On Tooting Common the oaks are swollen with acorns. Some are large and hearty, almost bursting out of their cups; others small and nut-like. Some are subsumed almost entirely within galls.
Oak galls are very high in tannin, and were used for centuries to make dyes and inks. In fact, traditional oak gall ink can still be bought today.
Beneath some of the oaks on the common today I saw that the ground was stained black where the galls had fallen and leached their tannin into the soil.