Y: I don't think I've ever seen a plant as hairy as this one.

D: Plants don't have hair, Yaël. Hair is an animal thing.

Y: The technical term for plant hair is trichomes, but nonetheless, hair is basically what it is.

D: No way. These trichomes may resemble hair, but they're not the same as what we mammals have.

Y: Hold up. You're right that trichomes are not the same as our hair, but insofar as the definition of hair is that it is an outgrowth of the epidermis, then trichomes are for all practical purposes, a kind of hair. Unlike animal hair, though, trichomes are often living cells.

D: Well, if you had only said what you meant in the first place!

Y: Whatever, Don. Now, this plant has downy trichomes that feel like wool. But trichomes can run the gamut in structure, appearance, and texture. Some trichomes are frail, some coarse; some are branched like tree limbs, others star-shaped; some are long and straight, others are short and curly. 

D: Do these plant hairs serve any purpose other than ornamentation?

Y: Indeed. Just as mammal hair serves various protective purposes, including insulation and camouflage, so do trichomes. Trichomes can be insulating by keeping frost away from leaf cells. They can help reduce evaporation by protecting the plant from wind and heat. In many cases, trichomes protect plants from herbivorous insects that may want to feed on them. And in some cases, if the trichomes are especially stiff or irritating, they may protect a plant from larger herbivores. 

D: Like me! I sure don't want to eat a plant with stiff spikes on it.

Y: That makes two of us.