Aerial roots (green) of the Christos orchid (Epidendrum elongatum). This orchid is an epiphyte. This means that it grows on a support as opposed to growing in the soil (terrestrial). The support is usually (as here) a tree. The orchid is not a parasite on the tree, but develops its own system of aerial roots. These obtain nutrition from the air, rainwater and the humus in the crevices of the host tree's bark. The orchids (Orchidaceae) are one of the most diverse families of flowering plants. There are over 20,000 species worldwide. They produce spectacular flowers and are widely grown for ornamental purposes.