Trichome

Trichome is one of those words you’ve probably never heard before, but you’ve seen what it means your whole life.

Trichomes are plant hairs. Trichome can also refer to plant scales, such as those seen on the outside of pineapples. These hairs or scales can be seen on leaves or stems. Understanding the vocabulary related to trichomes can help you identify unknown plants.

Trichomes on borage

Sticky trichomes trap and begin digesting insects (Noah Elhardt) CC BY-SA 3.0

When a plant is covered with hairs, that covering is called an indumentum. The presence of trichomes provides a physical barrier against grazing, as in the case of nettles. In other cases, there is a sticky secretion that traps insects as food.

Anatomy of trichomes

Trichomes can be unicellular or multicellular. Unlike thorns and spines, which grow from shoots and leaves, respectively, trichomes are more similar to root hairs, both being outgrowths from epidural plant cells. Each of these cells, or groups of cells, may turn into thread-like extensions that can be long or short, stiff or soft, straight or curved. Some trichomes are glandular, meaning they secrete fragrant essential oils or toxic histamines. Plants with hairs or scales are called pubescent. If a plant lacks hairs or scales, it is said to be glabrous or glabrate.

Electron micrograph of single-celled branching trichome (Heiti Paves) CC BY-SA 3.0

Types of trichomes

Trichome hairs can be single strands or they can branch. These branchings can look like a tree (dendritic), star-shaped (stellate), or be tufted. There are different words used to describe the various forms of indumentum:

• downy - a wooly covering of long hairs
• hirsute - coarse hairs
• hispid - bristly hairs
• pilose - long, straight, soft or erect hairs
• puberulent - fine, short, erect hairs
• scabrous - coarse scales, similar to, well, scabs
• scurfy - small scales, similar to dandruff
• stellate - star-shaped
• spinose - spines
• strigose - straight hairs all pointing in the same direction along a margin or midrib
• tomentose - dense, matted, wooly hairs
• villous - long, soft hairs that are not matted, but often curved

Take a closer look at your plants to see how they use trichomes to defend themselves.

Did you know that bean leaves have historically been used in Europe to trap bed bugs? Apparently, the spiky trichomes found on bean leaves puncture tiny bedbug feet, trapping them in place.