There are two basic types of orchids: epiphytic (a plant that grows upon, but does not receive its nourishment from, another plant, such as a tree), like the familiar cattleya orchid; and terrestrial (a plant that lives and grows in soil), like the spray-type cymbidium orchids. In each type you have a wide choice of plants that will thrive under pretty much the same conditions of heat and humidity. When you buy plants (and this will be mainly from mail-order specialists), find out whether they are epiphytic or terrestrial so you can handle them accordingly.
In nature, epiphytic orchids establish themselves by their fleshy roots on branches or bark of trees. Organic matter that accumulates between the roots and branches furnishes nutrients. There are also some species that cling to rocks for survival. Suspended from trees or on lofty rocks, the epiphytes dwell in filtered light and always in open, airy situations. Some also accommodate themselves to extremes of torrential rain and extended drought. To produce their exotic blooms in profusion in your greenhouse, the epiphytes need to find there an approximation of their natural conditions, particularly in regard to light and air. It is also claimed that a period of dryness between the months of growing and blooming is essential for flowers. However, I have never dried out my cattleyas and oncidiums and, except in dull weather, I water these almost every other day. I do not claim this is ideal practice for all epiphytic orchids, but it has certainly worked for these two, which bloom profusely for me on this schedule.
Roots of epiphytic orchids are constructed to sustain the plants through wet and dry periods. The tough, stringy, fiberlike core is encased in a spongy off-white covering that is highly water absorbent. When it rains, this cover soaks up water; when saturated, it turns light green. Roots retain moisture and release it gradually to plant tissues.
The terrestrial, or earth-bound, orchids have roots that grow beneath the surface of the soil, as with most other kinds of plants. Cypri-pediums, the lady-slipper orchids, are mainly of this type, though some are semi-terrestrial, having both underground and aerial roots. The lovely spray orchids, cymbidiums, are terrestrial; they grow in the loose humus of rotted wood or decayed leaves.