Trichomes are plant hairs that can occur on any part of the plant including leaves, stems, petals, etc. These hairs can vary greatly in size, shape, abundance. They may be branched, unicellular or multicellular, and may vary with the plants age or environmental conditions. Often trichomes play an important role in defending plants against getting chewed on by insects or animals. They may also aid in water retention, attracting pollinators, and preventing fungal pathogens (1). Trichomes can be useful in identifying plants, for example, dense trichomes on the stems and leaves of Staghorn sumac (Rhus typhina) are an easy way to distinguish this plant from its similar looking relative, Smooth sumac (Rhus glabra).
Trichomes can be glandular, meaning they exude chemicals that may make them toxic or less palatable to herbivores, or glandless (eglandular). They can also be branched or hooked causing insects to get stuck on the plants surfaces. Bedstraws (Galium sp.) have eglandular hooked trichomes on their stems, leaves, and fruits and are notorious for their ability to stick to things, such as the clothing of an inattentive hiker.
Glandular hairs, like the peltate hairs common in the mint family, can store chemical compounds which help deter insect pests (2). Some plants in the nettle family, Urticaceae, have needle-like hairs capable of breaking off and injecting stinging compounds when their leaves are touched. Stinging nettle (Urtica dioica) can even increase the number of trichomes it has one its surfaces to provide extra defense after being grazed by herbivores (3). Carnivorous plants such as Sundews (Drosera spp.) have glandular hairs that not only exude sticky substances that attract and trap insects, but also use these hairs to exude chemicals that digest the insect and absorb the vital nutrients (4). Those are very talented trichomes!
Trichomes can aid in attracting pollinators. The trichome covered staminodes that give Beardtongues (Penstemon spp.) their name function to attract the right pollinator and prevent unfriendly insects from stealing nectar (5). The trichomes on the staminode of Common Beardtongue (P. digitalis) are thick and bristle-like, but staminode morphology varies widely in this group (6).
With so many hairy plants out there, it may seem that hairiness is the rule rather than the exception. The ubiquitousness of trichomes could possibly be due to the evolution of root hairs, which aid in water uptake. Once plants evolved root hairs, it may have been just a small change in the genetic code to grow those hairs on other structures (1). Who needs a pet dog or cat when you could buy a nice fuzzy houseplant companion like Panda plant (Kalanchoe tomentosa) or Rabbit foot fern (Davallia fejeensis)? This is nowhere near a comprehensive overview of all the cool stuff there is to know about trichomes but you can see these structures for yourself by getting out your hand lens and peering lovingly at the plants in your backyard.
Other photos of trichomes on plants: