Morphology

Solanum species can assume a bewildering array of growth forms, from annual herbs to shrubs, trees, or woody vines. Many species of the Potato clade bear starchy underground tubers used as food by humans. Growth is by sympodia, where an inflorescence terminates an axis and further growth of the shoot occurs via axillary shoots from leaves proximal to (below) the inflorescence. Leaves of all Solanum species are alternate, but their phyllotaxis can be altered by complex growth patterns so that they may appear to be opposite or paired; this arrangement is commonly called 'geminate'.

Leaf blades can be simple, lobed, or compound, with leaf bases ranging from tapering to truncate or cordate. Species can be glabrous or pubescent, and pubescence can take many forms, with hairs simple (unbranched) or branched depending on the species. Stellate (star-shaped) hairs occur commonly in the Leptostemonum and Brevantherum clades, and branched hairs of various types are frequently found in the Dulcamaroid and Geminata clades as well as scattered throughout other subgroups of the genus. Prickles (often called spines) are a hallmark of the Leptostemonum clade. These most often occur on the stems, leaves, and calyx and can be straight or recurved, thin and needle-like or broad-based. Hair and spine morphology are diagnostic for many species and groups within Solanum and are very important characters in taxonomy.

An example of a compound leaf blade.	An example of a simple leaf blade.	An example of a spiny leaf blade.

Flowers of Solanum generally are actinomorphic with a 5-lobed calyx and corolla, 5 stamens, and a 2-carpellate superior ovary. Solanum anthers dehisce (open) by terminal pores, although in many species the pores elongate into slits with age. The filaments are usually very short, and in many species the anthers are connate into a tight cone around the style. This arrangement facilitates buzz-pollination by bees that cling onto the anther cone and discharge the pollen through the pores by vibrating their indirect flight muscles. Nectar is absent in Solanum flowers, with pollen as the sole reward in most species.

The flower of Solanum inelegans.	The flower of Solanum macbridei.	The flower of Solanum peruvianum.

The related genus Lycianthes also has 5-merous flowers with poricidal anther dehiscence, but the two genera can be distinguished by their calyx structure. Lycianthes has a truncate calyx, frequently with 5 to 10 linear teeth that emerge below the calyx margin. The calyx in Solanum is 5-lobed and lacks submarginal teeth.

Several species or clades of Solanum exhibit floral modifications. 4-merous flowers occur in some species, particularly in the Leptostemonum clade. The species of the tomato group (Solanum section Lycopersicum) have anthers with long sterile beaks that dehisce by elongated cracks rather than terminal pores. Species of Solanum section Pachyphylla (the former genus Cyphomandra) have a swollen area of tissue on the dorsal face of the anthers that functions as an osmophore to produce floral scents. Certain species in the Thelopodium, Normania, Dulcamaroid, Wendlandii, and Leptostemonum clades have zygomorphic flowers, unequal anthers, and/or enantiostylous flowers, in which the style is deflected to the right- or left-hand side of the flower. Floral zygomorphy has evidently evolved multiple times within Solanum, presumably to aid in pollen placement during pollination.

The generalized fruit in Solanum is a 2-carpellate berry, but fruit morphology can show great variation across the spectrum of the genus. Solanum forms assume several common forms:

1. soft, brightly-colored juicy berries (e.g., S. dulcamara),brightly colored relatively dry berries (e.g., S. pseudocapsicum),
2. soft greenish berries (e.g., S. mite),
3. hard greenish or yellowish berries (e.g., S. carolinense), or
4. dry berries often enclosed in an accrescent calyx (e.g., S. rostratum).

The fruit of Solanum mammosum.	The fruit of Solanum pseudocapsicum.	The fruit of Solanum scabrum.

Mature fruits can contain high levels of glycoalkaloids (as in many species of the Leptostemonum clade) or may virtually lack them and have a sweet or bland taste (e.g., Solanum section Solanum).