Warts/discs/"blobs" on the underside of oak leaves

Warts/discs/

Observation - Warts/discs/"blobs" on the underside of oak leaves. Description: I've seen light-coloured, convex discs on the underside of fallen oak leaves for years, but never found out what they are. Can anyone enlighten me? The differently coloured/textured ones shown in the first two pictures were something that I hadn't seen

I've seen light-coloured, convex discs on the underside of fallen oak leaves for years, but never found out what they are. Can anyone enlighten me? The differently coloured/textured ones shown in the first two pictures were something that I hadn't seen before, so they particularly piqued my curiosity.