Scientists are fascinated by rain forests. They are one of the Earth's last areas about which they have inadequate knowledge. Inquisitive travelers also find them fascinating — they offer one of the last opportunities to explore areas about which much is yet to be discovered.


THE FOREST'S FOUR LAYERS

Rain forests are complex and incredibly diverse. Life in one divides into four distinct layers.

The main layer, and one of the rain forest's distinctive features, is the canopy. Most forest trees grow close together, rising about 100 feet (30 meters) from the ground. Their trunks are mostly free from limbs. But on the upper 20 feet (6 meters) branches grow out almost horizontally, giving the forest a flattened crown. This canopy of treetop branches and leaves is so thick and continuous that it blocks out the sun. Less than 2 percent of the sunlight falling on the canopy filters through it.

On the light-starved forest floor, another layer, few plants grow. Just below the canopy, though, mature, low-light tolerant trees make up a layer called the understory. The fourth layer is composed of scattered trees shooting above the canopy for another 50 or so feet (15 or so meters).

This habitat is not called a "rain" forest for nothing. At least six feet (1.8 meters) of rain falls each year. In some forests up to 25 feet (7.6 meters). Intense sunshine, high humidity and year-round temperatures ranging between 70 degrees F. (21 degrees C.) and 90 degrees F. (32 degrees C.) are also typical.


WHERE TROPICAL RAIN FORESTS ARE LOCATED

Rain forests hardly change. They are located in a belt around the equator. So there are no seasons. The length of a day remains almost the same year round. Daytime itself in the forest is merely an adjustment from dark to twilight. Rain fall amounts change little within a particular forest. And the forests themselves have not changed in the tens of millions of years they have filled the tropics.

The world's major forests are located in three areas: South America's Amazon basin, which occupies about 40 percent of the continent; equatorial Africa, especially the Congo basin; and Southeast Asia, including the Indonesian islands and New Guinea. Less-extensive forests grow in Central America and along Australia's Queensland coast.

Rain forests differ depending upon their location. The forest canopy in Asia is less dense than the Amazon's, so undergrowth on the forest floor is thicker. And some plant and animal species are unique to particular forests. The African sets of the old Tarzan movies would often neglect differences and mix in Asian reticulate pythons, South American capuchin monkeys and generous helpings of Asian and American plants.


RAIN FOREST OR JUNGLE

The old Tarzan movies took place in the jungle. A few decades ago you only heard about jungles, not rain forests. Today you don't hear much about jungles, just rain forests.

Of course there were rain forests then. And today there are still jungles. We're just playing a name game.

The name "rain forest" generates a more favorable image than does the term "jungle." And if you are trying to gain support to promote preservation of tropical forests, which since World War II have been cut down at an alarming rate, a rain forest sounds worth saving, not so a jungle.

Actually, though, the two terms are not interchangeable. There is a difference. A jungle is made up of thick, dense, tangled vegetation at the ground level. The rain forest's thick canopy, on the other hand, keeps the forest floor relatively plant free.

Jungles and rain forests are usually neighbors. Jungle takes over along the edges of cleared areas or alongside rivers, where sunshine reaches the ground to spur the thick vegetation growth.


FOREST DIVERSITY

Rain forests represent the world's most lush, diverse and interdependent environment. While they cover just six percent of the Earth, they are home to, depending on who's counting, between one-half to two-thirds of all known plants and animals. Literally millions of species live in the rain forests -- and researchers are discovering more all the time.

One of the rain forest's mysteries is why such a diverse proliferation of life thrives there. Scientists are still working on the answer.

Diversity is what makes the rain forest so valuable to scientists, especially in the area of medicine. About 25 percent of medicines contain compounds originally derived from rain forest plants.

In the film Medicine Man, Sean Connery's character called the rain forest canopy in which he was searching for a cancer cure the "top shelf of the pharmacological superstore."

And in real life, scientists have reported leaves from a vine discovered in a Cameroon, West Africa, rain forest contain a chemical that blocks the AIDS virus from reproducing in the test tube.

Just how diverse is a tropical rain forest?

• Researchers identified 283 tree species on a single 2.5 acre (1 hectare) plot in Peru.

• A single tree in the Amazon was home to 43 ant species.

• California has some 5,000 flowering plants. The rain forests of Ecuador, one-third California's size, have 20,000.

• Some Amazon regions contain 1,200 types of butterflies.

• A single tree in Panama's rain forest yielded 950 beetle species.


DEFORESTATION

In 1950 rain forests covered 15 percent of the earth. Today, only a shrinking six percent. The causes of deforestation are tied to Third World economic and demographic problems. Among the reasons for rain forest loss are:

• Meat production. To boost their economies, countries (especially Costa Rica) clear land for cattle pasture so they can become meat producers and exporters. The nutrient-lacking soil, however, can only support cattle for a year or two without heavy use of fertilizers, so old pastures are abandoned. More rain forest is then cut for replacement pastures.

• Demand for wood. Global demand for new housing, furniture and paper is rising. As temperate forests become exhausted, the logging industry is turning increasingly to tree-rich rain forests.

• Population explosion. Most developing countries are undergoing a population explosion. As more people crowd the land, forests are cut for firewood and to provide land for farms.


EXPLORING THE FOREST

You'll find exploring a rain forest -- examining processes, identifying and even just sighting plants and animals -- difficult, because much life and activity takes place in the canopy. This has also frustrated researchers who have had to resort to examining the tops of felled trees or peering at the canopy through binoculars from a nearby high ground.

Recently, however, scientists have become creative in gaining canopy access. They have used mountain-climbing ropes and gear, towers, gondolas suspended from construction cranes and nets suspended from dirigibles.

They have also constructed walkways in canopies. Tour companies have followed their lead and built walkways in the Amazon to give travelers canopy access.

But even if you can't visit the canopy, there are still discovery opportunities.


TREES

The trees in a tropical rain forest are broadleaf, meaning the leaves are wide in relation to their length. Most of the world's broadleaf trees are deciduous, losing their leaves in winter. Not so, though, in the single-season rain forest. Here deciduous trees flower, bear fruit and shed leaves at random.

Leaves also tend to have a slick surface and taper at the end. This allows rapid runoff during the heavy rains, preventing the leaves from becoming waterlogged or even breaking off.

Despite a vast variety of tree species in the forest, their trunks and leaves do not differ significantly. To distinguish types, examine their flowers which do vary greatly. You will have to have canopy access, though. Flowering typically occurs at the canopy level.

One easily spotted tree feature is the buttressed trunks of the larger trees. Rain forest soil is so shallow many trees need the additional support the buttresses give them.

Another observable feature is the strangler fig trees growing on a host tree. Strangler figs are common. They sprout in the canopy from seeds dropped by bats and birds who have eaten their fruit. The figs send roots down the outside of the host tree to the ground.

The strangler fig lives on the host, but it is not a parasite. Nonetheless, eventually it will probably "strangle" the host tree. The fig's branches and leaves compete with the host for sunlight. Its roots compete for nutrients. Over the years, the fig usually wins.

You may be able to spot a dead host tree, standing upright, still wrapped in the embrace of the strangler's roots.

Among the more common trees found in the world's tropical rain forests are ceiba, mahogany, Pará nut and oil palm. The Amazon also has giant kapok trees that tower 30 feet (nine meters) or more above the canopy.


SOIL

Looking at a rain forest you can't help get the feeling that its soil must be the planet's most fertile. Its lush appearance contributes to its destruction by would-be farmers and cattle ranchers with visions of abundant crops and pastures.

But the soil is actually shallow and nutrient poor. So poor that once the forest is cut, the soil can only support crops and pasture for a couple of years without massive doses of fertilizer.

The rain forest's lushness derives from nutrients at the soil's surface. Heavy rains, high humidity and high temperatures lead to rapid decay of leaves and limbs that fall from the canopy to the floor. But the fast rate of decay keeps humus, the dark-colored, nutrient-rich upper soil from forming. And to be fertile, soil needs humus.

The heavy rains also wash out the soil's nutrients. In a process called leaching, the rain water dissolves nutrients and takes them deep into the ground as the water percolates down through the soil. At the surface only iron and aluminum oxides remain, giving tropical soil its reddish color.

The forest itself is actually growing on its own decaying matter. In addition to their soil-bound roots, some trees send out secondary roots to take in the nutrients found in the decay on the forest floor. Using this nourishment, the trees grow and produce more leaves and branches that will fall to the forest floor to begin the cycle again.


UNDERGROWTH FLORA

What little undergrowth vegetation develops tends to grow poorly. Epiphytes and lianas are common. Epiphytes such as ferns, mosses and lichens grow on other plants, but use them only for support not food. They obtain nourishment from the air and the moisture around them.

Lianas are rain forest creeping vines. They wind around and climb trees to get to the canopy and its sunshine. Examine some of the lianas you see. They use different methods for climbing. Some use a small tendril that wraps around its host like a corkscrew. Others use a three-pronged tendril to grab the bark. Still others use spines to cling to a tree.

One ground plant you may recognize is the "Monstera" -- some of its family members are sold as house plants. When a Monstera sprouts it grows toward the dark rather than the light. This leads the plant to the deep shadows around tree trunks. Once it attaches to a trunk it reverses its orientation and grows upward, toward the light in the canopy.


CANOPY FLORA

Among the most common rain forest plants are orchids and bromeliads which are also epiphytes. They thrive high in the canopy where they can get the sunlight. Seedlings take root in decaying vegetation or on leaves in small deposits of wind-borne dirt.

Bromeliads house their own little ecosystems. Their leaves form a water-holding cup at the base. Mosquitoes and other insects lay their eggs in the cup's water. Some frogs place their tadpoles into these miniature ponds.


ANIMALS, BIRDS AND INSECTS

Most rain forest explorers find that animal life tends to be elusive. Unless you make a determined effort, patiently waiting and exploring, you will probably not see any animals. If you are lucky, you might spot some monkeys. Insects and birds will give you the most viewing opportunities. Of course, insect life will be plentiful.

You are much more likely to hear the animal life rather than see it. Monkey and bird calls can be frequent. Occasionally you will hear an animal crashing through the branches overhead. Most animals live in the canopy. Many spend their entire lives in the canopy, never making their way to the ground.

The camouflage used by many creatures also makes viewing difficult. Some are so well camouflaged that you won't notice them unless they move. A mantis may resemble a piece of tree bark. Some grasshoppers appear to be leaves. Other insects take the appearance of dead leaves. Frogs are typically mottled in greens and browns so they will blend easily.

Among the more common animal life in the rain forests are: deer, tapirs, sloths, spider and howler monkeys, iguanas, snakes, tree frogs, toads, spiders, caterpillars, butterflies, harlequin beetles and -- one of the forest floor's deadliest predators -- army ants. Bird life includes: curassows, eagles, hornbills, macaws, parrots and toucans.

African ran forests include some spectacular wildlife in the form of mountain gorillas.