Plants in the rainforest understory must survive in an environment that is darker and more humid than the canopy.

Only 2 to 5 percent of the sunlight that falls on the canopy reaches the understory. The rest is intercepted and partly absorbed by the leaves of the canopy trees and lianas.

Isolated flecks of sunlight reach the forest floor, but most of the light is reflected from, or transmitted through, leaves, and tends to have a strong greenish tinge.

Plants rely mainly on the red and blue ends of the light spectrum for photosynthesis, so this reduced light, from which much of the red and blue has already been absorbed, is not as useful to them as direct sunlight.

In addition, the wavelengths just beyond red are sensed by plants, and particularly by seeds, upon which they have a strong inhibitory effect.

These near infrared wavelengths are transmitted by the canopy, so that the light in the understory is relatively richer in them than sunlight.

When a clearing is made by people, or by a fallen tree, direct sunlight streams down to the ground and allows the waiting seeds to germinate.

The canopy not only alters the quality and quantity of light that reaches the understory, it also decreases the understory's air circulation.

The air in the understory hardly moves, and the humidity is always high. Temperatures, although not as extreme as they are above the canopy or outside the forest, are still high and fluctuate little.

Despite the difficult growing conditions in the understory, it is well populated with seedlings of the canopy trees. True shrubs-woody plants with several stems from ground level-are rare, but tiny single-stemmed miniature trees, up to 10 feet (3 m) high at maturity, are common.

Dwarf palms, of similar habit, are often found in South American and Southeast Asian forests.

Other low-growing plants that tend to be well represented in this environment are ground herbs -soft-stalked plants-especially those belonging to the arrowroot, the ginger, the acanthus and spiderwort families.

These herbs are often similar in appearance, with features that help that absorb light as efficiently as possible. These include broad leaves that long, pointed tips and are often reddish beneath, with a matte green upper surface.

Some plants that grow wild in the understory have become popular houseplants. Examples are Calathea and Maranta (prayer plants) and Fittonia (mosaic plants).