Around 40% to 75% of all biotic species are indigenous to the rain forests. It has been estimated that there may be many millions of species of plants, insects and microorganisms still undiscovered in tropical rain forests. Tropical rain forests have been called the "jewels of the Earth" and the "world's largest pharmacy", because over one quarter of natural medicines have been discovered there. Rain forests are also responsible for 28% of the world's oxygen turnover, sometimes misnamed oxygen production, processing it through photosynthesis from carbon dioxide and consuming it through respiration. Tropical rainforests are characterized by a warm and wet climate. Mean monthly temperatures exceed 18 °C (64 °F) during all months of the year. Average annual rainfall is no less than 168 cm (66 in) and can exceed 1,000 cm (390 in) although it typically lies between 175 cm (69 in) and 200 cm (79 in).
The main plants in this biome are trees. A lot of the rain that falls on the rain forest never reaches the ground. It stays on the trees because the leaves act as a shield, and some rain never gets past the trees to the smaller plants and grounds below. Trees in this climate reach a height of more than 164 feet. They form a canopy. The forest floor is called understory. The canopy also keeps sunlight from reaching the plants in the understory. Between the canopy and understory is a lower canopy made up of smaller trees. These plants do receive some filtered sunlight.