Gall

Galls are abnormal plant growths caused by various organisms (insects, mites, nematodes, fungi, bacteria, and viruses).

These growths (a swelling on a leaf or stem ) are called galls because they contain large amounts of tannin, which has a very bitter taste. These unusually shaped growths and swellings are a plant's reaction to an irritation. These irritations can be physical or chemical, and usually are caused by developing insects, that use the plant for food and protection. Euomymus is a plant that is prone to Galls.