While rainforests cover a small percentage of the Earth's surface, they contain the most diverse groupings of plants. Webster's dictionary defines the rainforest as a place that receives at least 100 inches of rain per year. It is a woodland forest with a dense evergreen canopy. The category includes tropical and temperate rainforest regions.
The tallest trees in the tropical rainforest form what is called the emergent layer. They create a dense canopy above shade-loving plants. Without this layer entire eco-systems can be destroyed. This dense layer holds in moisture and creates a terrarium-like environment. Vast numbers of epiphytes, like orchids and bromeliads, live at the tops of these trees. They are not parasitic, they simply need to hold on to the elevated structures. Orchids draw moisture and nutrients from the air. Bromeliads have cups that collect water and nutrients from humidity and rain. Kapok trees (Ceiba pentandra) are emergent trees whose flowers draw honey bees to the rainforest. Another emergent tree, the Brazil nut (Bertholletia excelsa), relies on a specific orchid (Coryanthese vasquezii) to draw its pollinator, the orchid bee. The symbiotic relationships among the plants and animals of the rainforest are extremely important.
Just below the emergent layer is the upper canopy of trees. In temperate rainforests this is the first layer. The Olympic Rainforest found in Washington state contains some of the most stately conifers in existence. The trees reach this size due to the moist, mild conditions created by the ocean, rain and transpiration from the conifers themselves. In a tropical rainforest the upper story is where the bulk of the food plants are found. Animals linger there eating tree fruits like guava, papaya and dates.
The understory level of a forest is comprised of shorter trees and shrubs. Tropical vines are included in this layer because they begin life as small shrubs before scaling the tall forest plants. "Strangler" vines wind their way up trees and eventually take over as the tree dies. The understory in a tropical rainforest is densely shaded. Fruiting vines like passion fruit and akebia grow freely. In the temperate rainforest there may be filtered light and more of a shrub layer. You may find epiphytic shade plants, like tassle fern (Huperzia phlegmania) or licorice fern (Polypodium), hanging from the trees. The Australian king fern (Angiopteris evecta) has the longest fronds of all, reaching lengths of up to 16 feet.
Forest floor plants range from tiny mosses, lichens and fungi to ground-covers and trailing vines. In the Olympic forest there are lush ferns such as maidenhair fern (Adiantum pedatum) and western sword fern (Polystichum munitum). There are numerous wildflowers such as western white trillium (Trillium ovatum), bleeding heart (Dicentra formosa) and lady's slipper (Cypripedium). Rainforests contain some of the world's rarest, most delicate wildflowers.
Numerous medicinal plants are found in the rainforest. The Pacific yew (Taxus breviflora), used as a cancer treatment, grows in the Olympic rainforest. The tropical chinchona tree (Chinchona caisaya) was the original source for quinine. It is used to combat malaria and to calm lupus and other rheumatic diseases. These are well-known plant medicines, but many possible cures are still undiscovered among rainforest plants.