The species of Prosopis belongs to the subfamily Mimosaceae in the family Leguminosae. They can be distinguished from other genera in the subfamily, by (1) the more or less fleshy pod that does not open to release the seeds when it matures (indehiscent), and (2) the characteristic of releasing pollen in single grains rather than in clusters (as in the case with most members of the Mimosaceae family). They share the common features with other members of the family of having small flowers clustered into inflorescences, pinnately compound leaves with stipules, and regular flowers with a five-parted corolla.
Throughout the genus, there is great variation in the origin and form of the spines which are important taxonomic features. Some species are spineless, while others include individuals that have occasional spines. The spines on some species are modified axillary branches and can occur singly or in pairs. On other species, they may arise from hardened leaf stipules which are always paired. Some species that are not native to Mexico, Chile or Peru have spines developed from modified branch tips, while others have spines that arise as prickels formed by localized cells scattered along the branches.
The leaves of all Prosopis species are compound but have leaflets that vary in shape, in the spaces between them, and in size. Inflorescences can be ball-shape, stubby cylinders, or densely flowered catkins or spikes.
There are 40, New World species of Prosopis distributed in the arid and semi-arid areas of North and South America. In comparison, there are only four Old World species, found in Africa and Asia. Nine species are native to North America, including Mexico. Two of the Mexican species also occur in South America. Therefore, 31 species of Prosopis are endemic to Central and South America and the total number of Prosopis species that occur there naturally, is 33.
There is great morphological diversity in the South and Central America species of Prosopis, and those same morphological characteristics are also often found in the North American species. This has been cited as evidence that Prosopis is most probably of South American origin, and has spread from South to North.
Prosopis is an old genus (as indicated by the many distintive groups of species that have developed and the frequency of hybridization within these groups) that diverged early into several lineages from which a great variety of forms and a high degree of specialization have arisen.
