Stomates.
I wondered if I could see them when I took this photo.
Then I wondered if I had the name right.
Turns out: probably not; and, yes.
Stomates are stoma/stomata, the wee pores that open/close and allow the plant to intake carbon dioxide (breathing, essentially, to a plant); the downside is that by opening, this facilitates transpiration (loss of water vapor analogous to sweating in critters).
In general, a plant has more stomata on the bottom-sides of leaves than the tops.