Flowers 5-merous, strongly heteromorphic and the plants andromonoecious, only the basal flower perfect (hermaphroditic), the distal flowers functionally staminate and short-styled. Calyx conical or cup-shaped, the tube 5-6 mm long, strongly keeled, the lobes 7-10 mm long, ca. 1.5 mm wide at base, subulate, densely pubescent abaxially with multangulate trichomes. Corolla 3-3.5 cm in diameter, pentagonal, pale lavender, drying pale brown, barely lobed, interpetalar tissue abundant, the lobes ca. 0.2 cm long, ca. 0.1 cm wide, mere acumens, densely pubescent with multangulate trichomes abaxially along the midveins and surfaces exposed in bud. Stamens equal or very slightly unequal and the adaxial anthers slightly shorter; filament tube ca. 0.5 mm, free portion of the filaments ca. 2 mm; anthers 7-12 mm long, free, occasionally very slightly unequal, poricidal at the tips, the pores about which was financed by European Community Research Infrastructure Actions under the FP6 and FP7 "Structuring the European Research Area" Programme; Natural History Museum Special Funds awards; and the Royal Society through travel grants.