Published on 04/28/99

They're Not Really Big, Hairy Trees

Trees have many types of thread-like growths on buds, leaves and roots, but none of them are hairs. Mammals have hair and fur. Trees have "trichomes."

Fuzzy trees

R. Crang, University of Illinois, Urbana-Champaign

THEY'RE TRICHOMES, NOT HAIRS, begs Kim Coder, UGA forester. These are sycamore leaf trichomes that are detached from the leaf. These tiny structures, under extreme magnification here, serve all kinds of purposes for the plant they're on; they hold water, gather nutrients, absorb sunlight and protect the plant from insects or disease-carrying organisms.

Take a close look at a sycamore leaf. It can be so fuzzy on its underside that the dense trichomes can be rubbed off into small balls of fluff.

Sycamore leaf trichomes can cause allergic reactions and respiration problems. The fuzzy materials look like hairs, but they're not.

All kinds of trichomes

Trichomes can be all over the tree -- from leaves to root tips. They're part of the surface structure of tree parts. They are formed from the outermost layers of leaves, buds and roots.

Some remain alive for long periods. Others quickly die, leaving an empty shell behind. Trichomes are unique for most taxonomic groups of trees and can be used for identification.

Trichomes can be tall or short, thin or fat, big or tiny. They develop from a single cell or many cells on new tree surfaces like absorbing roots and leaves. They can be thickened at the base or have a large bulb at the end. They can stick straight up above the tree surface or recline on the surface.

Some trichomes are temporary, lasting just weeks, while some are permanent fixtures on tree surfaces. Trichomes can be disposable, breaking apart or falling off over time.

On new roots, trichomes are found just behind the growing tips. These root trichomes are sometimes mistakenly called "root hairs." Trichomes can form round containers that hold water on leaf surfaces.

Trichomes' purpose

Some trichomes are glandular. These have various materials that accumulate in or on their tips. The stickiness of butternut leaves and fruits come from glandular trichomes exuding materials.

Glandular trichomes also serve important waste removal functions in trees. Some species of trees which grow on alkaline soils or near the ocean transport salts and heavy metals onto the trichomes' ends. This secreted material prevents tissue damage and helps ease the washing away of excessive salts.

The underlying purposes for trichomes are as diverse as the trichomes and tree species themselves. Roles revolve around light absorption and reflection, tissue protection, water conservation and microbial interactions.

Young leaves of many species use trichomes to shade photosynthesis cells until they are fully functional. Some trichomes tangle, disrupt and confuse bugs and prevent some types of insect injury. Others, with defensive materials at their ends, touch and stab at insect visitors.

The tangle and mass of trichomes interfere with chewing- caused injuries. Trichomes can help slow water loss from leaves, too, by forming a thick layer of higher humidity around each leaf.

Trichomes on absorbing roots help take up water and essential elements. Some root trichomes act as avenues of colonization for beneficial fungi and bacteria in the soil. Unfortunately, some pathogens use trichomes for attaching to the root.

Go feel some trichomes

Feel some of our native trees. Many have trichomes of one form or another. If a tree has trichomes at all, it usually has them on the underside of leaves.

Green ash, sycamore, Southern magnolia, red mulberry, red elm, live oak, black oak, chestnut oak, post oak and river birch are just a few of the many trees that have trichomes.

One common exotic tree, the royal paulownia, is so densely covered with thick trichomes that its leaves feel like thick felt. An old common name for paulownia is "cottonwood" because of the dense, cottony texture of the leaf surface.

When you feel a leaf surface on a tree and the texture is hairy, rough, bristly, or silky, you're touching trichomes. Try not to call them hairs.

Kim Coder is a forester with the University of Georgia Warnell School of Forestry and Natural Resources.