Terrestrial orchids live on the ground, usually in a water-retentive humus of rotting wood and leaves. These orchids need a constantly moist, but never soggy, medium in which to grow. Paphiopedilums and cymbidiums are examples. Epiphytic orchids live on the branches of other plants -- usually trees. They are not parasites -- they obtain no food from the trees on which they grow. Epiphytic orchids have "air-roots" which are accustomed to drying out in between periods of rain. These orchids need an open medium in which to grow and the roots should be allowed to become moderately dry between waterings. Cattleyas, vandas, phalaenopsis, and dendrobiums are common examples.