Trees do not have hair-trees have trichomes. Trees have many different types of thread-like growths on buds, leaves and roots, but none of them are hairs. Mammals have hair and fur. Trees have trichomes.
Animal hair come in two general forms: set length and always growing. Both types of hairs grow from the bottom where they form within the skin of an animal. Trichomes are developed from outer surface layers of cells.
Trichomes can be tall or short, thin or fat, and big or tiny. They develop from a single cell or many cells on new tree surfaces like absorbing roots and leaves. Trichomes 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. Trichomes can be temporary, lasting weeks, or permanent fixtures on tree surfaces. Trichomes can also be disposable, breaking apart or falling-off over time.
Some trichomes are glandular. These trichomes have various materials which accumulate in or on their tips. The stickiness of butternut leaves and fruits come from glandular trichomes exuding materials. Some materials are defensive compounds to prevent animals consuming leaves. Other materials are allelopathic chemicals that are rinsed-off by rain into the soil.
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 into the trichomes ends. This material is moved or secreted to prevent tissue damage and help ease the washing away of excessive salts.
A special type of oozing trichome is called a colleter. Colleters are found on the surfaces of new formed leaves inside buds. They ooze a sticky material that permeates the leaves and buds. This process helps strengthens the new, succulent tissues and help prevent some types of pest damage. Buckeyes, hickories and birch are some of the common trees to have these special trichomes.
Young leaves of many species use trichomes to shade photosynthetic cells until they are fully operational. As leaves expand, the effective density of these trichomes declines. Trichomes selectively block ultra-violet wavelengths like a translucent or transparent coating. Trichomes also shade tissues from other wavelengths and reflect heat energy away from leaf and bud surfaces. Trichomes help elevate the primary energy exchange interface of the leaf.
Trichomes tangle, disrupt, confuse, and prevent some types of insect injury and use. Densely wooly trichomes prevent insects from contacting the leaf service. Trichomes with defensive materials at their ends touch and stab at insect visitors. The tangle and mass of trichomes interferes with chewing caused injuries. Trichomes also provide an elevated platform upon which dust and fungal spores can be swept away by wind and water before they would touch the leaf surface.
Trichomes can help minimize water loss. Tufts of trichomes are positioned around stomates, the water control valves in a leaf, and slow water evaporation. Trichomes also form a thicker boundary layer of higher relative humidity around a leaf which slows water loss. Shading and reflectance by trichomes also lowers tissue temperatures which lessen food use and decreases water evaporation from tree surfaces.
Trichomes on absorbing roots assist with water and essential element uptake. These root trichomes increase surface area and the interaction with the soil. Some root trichomes act as avenues of colonization for beneficial fungi and bacteria in the soil. Unfortunately, some pathogens use these trichomes for attacking the root.
Trichomes come in many shapes. Some trichomes are long and thin, matting down in a thick felt-like surface. Other trichomes have multiple branches that looks like a tree in miniature. Still other trichomes have a circular crown of branches which are star-shaped. Some trichomes are colored with a pigment or develop a color from weathering and from the environment, but most trichomes have no color. Trichomes do not have chlorophyll.
Some trees bear trichomes in selected locations. Black cherry develops reddish-brown trichomes on the underside of the leaf along the main vein. A number of oaks have trichomes in the junctions between side veins and the main vein on leaf undersides. Turkey oak and northern red oak are good examples. One native tree with dense, glandular trichomes which smell very aromatic is mockernut hickory. Oglethorpe oak has unique five-pointed, star-shaped trichomes. Black walnut has glandular trichomes which are swollen at the top and dispense an allelopathic material that damages other plants. Figure 1 presents a diagrammatic representation of various forms of trichomes on the leaves, buds, and roots.
Figure 1. Diagrammatic representation of various forms of trichomes on leaf, bud and root outer surfaces.
The first word most people understand about trichomes on leaf surfaces is "pubescent," meaning clothed with soft, short hairs. "Downy" means clothed with soft, fine hairs. "Puberulous" means minutely pubescent, while "purberulent" means minutely pubescent but with soft, straight, erect, and tiny hairs. "Tomentose" means densely pubescent with matted, wool-like hairs. "Tomentulose" means slightly pubescent with matted wool-like hairs. "Velutinous" signifies a velvety surface texture of the leaf's surface while "sericeous" means a silky surface texture.
"Pilose" means a tissue surface is hairy with soft distinct hairs. "Villous" represents a surface which is hairy with long, soft, shaggy hairs. "Fimbriate" present a definition problem because it means a surface with tread-like hairs-are hairs thread-like or are threads hair-like? "Wooly" is clearly an animal term meaning covered with long, matted or tangled hairs. "Lanate" is wooly with long, curled or wavy hairs. Describing tree trichomes, we continue to come back to human and animal terms for hair or fur. It is important to remember that trees do not--cannot--have hairs.