What Is the Waxy Coating on a Plant?

When someone admires shiny green leaves on a plant, he or she might not realize that the plant did not develop the shine merely to be attractive. The shiny, waxy substance performs a vital function. Plants evolved from one-cell, green algae and began their transition to dry land approximately 450 million years ago. A main challenge of plant evolution was for plants to develop ways to survive out of water.

The waxy coating, called the cuticle, on plant leaves and on young plant stems, also gives fruits their sheen, according to the New Mexico State University Cooperative Extension Service. The cuticle is composed of cutin, a wax-like material that is a hydroxy fatty acid and fills in spaces in a plant's epidermis, or outermost layer of cells. The cuticle prevents too much of the water from the plant's interior from escaping, protecting the plant from drying out. The Extension Service says that a plant's cuticle can form flat plates or be a mass of threads. It also notes that the cuticle might be a loose covering, to allow gases and water vapor to pass through the plant easily, or a tight one, slowing gas exchange, according to the plant's needs.

In a process known as transpiration, water from a plant's interior evaporates through stomata, which are microscopic pores on the undersides of leaves. When a plant transpires, water is exchanged for carbon dioxide for photosynthesis, which needs a liquid to enter the plant. Transpiration requires a good deal of water. The amount lost depends on the temperature, humidity and air movements around the plant. The cuticle makes it impossible for water and carbon dioxide to pass through the leaves anywhere other than through the stomata, which open and close to regulate passage of water and carbon dioxide.

The cuticle is especially important in regions where heat and evaporation rates are high. According to Michael E. Ritter, author of "The Physical Environment: an Introduction to Physical Geography," the Havard oak, a shrub found in the dry southwestern United States, provides one example. The shrub adapted to the conditions by developing tap roots that extend 15 to 20 feet deep to locate and absorb moisture. It also has exceptionally thick cuticles to reduce the amount of water lost by evaporation from the leaf surface through transpiration.

A water lily's flat leaves have a thick cuticle on their upper sides, but not the undersides, which are supported by the water's surface. Stomata, the tiny pores through which water and carbon dioxide are exchanged, are also found only on a water lily leaf's upper side. Water lilies usually have thick, waxy cuticles to repel water, keeping the stomata clear for performing photosynthesis, according to the Offwell Woodland and Wildlife Trust.