The overwhelmingly dominant feature of the Amazon basin is the tropical rain forest, or selva. From the air the Amazon forest appears to stretch unbroken to the horizon like a tufted green carpet. Closer inspection reveals its bewildering complexity and prodigious variety of trees; as many as 100 arboreal species have been counted on a single acre of forest with hardly any one of them occurring more than once.

The Amazon forest has a strikingly layered structure. The sun-loving giants of the uppermost story, the canopy, soar to as much as 120 feet above the ground; occasional individual trees, known as emergents, rise beyond the canopy, frequently attaining heights of 200 feet. Their straight, whitish trunks are splotched with lichens and fungus. A characteristic of these giant trees is the buttresses, or basal enlargements of their trunks, which presumably help stabilize the top-heavy trees during infrequent heavy winds. Further characteristics of the canopy trees are their narrow, downward-pointing "drip-tip" leaves that easily shed water and their cauliflory (the production of flowers directly from the trunks rather than from the branches). Flowers are inconspicuous. Among the canopy species, which capture most of the sunlight and conduct most of the photosynthesis, prominent members include the rubber tree (Hevea brasiliensis), the silk-cotton (Ceiba pentandra), the Brazil nut (Bertholletia excelsa), the sapucaia (Lecythis), and the sucupira (Bowdichia). Many creatures, including monkeys and sloths, spend their entire lives in this sunlit canopy. Below it are found two or three levels of shade-tolerant trees, including many species of palms, such as Mauritia, Orbignya, and Euterpe. Myrtles, laurels, bignonias, figs, Spanish cedars, mahogany, and rosewoods are also common. They support a myriad of epiphytes (plants living on other plants)--such as orchids, bromeliads, and cacti--as well as ferns and mosses. The entire system is laced together by a bewildering network of woody ropelike vines known as lianas.

In addition to the rain forests of the terra firme, there are two types of inundated rain forests, várzea and igapó, which constitute about 3 percent of the total Amazonian rain forest. Várzea forests can be found in the silt- and nutrient-rich floodplains of whitewater rivers such as the Madeira and the Amazon, with their ever-changing mosaic of lakes, marshes, sandbars, abandoned channels, and natural levees. They are generally not as high, diverse, or old as those of the terra firme, being subject to periodic destruction by floods and human manipulation. (The várzea and its flood-free margins are the principal rain-forest habitat of human beings.) Wild cane (Gynerium) and aquatic herbs and grasses, as well as fast-growing pioneer tree species such as Cecropia, Ficus, and Erythrina, are conspicuous.

Igapó forests grow along the sandy floodplains of blackwater rivers such as the Negro, the Tapajós, and the Trombetas. Because human settlement is limited in these plains, there may be undisturbed, seasonally flooded forests that stand in water for up to half the year, the water reaching heights of up to 40 feet. A canoe can often be paddled between the trunks of trees adapted to such an aquatic environment.

The lowland rain forest on the Andean fringe grades into a discontinuous, tangled montane or cloud forest of misshapen trees cloaked with mosses, lichens, and bromeliads. Here one encounters the cinchona or fever-bark tree, once exploited for its antimalarial agent quinine. At still higher elevations is found the grass and shrub growth of the cold puna and páramo regions.