Their high concentration of tannic acid has led to some of them being used as medieval sources of ink.
They are more prevalent in tropical regions due to longer growing seasons, and thus more available active meristematic tissue. A wide range of organisms have the ability to generate them, including fly larvae, bacteria, mites, and fungi. Some very complex structures can be formed from the parasites' secretion and manipulation of growth factors. Though some types are found on leaves, the best-known ones grow on branches or trunks. Their high concentration of tannic acid has led to some of them being used as medieval sources of ink. For 10 points, name these parasitically-induced abnormal growths on plants, types of which include "oak apple," "Aleppo," and "crown."
They are more prevalent in tropical regions due to longer growing seasons, and thus more available active meristematic tissue.
{galls}