Persea

Persea is a genus of about 150 species of evergreen trees belonging to the laurel family, Lauraceae. The best-known member of the genus is the avocado, P. americana, widely cultivated in subtropical regions for its large, edible fruit. They are medium-size trees, 15–30 m (49.2–98.4 ft) tall at maturity. The leaves are simple, lanceolate to broad lanceolate, varying with species from 5–30 cm (1.97–11.81 in) long and 2–12 cm (0.79–4.72 in) broad, and arranged spirally or alternately on the stems. The flowers are in short panicles, with six small greenish-yellow perianth segments 3–6 mm (0.12–0.24 in) long, nine stamens and an ovary with a single embryo. The fruit is an oval or pear-shaped drupe, with a fleshy outer covering surrounding the single seed; size is very variable between the species, from 1–1.5 cm (0.39–0.59 inches) in e.g. P. borbonia and P. indica, up to 10–20 cm (3.94–7.87 inches) in some cultivars of P. americana. The species of Persea have a disjunct distribution, with about 70 Neotropic species, ranging from Brazil and Chile in South America to Central America and Mexico, the Caribbean, and the southeastern United States; a single species, P. indica, endemic to the Macaronesian islands, in the North West coast of Africa area, including Madeira and the Canary Islands; and 80 species inhabiting east and southeast Asia. None of the species are very tolerant of severe winter cold, with the hardiest, P. borbonia, P. ichangensis and P. lingue, surviving temperatures down to about −12 °C (10.4 °F); they also require continuously moist soil, and do not tolerate drought. A number of these species are found in forests that face threats of destruction or deforestation; for example, P. meyeniana in Central Chile. The family Lauraceae was part of the land flora of Gondwana, and many genera had migrated to South America via Antarctica over ocean landbridges by the time of the Paleocene. From South America they spread over most of the continent. When the North American and South American tectonic plates joined in the late Neogene, volcanic mountain building created island chains which later formed the Mesoamerican landbridge. Pliocene elevation created new habitats for speciation. While some genera died out in increasingly xerophytic mainland Africa, starting with the freezing of Antarctica about 20 million years ago and the formation of the Benguela current, others, which also reached South America and Mesoamerica, such as Beilschmiedia and Nectandra are still surviving today in Africa in a number of species. The genus, however, died out in Africa, except for P. indica, which is, today, a threatened species that survives in the fog-shrouded mountains of the Canary Islands. Fossil evidence indicates that the genus originated in West Africa during the Paleocene, and spread to Asia, to South America, and to Europe and thence to North America. It is thought that the gradual drying of Africa, west Asia, and the Mediterranean from the Oligocene to the Pleistocene, and the glaciation of Europe during the Pleistocene, caused the extinction of the genus across these regions, resulting in the present distribution.