Orchids (Orchidaceae) first appeared on Earth about 100 million years ago and adapted over time to include both terrestrial types growing in soil and epiphytic types thriving on only air and water. Close to 25,000 naturally occurring orchid species exist on every continent, and all have their own distinctive characteristics or adaptations. Even though they have diversity among their species, orchids share common strategies that ensure their survival.
Orchids developed a variety of unusual flowers that attract the creatures that pollinate them. Some flowers resemble their pollinators, such as those of moth orchids (Phalaenopsis group), which are hardy in U.S. Department of Agriculture plant hardiness zones 10 through 12. The wide petals at the top of those flowers are reminiscent of moth wings. The flowers of Venus' slipper orchids (Paphiopedilum group) each has a pouch-shaped petal that directs bees and birds straight to the source of pollen. Venus' slipper orchids are perennial in USDA zones 9 through 12.
The ghost orchid (Dendrophylax lindenii), an endangered species that is hardy in only southern Florida in USDA zone 10a, is a leafless orchid that relies on a symbiotic relationship with fungi to move nutrients throughout the plant. A specific fungus lives inside this orchid's roots and also protrudes outside the roots and gathers nutrients from soil.
Epiphytic orchids' roots have evolved to serve multiple functions. Such an orchid's roots work as roots normally do by transferring water and nutrients to the rest of the plant, but they also anchor the air plant to a tree and perform the photosynthesis that leaves normally do on other kinds of plants. An epiphytic orchid's roots also can store water in the same way as a succulent plant's stems, allowing the orchid to exist high in a tree top to get more sunlight than plants living below.
Because epiphytic orchids live above soil level, they developed bulblike structures that perform the same work of storing water and nutrients that underground bulbs perform for other types of plants. In an epiphytic orchid, one or more sections of a stem bulge outward and become darkened areas, which are the pseudobulbs. Orchids with pseudobulbs can thrive in regions that have dry months between times of seasonal rainfalls.