What Is That Lump on the Leaf? A Plant Building a House for Something Else

A gall is plant tissue, grown by the plant from its own cells, in a form the plant would never produce by itself, and the shape is determined by whatever is inside rather than by the plant. The inducing organism, most often an insect larva, releases chemicals that redirect the plant's growth, causing cells to divide and differentiate into a structure that encloses and feeds it. The resulting structure is frequently elaborate, with distinct layers including a nutritive tissue the larva eats and a protective outer wall, and it is specific enough that the species responsible can usually be identified from the gall alone without seeing the occupant. How the chemical control works is not fully understood and is an active research question.

Several kinds of organism produce galls and the insects dominate:

• •Gall wasps, which produce many of the most elaborate galls, particularly on oaks
• •Gall midges, which are the most numerous group
• •Aphids and their relatives, producing distinctive pouches and twists
• •Mites, producing felty patches and small pimples
• •Fungi and bacteria, including the bacterium used in genetic engineering
• •Nematodes, producing root galls of considerable agricultural importance

A gall is a resource and other organisms exploit it, producing communities of surprising complexity. Inquilines are species that cannot make galls themselves and live inside those made by others, feeding on the tissue and sometimes killing the original occupant incidentally. Parasitoids lay eggs into the gall and their larvae consume the occupant, and several layers of these exist, with parasitoids of parasitoids documented. Predators break in from outside. Studies of single oak gall types have identified communities of dozens of associated species. The result is that rearing a gall in a container frequently produces something other than the species that made it, which is a standard experience for anyone attempting it and a reason identification from the gall rather than the emerging insect is preferred.

Galls are among the easier things to record because the structure stays put and identifies its maker. Oaks host more gall-inducing species than any other plant in Europe and are the obvious place to start, with recognisable galls on leaves, buds, acorns and roots at different seasons. Roses, willows, brambles and knapweeds all carry distinctive ones. Identification works from the plant species, the part affected, the season and the structure's form, and guides are organised on exactly that basis rather than by insect taxonomy. Many gall wasps alternate between two generations that produce completely different galls on different parts of the plant, which confused naturalists for a long time and means a single species may appear twice in a guide under different descriptions.

One gall had enormous historical importance. Oak galls induced by a particular wasp contain very high concentrations of tannins, and boiling them with iron salts produces a dark liquid that penetrates and bonds to paper and parchment, which is iron gall ink. That ink was the standard writing medium across Europe and the Middle East for well over a thousand years, used for most surviving medieval manuscripts, for legal documents and for the work of essentially every European writer until the nineteenth century. It is also destructive over time, since the acid and residual iron attack the paper, which is why archives spend a great deal on conserving documents written in it. The galls were traded internationally as a commodity.

A gall is plant tissue grown into a form the plant would never make, with the shape determined by chemicals released by the organism inside and with a nutritive layer it feeds on. Wasps, midges, aphids, mites, fungi and nematodes all induce them. Oak galls rich in tannin produced the ink used for most surviving medieval manuscripts, which attacks the paper over time.