Proteas are evergreen trees and shrubs belonging to the dicotyledonous plant family Proteaceae and, in particular, to members of the genus Protea. They grow mostly in dry regions of the southern hemisphere, especially in Australia and South Africa. The family is divided into five subfamilies, 75 genera, and 1,350 species.
The Proteaceae are distinguished from closely related families by having one stamen attached to the center of each of four petals, seeds attached to the wall of the fruit, and flowers often aggregated into heads and enveloped by large densely hairy or showy bracts. The flowers of many species are pollinated by birds, bats, and small marsupial mammals.
The species of Proteaceae have two important adaptations to the dry habitats in which they grow. First, their leaves are thick and hard, a condition called sclerophylly. This prevents moisture loss and decreases damage should wilting occur. Second, their roots are clumped and very thin for efficient absorption of water and mineral nutrients. These special roots, called proteoid roots, lack the symbiotic mycorrhizal fungi found in the roots of most other plants.
Because the Proteaceae occur naturally only in the southern hemisphere, it is believed that the family originated on the ancient supercontinent of Gondwana. During the early Mesozoic Era, this continent was formed by the union of South America, Africa, Antarctica, India, and Australia-those continents where the family is found today. Until recently, these continents have been separate from the northern continents of North America, Europe, and Asia. For this reason, the family is not found naturally in the northern hemisphere.
The Proteaceae contain several economically important species. The Macadamia nut (Macadamia integrifolia) is considered by many people to be the most delicious nut in the world and consequently is one of the most expensive. It is native to Australia but primarily cultivated in Hawaii and southern California. The showy flower clusters of many species of Proteas are sold in the florist trade. The most important species (Protea cynaroides) comes from South Africa and has long-lasting cut flowers with heads to 8 in (20 cm) across. The silk-oak (Grevillea robusta), native to eastern Australia, is a commonly cultivated ornamental in California and the southern United States; it has become naturalized in waste places in Florida.