Wool sower gall
This species is a bug, not a plant. Wool sower galls—all galls—are caused by insects that reprogram a plant’s growth mechanisms to grow a house around themselves. That just blows my socks off!
These particular galls are caused by Callirhytis seminator gall wasps, tiny little brown fellers ⅛" (4.8 mm) long. The grubs (immature insects) secrete plant growth regulators on oak branches in their saliva, forcing the plant to build a home for them. Each gall has an outer wall, a spongy fiber layer, and hard, seed-like structures that house developing grubs. The commandeered plant machinery even secretes nutrients for the developing wasps! The galls appear in the spring, and do not harm the host oak.
Callirhytis seminator description by Thomas H. Kent, last updated 22 Sep 2020.