The variety of fascinating plants at Palomar College never ceases to amaze me. Here is another remarkable tree that campus horticulturist Tony Rangel recently showed me near the campus greenhouse. Tony is a horticultuist extraordinaire and is very modest. In fact, he said: "the Banana fig was donated to us by Tim Hoehn-Boydston. Actually he and Seth Menser, the San Diego Zoo, along with a few others have donated much to our Ficus collection in recent years. We really owe so much of what we have to them."

The banana fig Ficus pleurocarpa is endemic to NE Queensland, Australia, from Cape Tribulation south to the Atherton Tableland. It grows in lowland and upland wet tropical rainforests from sea level up to 1,000 m (3,300 ft) above sea level. It is a monoecious species with separate unisexual male & female flowers on the same tree. The flowers are produced inside structures called syconia. Unlike most figs with round or oval syconia, the syconia of banana figs are elongate and superficially resemble small bananas. Its life begins as a hemiepiphyte. Seeds are deposited in the excrement of birds & bats high in the branches of rainforest trees. Germinated seeds develop adventitious roots that grow to the ground. Eventually the tree develops into a strangler fig that envelops it host tree. See my discussion of strangler figs later on this page.

Banana figs depend on symbiotic fig wasps for pollination, and the wasps can only reproduce within the fig flowers. They truly depend on each other for the perpetuation of their species. Many animal residents of the rainforest depend on the fleshy, nutritious fig syconia for food, including birds, bats and marsupial mammals, such as musky rat-kangaroo and flying-foxes. There are many excellent references on the Internet that describe the ecology of banana figs.

As I stated in a Facebook note, the coevolution of more than 800 species of figs, each with their symbiotic wasps, is the most amazing story of my biology career. It is difficult to summarize in a lecture because of its complexity. In fact, oversimplification by authors has contributed to errors in their publications.

Unlike most species of figs that I studied, Ficus pleurocarpa is pollinated by two species of fig wasp, Pleistodontes regalis and P. deuterus in the same syconium. The assumption that fig species are usually pollinated by just one species of fig wasp has been challenged by the discovery of species like the banana fig. We also have Pleistodontes froggatti in the Edwin & Frances Hunter Arboretum that pollinates the Moreton Bay fig (F. macrophylla) and P. imperialis that pollinates the rustyleaf fig (F. rubiginosa) on campus. Without pollination there will be no viable seeds in the syconium and no seedling trees. When I taught general biology in the old science building I climbed to the roof top and collected branchlets of the rustyleaf fig during wasp mating season. My students were fascinated and this turned out to be the best lab class of the semester!

All figs rely on tiny symbiotic wasps for their pollination. Minute male and female fig wasps are borne inside hollow, fleshy, flower-bearing structures called syconia. The syconium is what most people associate with the tasty fruit of a fig, but technically it is not a true fruit. The syconium is lined on the inside with hundreds of tiny male and female flowers. The flowers are greatly reduced and do not have petals. Male flowers consist of pollen-bearing anthers. Female flowers are of two types: Long-style, seed-bearing flowers and short-style flowers that bear fig wasps instead of seeds. Wasp eggs are not laid in the ovaries of long-style flowers because the wasp's ovipositor cannot reach the ovary.

It is interesting to speculate on how these symbiotic wasp species were introduced at Palomar College. Were wasp-bearing syconia already present on the original plantings, or were wasps carried here by the wind from trees in neighboring cities? The Moreton Bay fig is native to eastern Australia where it grows in the rain forest and often starts out as an epiphytic vine that develops into a strangler fig. Eventually it shades out and kills its host, becoming a massive tree with a buttressed trunk and huge spreading surface roots. In Hawaii and northern New Zealand it has become naturalized because of the introduction of its pollinator wasp (Pleistodontes froggatti). In Hawaii the wasp was deliberately introduced in 1921. In New Zealand it was first recorded in 1993, having apparently arrived by long-distance dispersal from Australia, a 3,000 km (1900 mile) journey. According to R.O. Gardner and J.W. Early (1996), adult female wasps usually live 2-3 days; there are an average of 21 days over the course of a year during which the wasps could make this trip in 1-3 days on air currents. Fig wasps have been caught in light traps aboard ships up to 99 km (60 miles) offshore in the Pacific (J.C. Harrell and E. Holzapfel, 1966). Ahmed, at al. (2009) reported pollen transfer by the African fig wasp Ceratostolen arabicus between known host trees of Ficus sycomorus separated by a distance of 160 km (100 miles). Based on these records for long distance dispersal, reaching Palomar College in San Marcos from known wasp-bearing trees in coastal San Diego County is certainly plausible. Once in the vicinity of host trees, they are attracted to the correct syconia by mixtures of species-specific chemical attractants, including volatile terpenoids. For example, receptive syconia of F. hispida release blends of three fragrant monoterpenes, including linalool (major constituent), limonene and pinene (C. Chen and Q. Song, 2008).

In their native tropical habitats, many figs are called "stranglers." Seeds germinate high on the moist branches of rainforest trees, sending numerous aerial roots to the ground. The sticky seeds are dispersed by a variety of fruit-eating birds and bats. Like botanical boa constrictors the serpentine roots gradually wrap around the host's limbs and trunk, crushing the bark and constricting vital phloem and cambial layers. The network of roots, resembling a tangle of writhing snakes, also fuse together (anastomose) forming a massive woody envelope or "straightjacket" encircling the host. Expansion of the host trunk as it grows in girth may accentuate the death grip and subsequent girdling process. Eventually the host tree dies of strangulation and shading, and the strangler fig stands in its place. In many cases the host tree may actually succumb from shading and root competition rather than strangulation. When strangler figs start in the ground, as in cultivation, their trunks develop from the ground upward like other "conventional" trees.

I have seen many strangler figs on field trips to Mexico and central America. One of the most photogenic was a massive strangler fig (Ficus cotinifolia) in Yucatan, Mexico. Actually, several figs on campus, such as Moreton Bay fig (Ficus macrophylla) are stranglers in their native rainforests of Australia.

In spite of their sinister common name, strangler figs are one of the most important components of tropical forest ecosystems. During the day hundreds of animals feed on the sweet fruits, including many species of pigeons, parrots, hornbills, toucans and monkeys. As night falls, the day foraging shift retires and flocks of fruit-eating bats descend upon the branches. Fig trees typically produce three or more crops of fruit a year, providing food throughout the year when other sources are in short supply. The fleshy, juicy fruits are full of small seeds which readily pass through the digestive tract of animals. In fact, the purgative effect of fig fruits on the bowels encourages its seeds to be widely dispersed--good strategy for an epiphytic opportunist. In addition to the prodigious fruit source, hundreds of animal species make their homes in the hollow trunk where the strangler fig has enveloped the decayed host tree. The cavities provide housing for myriad creatures, including geckos, frogs, anolis lizards, bees, wasps, beetles and ants. These trunk-dwellers in turn provide an additional food source for higher levels of the fig food web.

Although several species of wild figs are native to southern Florida and Baja California, fossil evidence indicates that strangler figs once grew wild in California about 40 million years ago, in a tropical forest much like present-day Yucatan. Today they are represented by several spectacular Asian and Australian trees originally planted by farsighted gardeners with a vision, almost a century ago. With a warming climatic trend during the next 10,000 years, accompanied by an increase in humidity and rainfall, the progeny of some of these exotic species quite possibly could assume their inherent epiphytic growth habit in southern California. Alien strangler figs in California might prove to be a horticulturist's nightmare, as they are today in some areas of southern Florida.

Many tropical figs develop slender aerial roots hanging from the branches; however, in true banyan-type strangler figs enlarged aerial roots extend from the branches to the ground, giving the tree the unusual appearance of being supported by pillars. By this manner of growth the tree is able to spread outward almost indefinitely, and many Indian banyans (Ficus benghalensis) are of immense size and very old. One of the largest trees on record grew at the Calcutta Botanic Garden. It is listed in the Guinness Book of World Records (1985) as the world's largest (spreading) tree crown, with 1,000 prop roots and covering an area of four acres. The canopy of some banyans provides shade for entire villages. Alexander the Great reportedly camped with an army of 7,000 soldiers under a banyan. The Hindus regard the banyan as sacred, for it is said that Buddha once meditated beneath one. Probably the most revered tree in the world is F. religiosa, the sacred bo tree of Burma, Ceylon and India. It is said that Buddha sat under its shade for six years while he developed his philosophy of the meaning of existence. The striking heart-shaped leaves of the bo tree tremble in the slightest breeze like a cottonwood--a legendary tribute to the divine meditations of Buddha. The English name "banyan" comes from the "banians", or Hindu merchants who set up markets in the shade of these enormous trees.