The 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.

Thousands of plant species from throughout the world produce seeds with an attached fat body called an elaiosome. [Note: Some botanists refer to the fleshy attached structure on a seed as an aril.] The elaiosome is rich in lipids (elaio=oil) and may be variously shaped, depending on the species of plant. In some Acacia species, such as the Australian A. verticillata, it resembles a thickened, coiled stalk attached to the seed. According to S.N. Handel and A. J. Beattie (Scientific American Volume 263 (August 1990), elaiosomes attract ants, which carry both the seed and its elaiosome back to their nest. There the colony eats the nutritious elaiosome and discards the seed unharmed. This type of dispersal is termed myrmecochory from the Greek "ant" (myrmex) and "dispersal" (kore). The interaction does not sacrifice any seeds to predation and appears to be mutualistic to both the ant and plant. The plant benefits from the relationship because its seeds are dispersed to favorable germination sites, and are also planted (carried underground) by the ants.

In addition to nutritious food morsels gathered by ants, the elaiosome may elicit other innate behaviors in ants. For example, oleic acid is known to stimulate corpse-carrying bevavior (necrophoresis) in some ants. This may explain why the ants carry the elaiosome as well as the attached seed, instead of removing the seed which they apparently do not use.

Acacias belong to the legume family (Fabaceae), the third largest family of flowering plants, including three subfamilies, 630 genera and 18,000 described species. All three subfamilies produce typical legume seed pods that either split open or remained closed at maturity, but their flowers are quite different. Acacia blossoms are not pea-like, and for this reason the genus is placed in the subfamily Mimosoideae, along with silk tree (Albizia), fairy duster (Calliandra) and mesquite (Prosopis). The flowers consist of an inconspicuous calyx and greatly reduced or no petals, with numerous, showy stamens. Acacia flowers are clustered together in small yellow or white globose heads, or in cylindrical spikes. In some species (A. baileyana) the flower clusters are produced in spectacular yellow masses, and in others (A. farnesiana) they are very fragrant, attracting numerous insect pollinators. The latter species is a spiny shrub native to the southwestern United States and Mexico. The flowers contain a essential oil used for perfumery in France. At the San Diego Wild Animal Park it grows in the Baja Garden and along Kilimanjaro Trail.

One of the most intriguing taxonomic features of the genus Acacia is its divergence into two major groups with entirely different leaf types. One group has fern-like, bipinnate leaves subdivided into numerous minute leaflets. It includes hundreds of species throughout Australia, Africa and the Americas. Another group has "simple" leaves that are not divided into leaflets. The leaves of this group are called phyllodes, and they are actually expanded or broadened petioles (leaf stalks) which have lost the upper pinnate portion. Seedlings of this group produce the ancestral pinnate leaf, gradually replaced by phyllodes. Pruned branches of some species often develop phyllodes bearing bipinnate leaves at their tips. The phyllode group also contains hundreds of species distributed throughout Australia and the Pacific Islands. In fact, one of these species is the magnificent "koa" tree (Acacia koa) native to Hawaii. The following chart shows the vegetative divergence in the genus Acacia:

This simplified dichotomous flow chart is not a computer-generated, monophyletc cladogram. It simply shows how the traditional genus Acacia was subdivided into groups in order to key out different species. Some of these species with prickles and stipular spines have been removed from the genus Acacia and placed in new genera. The original 1350 species now comprise 5 genera, with 960 (mostly Australian) spp. still retained in Acacia.

Most members of the genus Senegalia differ from Acacia by the presence of prickles and the absence of phyllodes. A naturalized acacia in San Diego County called "sweet acacia" (A. farnesiana var. farnesiana) has been placed in the genus Vachellia. Members of the genus Senegalia can be distinguished from Vachellia by the absence of stipular spines and the presence of prickles. The origin of stipular spines vs. prickles is quite different, and molecular taxonomists have concluded that the separation of these species into separate groups is warranted. Some of the Acacia species mentioned in this article are now placed in separate genera.

A. Prickles of cat's claw acacia (Senegalia greggii). B. Stipular spines of sweet acacia (Vachellia farnesiana var. farnesiana). Unlike stipular spines at the bases of leaves, prickles arise from the cortex and epidermis of plant stems. The classic thorns of roses are actually prickles.

Many of the pinnate-leaf acacias produce sharp spines, although some Australian phyllode acacias also have spines. The spines typically occur in pairs and are actually modified leaf appendages called stipules at the base of the leaves. Since "spines" are modified leaf stipules, they are referred to as stipular spines. Technically thorns are modified stems, but most botanists use the names thorn and spine interchangeably. The paired spines can be two-three inches long in the fever tree (A. xanthophloea), or short and curved in the knobthorn (A. nigrescens). Some species, such as the cats-claw (A. greggii) also produce thorns or prickles that arise directly from the bark as in roses. Cat's claw is also called "wait-a-minute bush" because clothing readily becomes entangled in the hooked prickles. This interesting shrub is native to the Colorado Desert of North America and also grows in the Baja Garden at the San Diego Wild Animal Park.

The "swollen-thorn" acacias of Central America and South Africa are truly remarkable trees. In the wild, their enlarged, hollowed-out stipular spines are occupied by fiercely biting-stinging ants that protect them from browsing herbivores and epiphytic plants that might shade them out. The swollen thorns are not galls, they are not produced in response to chemical or physical stimuli from invasive insects imbedded in their tissues. The amazing thorns are genetically-programmed structures that are formed with or without the presence of symbiotic ants.

Paired thorns of the Central American bullhorn acacias (Acacia cornigera and A. collinsii) resemble the horns of steers. They are commonly strung into unusual necklaces and belts, and in El Salvador, provide the legs of small ballerina seed dolls which can be worn as decorative pins. On wild trees they are inhabited by colonies of Pseudomyrmex ferruginea, very aggressive ants with a painful sting. Disturbed ants release an alarm pheromone and rush out of their thorn "barracks" in great numbers. According to Daniel Janzen (Costa Rican Natural History, 1983), livestock can apparently smell the pheromone and avoid these acacias day and night. In addition to protecting A. collinsii from leaf-cutting ants and other unwanted herbivores, the ants clear away invasive seedlings around the base of the tree that might overgrow it and block out vital sunlight. According to Dr. Janzen (Smithsonian Contributions to Botany Number 13, 1974), the Central American swollen-thorn acacias lack the chemical defenses of most other acacias to deal with their predators and competition, and symbiotic ants have taken over this vital role. For example, A. cornigera is apparently unable to synthesize bitter alkaloids that would normally discourage ravaging insects. This competitive disadvantage is particularly true in wetter areas where competing vegetation grows very rapidly. The acacias reward their ant helpmates with thorn "condos" to live in, carbohydrate-rich nectar from glands on the leaf stalks, and nourishing, protein-lipid Beltian bodies on the leaflet tips. There is no known function for Beltian bodies, except to provide food for symbiotic ants. Although they attack bird nests initially, the ants may grow accustomed to nests of orioles and kiskadees, and may even protect the nests from climbing predators.

Prior to settling on a thorn acacia, the winged virgin queen ant goes on a mating flight to the highest treetop or nearby hill. Here she gets inseminated by a winged male and then hunts for an acacia in which to lay her eggs. This "hilltopping" phenomenon also occurs in San Diego County, where sexually mature male and female butterflies fly upslope to a rendezvous point at the summit. The queen ant cuts an entrance hole into a green thorn, hollows it out and then deposits her eggs. Subsequent entrance holes are cut by the new generation of worker ants.

The whistling thorn of Kenya (Acacia drepanolobium) has bulbous thorns occupied by stinging ants of the genus Crematogaster. Like the Central American acacias, the ants hollow out the soft green thorns for living quarters. Giraffes and other herbivores normally eat thorny acacia foliage, but leave the whistling thorn alone. In fact, for most South African acacias, spines alone are no deterrent to giraffes. They simply reach high into the branches and carefully wrap their long prehensile tongue around the tender foliage, carefully avoiding the vicious spines. At the San Diego Zoo you can watch a giraffe delicately removing tender leaves from the winter thorn (A. albida) near the edge of its enclosure on Horn and Hoof Mesa. Their feeding actually produces the characteristic flat-topped shapes of many acacias on the vast African savannahs. In the case of the whistling thorn acacia, a mouthful of stinging ants is an excellent deterrent to hungry herbivores. Old thorns which have been vacated by the ants make a whistling sound as wind blows across the entrance hole.

The San Diego Wild Animal Park has some marvelous examples of African thorn acacias. A splendid knobthorn acacia (Acacia nigrescens), also listed in some references as A. pallens, grows in the planter in front of the Administration Building. The trunk is covered with large, woody knobs each tipped with a sharp, curved thorn. The knobthorn is a common tree in Kruger National Park and produces a dark, heavy "ironwood" that sinks in water. Its density rivals the "quebracho" (axe-breaker) Tabebuia serratifolia of South America. At the San Diego Wild Animal Park, beautiful specimens of the fever tree (A. xanthophloea) grow at Thorn Tree Terrace and around Mombasa Lagoon. This acacia is unmistakable with its greenish-yellow bark and long, white stipular spines. Its native range is the lowland savannahs of South Africa and northward, often in swampy surroundings. Because its range coincides with regions where malaria is prevalent, it is called "fever tree." Some people actually believe that it may transmit malaria, but we know the vector is a mosquito.

Another common acacia at the San Diego Wild Animal Park is the paperbark (A. woodii), so named because the bark flakes off in papery sheets. In South Africa this is a striking indigenous tree with a large, flat-topped crown. In Natal it is known as "camel thorn," although that name applies to another species A. giraffae. Fever trees and paperbark acacias can also be seen at the Heart of Africa, along with dark-trunked mesquite trees (Prosopis glandulosa). West of the Trading Village is a small whistling thorn acacia, complete with its bulbous stipular spines, but without its stinging ants. This remarkable acacia also grows near the exit gate at the San Diego Zoo.

Phyllode acacias are common at the San Diego Zoo and Wild Animal Park, especially the Australian Rain Forest and Horn and Hoof Mesa. They include dense hedges (Acacia longifolia and A. veticillata), ground covers (A. redolens), black acacia shade trees (A. melanoxylon), everblooming A. retinodes, silvery weeping acacia (A. pendula), pearl acacia (A. podalyriifolia) and the unmistakable knife-leaf acacia (A. cultriformis). In Australia these fast-growing trees and shrubs are called "wattles." In their native habitats, some of them become huge forest trees over 100 feet tall with trunks four feet in diameter, including the black acacia and A. bakeri. Some of these huge phyllode acacias provide beautiful hardwood lumber for furniture, cabinets, boats and musical instruments. Wood of the Hawaiian koa is well known for the finest ukeleles and hand-carved canoes. Some Australian wattles have bipinnate leaves, such as the spectacular flowering A. baileyana and the beautiful silver wattle (A. dealbata).

When acacia stems are cut or injured, they often exude a clear, gummy substance which helps to seal the wound and prevent infection by fungi and bacteria. Chemically, this natural gum is a water-soluble polysaccharide composed of many sugar molecules joined together. Natural plant gums are used as thickening agents and emulsifiers in food products, cosmetics and pharmaceuticals. Gums produce the thick, rich consistency of sauces and syrups, prevent the formation of ice crystals in frozen desserts, and stabilize the foaming in beer.

Natural Gum Exudate

Jelly Beans Containing Gum Arabic

One of the most important natural plant gums is gum arabic, obtained from the spiny, shrubby Acacia senegal of northeastern Africa. In addition to its use in foods, hand lotions and soaps, it is used in fine water colors, inks, flavoring syrups of beverages and confections. If you enjoy soft drinks, marshmallows and hard gummy candies, then you undoubtedly have consumed gum arabic. It also produces the water-soluble adhesive on postage stamps and the "lace curtain" on the sides of your beer glass. Other spiny African acacias have been tapped for their gums, including A. nilotica and A. seyal. The latter species (also called shittim) is especially interesting because biblical scholars believe its wood was used to construct the Ark of the Covenant.

During the 1990s, news sources implicated the terrorist leader Osama bin Laden as a stock holder in a profitable Sudanese company and major world producer of gum arabic. Later reports confirmed by the State Department said that bin Laden divested himself of the holdings in 1996 when Sudan ordered him out of the country. Based on the current evidence (2004), waging a personal war on terrorism by boycotting products containing gum arabic is irrational and could harm innocent companies. Avoiding soft drinks and gummy candies for health reasons or to lose weight probably makes more sense.

Acacias are one of the largest and most diverse groups of trees and shrubs on earth. From picturesque silhouettes on African grasslands and lovely Australian shade trees covered with blossoms, to spiny, fragrant shrubs of the Colorado Desert, acacias are truly unforgettable.

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