What Are Trichomes in Plants?

Trichomes are appendages that emerge from plant organisms that take the form of fine outgrowths. While they can appear like hairs, they can be of varying structures, with some being long, short, scaly or stumpy, depending on the plant species. While they are associated with plants, they can also appear on lichen, algae and protists. These structures are crucial for the plant's interaction with its environment and can provide insights into their evolutionary adaptations.

There are other protuberances that can emerge from plant structures, but only those that come out of the epidermis are called trichomes. For example, rose thorns come from the sub-epidermis, so they are not classified as trichomes. Learn about the latter structure with our article on caring for a crown of thorn plants. This distinction emphasizes the unique nature of trichomes as compared to other plant outgrowths.

Depending on the plant, trichomes have different functions which allow them to survive in their environment. Some of these functions of trichomes include their ability to:

• Defend against attacks by herbivorous insects that want to eat the plant, because they are uncomfortable when the insect tries to approach the plant. They can even defend against larger predators, when the trichomes are rigid and long enough to affect them, hurting the animal's mouth if eaten.
  Allow the plant to absorb water. Water can lodge between the trichomes, being particularly beneficial in dry areas.
• Absorb nutrients, something which only occurs in certain plant species. This capacity can be vital for plants in nutrient-poor soils, enhancing their survival.
• Help the plant adapt to its environment by providing additional protection against environmental stressors.
• Secrete salts and sugars, which can deter herbivores and attract pollinators, aiding in the plant's reproductive success.
• Secrete essential oils in some species, contributing to the plant's defense mechanisms and sometimes used in human applications.
• Protect against the sun, allowing the plant to be protected against burns and dehydration, which is crucial in arid climates.
• Sting by releasing toxic and irritating substances on whoever attacks the plant. These substances are injected by the trichome, which acts like a needle for the toxins.
• Protect against wind in windy areas, reducing transpiration and water loss in the process, as well as maintaining the structural integrity of the plant.
• Secrete proteolytic substances in carnivorous plants that break down animal nitrogen molecules to assimilate them, aiding in nutrient acquisition.

Learn more about the classification of plants with our article which explains what are pyrophytic plants, and how these adaptations contribute to plant survival.

The structure, shape, position and even thickness of a trichome is a response to their environment. They serve as examples of how evolutionary adaptation allows plants to survive their environment. This presence or absence of trichomes and their structure helps determine their taxonomic position. In addition to facilitating botanical identification, it helps know their origin and phylogenetic relationship. Below are the main classifications of trichomes.

Glandular trichomes

Generally speaking, we can classify trichomes according to whether they have the presence of glands:

• Glandular trichomes: they have a secretory gland with which they can release substances resulting from metabolic processes. These are aimed at fulfilling particular functions for the benefit of the plant. They are the least common type of trichome, with only 30% of plants with trichomes being of this type. Some of the substances they secrete may be flavonoids, terpenoids, ketones or phenylpropanoids. These compounds can also play a role in protecting the plant from microbial attack and herbivory.
  
  
• Non-glandular or eglandular trichomes: these trichomes have a mechanical and not a chemical function, since they do not release any substance. Generally, the function is protection against environmental stressors, such as wind or temperature extremes.

Types of cells in trichomes

Depending on the number of cells that form trichomes, we can consider them to be:

• Unicellular trichomes: formed by a single cell, these are often found in less complex plant structures.
• Multicellular trichomes: formed by more than one cell, several arranging themselves to form the trichome. This type is common in more evolved plant species, offering enhanced functionality.

Learn more about the difference between unicellular and multicellular organisms in our related guide.

Shape of trichomes

Depending on the shape of the trichome, we can group them into:

• Hair: they are long and strand-shaped, often aiding in water retention and protection from herbivores.
• Cilium: they are short and rigid, providing a barrier against small herbivores and environmental challenges.
• Scale: they are lamellar trichomes, which can help in reflecting sunlight and reducing water loss.

Within the form, they can also be classified descriptively according to their morphology. Some examples of this are:

• Simple: they are straight trichomes, providing basic protective functions.
• Branched: they branch out from different levels, like the branch of a tree, offering increased surface area for various functions.
• Woolly: they are those that completely cover the surface and appear like fur, which can insulate the plant against cold temperatures.
• Starry: they are those that have several branches that start from a point, like a star, maximizing defense and surface interaction.

Location of trichomes

Depending on their location, there are the following types of trichomes:

• Abaxial: when they are located at the bottom of the leaf, often helping in reducing water loss.
• Adaxial: they are those that grow at the top of the structure, providing protection against sunlight and herbivory.

Quantity and density of trichomes

If you want to classify by quantity and density, some examples of types plants are:

• Hirsute: when they are many and rough, these trichomes can deter herbivores and reduce moisture loss.
• Glabrated: that is, it lacks trichomes altogether, which might occur in plants with other defensive strategies.
• Hispid: trichomes with the appearance of brush bristles, providing a tough exterior to deter small animals.
• Strigoso: when the trichomes are long and rigid, pointing towards the same place and maintaining the same margin of growth, aiding in defense and environmental adaptation.

Discover more about how plants are studied with our article explaining the difference between angiosperm and gymnosperm plants.

As we reviewed, the functions and structures of trichomes are very variable and this responds to the needs of each organism. To better understand this topic, we present some examples of organisms that have trichomes, including plants:

• Spirulina (Spirulina spp.): this is a genus of cyanobacteria, some of which is currently consumed as a dietary supplement. They have multicellular trichomes in a helix form. They only appear when they are in a liquid medium. In this post you can read about the proposed properties and benefits of spirulina. These trichomes play a role in buoyancy and nutrient absorption, crucial for their survival in aquatic environments.
• Cape sundew (Drosera capensis): this is a carnivorous plant that has very long red trichomes. Their trichomes secrete a sticky proteolytic substance that it uses to capture prey such as bees, flies or any flying insect. It also speeds up degradation for digestion. This is an example of plants with glandular trichomes, showcasing the diverse functions trichomes can have.
• Nettle (Urtica spp.): this plant is well known for causing severe irritation when rubbed on the skin. This is because their trichomes are glandular and release histamine, acetylcholine and serotonin into the skin, causing the aforementioned reaction. This defensive mechanism helps protect the plant from herbivores.
• Some cacti without spines: these cactus plants without spines have these hairy trichomes instead of the thorny structures common to most cacti. They are the ones that seem soft to us, appearing as if the plant has whitish hair. These adaptations help in water conservation and reflect sunlight, vital for survival in desert habitats.

After having learned all this about what trichomes are, we encourage you to continue discovering details about plants by reading about the parts of a leaf and their functions, enhancing your understanding of plant biology.

If you want to read similar articles to What Are Trichomes in Plants?, we recommend you visit our Biology category.