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he genus Acacia is well-represented in southern California parks and gardens with dozens of different species. In fact, it is one of the largest genera of trees and shrubs in the world, with nearly 800 species. It is rivaled in size by only a few woody genera, such as Ficus (1,000), Eucalyptus (500) and Cassia (500). Acacias are commonly cultivated throughout temperate, tropical and desert regions, and include numerous distinctive growth forms with a profusion of yellow or white flower clusters. Acacias also provide us with many useful products, from beautiful hardwoods to water-soluble gums used as a thickening agent in your favorite frozen dessert. In the wild, native acacias provide the food and habitats for a variety of animals, from hoofed mammals and birds to countless species of insects. Extensive browsing by giraffes can prune the understory branches of acacia trees. The characteristic flat-topped crowns of some species are genetically determined. For example, the widespread umbrella thorn (Acacia tortilis) on vast African savannahs is adapted to this environment. Apparently the exaggerated umbrella-shaped crown enables the trees to capture the maximum amount of sunlight, with the smallest of leaflets. Swollen thorn acacias of Central America and South Africa form one of nature's most remarkable symbiotic associations with ants.
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