Branches of Botany: Trichomes, or ‘hair,’ act as plants’ protectors

One of the most important tissues/organs in the human body is our epidermis. It acts as a barrier to pathogens we encounter every day in our environment. Equally important, it regulates water loss and acts as a waterproof barrier.

This tissue functions similarly for plants. In a way it is more important because it contains pores called stomata that regulate gas exchange and water loss through evapotranspiration. This process fuels the transportation of water throughout the plant without the need for a heart. Often, so-called “hairs” grow out of a plant’s epidermis, much like our own skin. These hairs are clinically referred to as trichomes.

Over the eons of plants’ existence on Earth, trichomes were modified in myriad forms. They exist in most lineages, but often have very different functions. The types of hairs, along with their density and distribution on the plant, can be essential to identification. Trichomes can be single or multicellular, and borne singly or in tufts. Some taxa express multiple types of hair mixed together on their stems and leaves.

One common use of trichomes is protection from the sun. Plants, like humans, need to protect themselves from too much exposure to the sun. Surprisingly, an organism so reliant on the sun can be susceptible to UV rays just like us.

Young tissue is most prone to damage. We often see new growth with a reddish maroon tinge. Anthocyanin, the chemical that produces this color, absorbs high-energy radiation functioning like a sunscreen. Many species that exist in exposed, high-light environments will have a dense coating of hairs that makes them appear silver.

The leaves of the iconic Silversword plants — members of the genus Argyroxiphium — are an example of this. These trichomes also protect the plant’s chloroplasts — the sites of photosynthesis colored green by chlorophyll. The chemical mechanism, used when a plant converts light energy into chemical energy, naturally becomes damaged over time through a process called photoinhibition. Increased light levels accelerate and amplify this destructive process. Dense tufts of trichomes block excess light, and in some cases reflect it away from the plant.

In some lineages of plants, hairs do not provide protection. They function as a tool to gather essential nutrients.

Carnivorous plants thrive in nitrogen-deficient habitats like swamps and bogs, or nutrient-deficient soils. These environments are surprisingly common. Therefore, these plants exist on all continents, with the exception of Antarctica, and some oceanic islands, including Hawaii.

According to a 2015 article by Dr. Thomas Givnish, the carnivorous condition has evolved independently nine times in 12 families.

In Sundews, the genus Drosera, their trichomes exude nectar to attract their prey. These unfortunate insects become trapped by the “dewdrops” of a sticky substance exuded at the top of the glandular trichome. The plant can sense once it has caught something, and curls the leaf around the prey to bring it into contact with more trichomes.

In addition to trapping their quarry, these hairs are also involved in digestion. They secrete digestive enzymes that break down the insect, which is then absorbed through the hairs and pores on the leaves.

While trichomes — in any forms — create interesting textures, resist the urge to touch them. A 2018 scientific paper observed that when touched, the plants’ response indicated that it was stressed. The plant perceived touch as if it was an attack by a pathogen.

Jesse Adams is a botanist at the Harold L. Lyon Arboretum, where he works to catalogue, propagate and conserve the plant diversity found there.