Oaks are hard. Hard wood, yes, but also hard to identify. This morning, I’m trying to key out a mystery oak on our dog walk, so I came home with a twig, buds, acorns and leaves. A few leaves are still green, and the undersides (the abaxial surface, thank you) gave me pause.
See the rusty-beige fuzziness wedged in the junctions between the main vein and side vein? My first thought was some kind of insect hideaway. Not a gall, but a woven, triangular tent to cloak an in-progress larva.
But no. The fuzziness is part of the leaf itself. According to one of my many dichotomous keys, the triangles are “axillary tufts of tomentum,” or just “axillary tomentum.”
Axillary = of or relating to the armpit
Tomentum = Latin for “cushion stuffing,” a.k.a. wooly or matted hair
Armpit hair, then.
My leaf has pits: hairy pits.
I had hoped hairy pits would be a unique feature of one oak species, or that the presence of hairy pits might narrow possibilities to a handful of candidates, but apparently, axillary tomentum is common.
I wonder what function it serves?
Human hairy pits wick sweat and bacteria from our bodies. But how would a leaf benefit?
And wouldn’t the tomentum interfere with the stomata: the “breathing” holes on the underside of a leaf?
Shumard, Black, Pin, Red, Scarlet are all in the running at this early stage. Or, the tree could well be a planted ornamental (and not native) or a squirrel/jay-grown tree hybridized from two species nearby. But it’s fun to try.
Does this matter? Knowing what oak we walk under every day? Noticing new details on leaves we’ve been kicking for years?
Yes.
A small voice of wonder about leaf armpits is still a voice.
(Notice. Wonder. Connect.)
Sidewalk Nature 
I’ve got that identical feature on leaves of trees that I know to be Shumard oaks, if that helps! And thanks, because I was trying to work out what sort(s) of gall-formers might be involved. Axillary tomentum is new to me.
Thanks, Jenn. I do vote for Shumard. Speaking of galls, my leaves have small, clustered galls, too, but I’ve only been able to get as far as the “vein pocket gall” of a midge.
Yes, I saw that you had one of those on the same leaf. My Googling suggests this one: http://bugguide.net/node/view/579482/bgpage . This one is being listed as “Macrodiplosis qoruca”, but that would presumably have come from “q-oruca” or “quercus-oruca” which would more clearly indicate the oak association. Thanks for cluing me in about the non-gall part!