Orchids which grow in trees are Epiphytes. The term comes from the Greek words, ipe-, meaning on and phyte, a plant. Orchids only use the tree or bush they grow on as a place to anchor their roots; all nutrients come from the air or from droppings from higher up in the branches. All their roots are exposed to the air in order to give them more opportunities to absorb water and nutrients.
The majority of orchids grown by man around the world are epiphytes. Many are grown mounted on wooden rafts such as cork; some are grown in baskets; and some can be grown in pots in an extremely porous mixture of bark, perlite and charcoal. Cattleya, Phalaenopsis and Brassia are common examples of epiphytes.