Why Do Plants Have Hair? (And What It Does)
Trichomes
Hairs in Botany are called Trichomes ( Greek). They come in every size, shape and color imaginable. Most importantly, what they look like tells you a lot about where and why they evolved to look that way.
Typical glandular hair - a trichome
Some have glands in their tips and others at their base, or no glands at all. Some are solid, others are hollow - and some of those have irritating oils or poisons inside the hollow space. They could be singular cells or multi cellular. Its endless!
Their special traits are directly related to the environment in which the plant evolved. The one characteristic that unites them all is they occur on epidermal tissue – the soft outer “skin” - and never on secondary tissue like bark.
Keep in mind that hairs may look similar in different species (below bottom) but may have widely distinct functions. The sheer number of functions they offer plants makes it clear how this is driven by being an environmental advantage.
Curved bristles on the underside of a Begonia leaf - keeps slugs and snails away !
Non glandular Trichomes
These are simple hairs with no substances in them. Their function is a reflection of the environment they are in.
In alpine plants hollow hairs insulate the central growing centers from the cold.
Lavender is very hairy and covered in a white wax. Not unlike dusty miller it evolved in a windy and sunny environment. The hairs, color, and wax, help reflect the intense sun.
Dusty miller comes from sunny, windy places in the Mediterranean
The hairs in African violets come in 2 sizes. The small ones are there to deter insects from eating the leaf. This is very helpful considering they live on the rainforest floor ! They also help reduce the loss of water through evaporation - great help when you live in a hot tropical place.
The larger ones help bead the water on the top surface of leaves. The rain falls, collects, forms a drop, then rolls off. A perfect advantage to keeping the leaf from getting constantly wet from the daily rains.
Tomatoes have hairs everywhere. They are there to ward off caterpillars of a specific size - young ones! Anywhere on the plant.
Hairs on tomatoes - an evolutionary advantage
Simple trichomes, bundled in mass, block the intense sun and the UV rays in all those hairy succulents we love! They also slow the effect of the wind, and reflect sunlight, keeping the plant cooler.
Glandular Trichomes
These types of trichomes are known to secrete various substances including water, nectar, resins, mucilage, salt, oils, perfume, poisons, irritants, and terpene among others.
Glandular trichomes not only vary in the type of substances they secrete, but also with regards to the mode through which they use these secretions.
For example:
The stinging hairs and glandular hairs on Urtica dioica (stinging nettle) is a good example. The hairs have basal bulb that gives rise to a protruding stiff and slender structure which will break off and penetrate the skin. This then secretes a poison that can burn or irritate.
In Marijuana, hairs function as a defense mechanism. When female cannabis plants begin to produce flowers they become vulnerable to various insects and animals as well as harmful UV rays. The hairs are a deterrent for animals because their bitter taste and strong aromas render cannabis flowers unpalatable. At the same time, they protect the plants from UV rays, damaging winds, and even fungal growth.
One group of trichomes secrete a type of mucilage (think slime) that serves to trap insects when they come in contact with the plant leaves.
Mucilage glands also helps prevent excessive water loss from the leaves. This happens in apples and sorghum where the unicellular hairs secrete droplets, ensuring the plant does not dry out.
Carnivorous plant leaves like in sundew forms trichomes with digestive glands at its tip used to eat insects!
A carnivorous sundew with glands that secrete a sticky substance made to trap insects
Dock and sorrel leaf glands excrete water. Basil secretes a fragrant oil - which we love ! Brackish or salt water mangrove plants excrete excess salt as crystals. They form inside epidermal leaf cells and then get extruded out to the leaf surface, where they fall off the leaf. Oils from Cymbopogon (lemongrass) hair glands repel insects.
Salt crystals forming on the leaf surface of a mangrove plant
Go seek out some hairy plants and figure out why they evolved that way!
You may find the answer most revealing!
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Article written by our Staff Horticulturist, Peter B Morris, BSc, MSc, MBA
All photographs used with permission from @SHUTTERSTOCK