The most prevalent plant species on Earth take up water from soil through a system of roots. Two exceptions to this are nonvascular plants and epiphytes, which either absorb rain water or sap humidity directly from the air. Due to these and other limitations, neither has ever been a prominent species on the planet.
Non-vascular plants are those lacking a vascular system, the xylem (or "woody" tissue) and phloem (tissue that transports and distributes nutrients and water throughout the plant); they are without roots, leaves or stems, though several contain other specialized tissues that convey water within the plant. The three categories of nonvascular plants are Bryophyta (around 10,000 species, including mosses and lichens), Hepatophyta (roughly 6,500 species of liverworts) and Anthocerophyta (100 or so species of hornworts). Although mosses and lichens lack true roots, they have rhizoids, with which they attach themselves to plants and rocks that act as roots in water absorption.
These plants favor damp, shady areas because they rely largely on pooled rainwater, which they absorb slowly through capillary action and cytoplasmic streaming (fluid movement within the plant cell). Without a nearly constant supply of water, they will dry up and die quickly.
Epiphytes (or Air Plants)
Epiphytes are vascular plants that cling to a host, usually another plant. They are often mistaken for parasites, but most do no actual damage to their host and are therefore "tenants," with certain exceptions such as mistletoe, which taps into its host tree for nutrients and water. Ferns, mosses, orchids, bromeliads and vines are examples of various epiphytes. Some have evolved specialized structures designed to conserve water while others can absorb water from the humidity in the air, at least as a supplement. However, like non-vascular plants, they generally rely on pooled rainwater as well, not only for hydration but also for the nutrients common to rainwater, especially in tropical areas. Rain from thunderclouds contains nitrogen, courtesy of lightning.
Orchids are by far the most successful of the epiphytes, with more than 18,000 known species and another 10,000 that have yet to be studied. They are found all over the world.