In botany, we can draw many analogies with the animal kingdom. The terms used are sometimes the same: epidermis, cuticle, conducting vessels and even hairiness. Indeed, if we compare animal epidermis with that of plants, we notice that some have hair, while others are completely hairless. As with animals, plant hairs have a variety of functions, from protection to temperature regulation. More comfortable than some humans when it comes to body aesthetics, plants have no qualms about proudly displaying their hair – or rather, their trichomes!
You’ve probably already noticed that some plants are hairy, others less so and some not at all. This hairy characteristic can make leaves soft to the touch, rough and sometimes even sticky. Plants can be classified according to the presence or absence of hair: those with hair are said to be pubescent, while those without hair are said to be glabrous. Moreover, when they have a very high density of fur-like hairs, they are said to be tomentose.
Photos: Mathieu Gaudreault Mathieu Gaudreault
What are these hairs for? Do they have a function?
The answer is yes, they have a function. Trichomes play several essential roles that vary according to species and their specific needs. Let’s take a closer look!
Let’s Start With a Little Anatomy
Trichomes, which consist of just a few cells, can be considered as modified epidermal cells. These specialized structures come in a variety of forms, and some are glandular, i.e. capable of secreting specific substances. Below is a diagram illustrating several types of trichomes and their distinct characteristics.
And Now Their Functions!
Protection Against Herbivores
Plant trichomes play an important role in deterring herbivores by making their leaves less appetizing, or even stinging. A typical example is stinging nettle (Urtica dioica), which has specialized hairs containing irritants that discourage animals from eating it. You may have experienced this when pulling up this plant in your garden: at the slightest touch, its trichomes, which resemble fine glass prickles, break and release an irritating substance. This substance, composed of histamine and formic acid among other things, causes a burning or itching sensation. This chemical defense strategy illustrates the ingenuity of plants in ensuring their survival in the face of predators.
Solar Protection
The hairs on plant leaves can also play a crucial role in reducing water loss. By creating an immobile layer of air on the leaf surface, they restrict air circulation and reduce evapotranspiration. The example of lavender (Lavandula angustifolia), a plant native to sunny areas of the Mediterranean, illustrates this adaptation. Its star-shaped trichomes, resembling tiny parasols, form a natural barrier against dehydration by reflecting sunlight and protecting internal tissues from excessive heat.
Edelweiss (Leontopodium nivale subsp. alpinum syn. L. alpinum), the emblematic plant of the Swiss Alps, is perfectly adapted to its extreme environment thanks to its tomentose bracts covered with dense, silvery hairs. These hairs reflect part of the sun’s rays, protecting the plant from the harmful effects of overexposure. This adaptation, common to plants growing in sunny, high-altitude environments, limits heat absorption and preserves fragile tissues, helping them to survive in hostile climatic conditions.
Harvesting Moisture
Some trichomes specialize in the ability to extract moisture directly from the air to help hydrate certain plants. This is typical of epiphytic plants such as Tillandsias, which use their specialized trichomes to capture ambient moisture and even assimilate nutrient particles. These trichomes also act by capillary action, allowing the collected water to spread evenly over the leaf surface and then be absorbed by the plant. When these peltate (disc-shaped) trichomes come into contact with water, they curl up and trap the water against the leaf surface. A fascinating and essential adaptation for these epiphytic plants, which have very few roots.
Essential Oil Storage
Some trichomes are specialized in storing essential oils or other chemical compounds, playing a protective role. For example, the glandular trichomes of mint contain aromatic oils responsible for its characteristic, refreshing scent. Most aromatic plants, such as basil, rosemary and lavender, have trichomes specialized in storing oils. Released by the bulbous trichomes, the oils not only protect the plant from herbivores, but also provide intense, refined aromas for perfumery and cooking.
Sticky, Glandular Hairs
Some carnivorous plants, such as those in the Drosera genus, use specialized trichomes to capture their prey. Their leaves are covered with glandular hairs that secrete mucilage, a sticky substance rich in polysaccharides. This natural trap immobilizes unwary insects. Once trapped, the prey is slowly digested by the enzymes released by these trichomes, enabling the plant to draw essential nutrients from an unusual meal, but vital to its survival in poor soils. Beware of insects that dare to put their paws on them: Drosera plants leave nothing to chance!
Similar glandular hairs are found in butterworts (Pinguicula sp.), carnivorous plants that also capture their prey with a sticky secretion. In this video, we can observe a sciarid fly trapped on the leaf’s viscous surface. To illustrate the viscosity of the mucilage secreted by trichomes, the end of a paper clip was used. The demonstration clearly shows the strength of this natural trap. This ingenious mechanism enables butterworts to capture and digest their prey, extracting nutrients essential to their survival.
Glandular trichomes are abundant on some plants, and play a crucial role in secreting chemicals such as cannabinoids. These hairs are particularly concentrated on the inflorescences, which explains the interest in harvesting this part of the plant. Indeed, it is the inflorescences that contain the greatest quantities of these active compounds, sought after for their psychoactive or therapeutic properties.
And Much More!
The list could go on and on about the many uses of plant hairs. They may be discreet, but their functions are no less extraordinary! In violets (Viola sp.), for example, a few trichomes are found at the entrance to the flower, more specifically at the base of the lateral petals.
These form a structure that acts as a guide, directing pollinating insects (such as bees and butterflies) towards the center of the flower, where the nectar and reproductive organs (pistil and stamens) are located.
These few hairs also form a partial physical barrier that limits access to insects that are too small or unfit for pollination, such as certain ants. In this way, nectar is reserved for efficient pollinators – the ones who deserve it after all! What’s more, this little tuft of hair helps reduce nectar evaporation, making it more attractive to insects while preserving this essential resource.
Trichomes, Incredibly Important
This is the end of our hairy journey. We hope it has given you a better understanding of the incredible importance of trichomes in plants and the fundamental roles they play in the survival of our flora. Whether they’re glandular, star-shaped or simple, hairs are as much a means of defense as they are a means of feeding. Present in both carnivorous and epiphytic plants, they fascinate and deserve our full attention.
In their own way, they remind us of our own hair, which also plays a role in our protection and thermal regulation. For plants and animals alike, hair is more than just a feature: it’s an ally in adaptation and survival. So be proud of your hair!
Fantastic and very interesting article. I was aware of trichomes but not the many types and uses for them. Thanks for this. Encourages me to do some more research on this interesting plant part.