These are not taxonomic classifications, but helpful guidelines to understanding their evolutionary adaptability and care requirements. Orchids have two main ways of growing. In monopodial growth, there is one, usually upright growth axis from which the leaves appear. In sympodial growth, the growth axis is a prostrate rhizome which sends off shoots. Many sympodials develop pseudobulbs, which are thickened shoots from which the leaves grow.
Pseudobulbs act as storage organs for water and nutrients.
Terrestrial roots resemble those of other plants. Epiphytes are the climbers. In their natural habitats they are found in the treetops, their roots clinging to the tree and supporting them, and absorbing nutrients and water. Lithophytes are orchids that grow on rocks or in very little soil. Both epiphytes and lithophytes have aerial roots which are thick and fleshy, resemble snakes, and are covered with a spongy, silvery crust called velamen that adds to their other-worldly look.