What is a Gall?

WHAT IS A GALL?

A gall is an abnormal growth on a plant caused by another organism, usually by the feeding of a parasitic insect. A wide array of flies, wasps, moths, aphids, and beetles make up the thousands of gall-inducing insect species. Gall insect larvae feed on the stems, leaves, twigs, fruit, or flowers of plants. The insect hijacks the plant’s cellular machinery, forcing it to form a tumor-like growth that will provide food and shelter for the parasite. Galls, especially oak tree galls, are rich in resins and tannins. 

These compounds defend the plant by being distasteful to insects and happen to be effective ingredients in dyes and inks. For instance, "Aleppo galls", a variety of oak galls caused by wasps in Syria, were prized in medieval times as ingredients for dyes. The Talmud refers to writing divorce documents in 'gall water'. The earliest known writer on galls was Hippocrates (406-374 BCE), who described the medicinal uses of what he called 'oak apples'. Pliny the Elder (23-79 CE) was the first to call oak apples galla in Latin, from which we get the English 'gall'. The Christian Barclay manuscript in the front case, which includes the recipe for Sad Purple, is written with oak gall ink. 

— Written by Robert Witkowski (Graduate Student, Plant Biology)