Lots of plants bear strange-looking swellings on their leaves and stems. These wart-like growths are called galls, and they’re the direct result of mites and insects injecting chemicals into the plant’s tissue during rapid cell division.
Some stem galls look like miniature apples, others like hairy spikes, and on leaves a few resemble flat woolly buttons. Oaks, willows, evergreens, and desert shrubs each have a unique and specific subset of galls.
Insects responsible for these motley forms include many distinct species of miniature wasps and flies. Larvae of these insects find within the bizarre growths both protection from enemies and a secure food source.
To be galled, if you are a plant, means to be irritated either physically or chemically by an insect you cannot avoid. The response of the plant’s tissue—making a growth that encases the larvae—only helps the intruder survive. Scientists know of no benefit plants get from the presence of galls.
Still, there’s at least one redeeming feature for humans. Galls are rich in resins and tannic acid which have been used for centuries to make permanent inks. The first recipe for gall ink comes from Pliny the Elder. Texts written with this ink include the Bible and other early religious documents. With the demise of fountain pens, only artists and a few eccentrics use gall ink today.