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The red things are galls produced by a gall mite on a leaf

Gall produced by an insect on a vine

Galls can also appear on skeletal animals and in the fossil record. Two galls with perforations on a crinoid stem from the Middle Jurassic of southern Israel

Oak apple or oak gall is a large, round, gall commonly found on species of oak, 2–5 cm in diameter. They are caused by chemicals injected by the larva of certain kinds of gall wasp in the family Cynipidae

Eucalyptus leaf gall

Oak marble galls, one with a gall fly exit hole and another with Phoma gallorum fungal attack

A gall is a growth on the surface of plants and other lifeforms. They are caused by various parasites, such as fungi, bacteria, insects and mites.

Plant galls are abnormal growths of plant tissues similar to benign tumors or warts in animals.

What causes a plant gall can often be known without the actual agent being there. Many types of gall have typical structures. This is true of many galls produced by insects and mites.