blackberry gall

A gall is an aberrant growth on a plant. Typically these are caused by an insect, and the growth acts as a rearing chamber for its larvae. How does the insect get the plant to grow it a nursery?

The insect, usually a tiny wasp or fly, injects the plant with a poison of sorts. This photograph hints at the underlying chemistry. The injected compound is likely a mimic of the plant's own growth hormones. In this case, it looks like the signal to grow a fruit was given to a blackberry stem. The fruiting hormone, when expressed in the proper tissue, triggers the production of a cluster of juicy orbs that will ripen into the black bear's favorite fruit. When expressed in the wrong tissue, the resulting growth is more tumorous than tasty.

Many galls appear to be fruits growing where fruits shouldn't grow, but sometimes they look more like an aberrant flower. The common oak gall, a hollow globe, may be what happens when you ask a leaf to grow an acorn. For an insect, a gall is a safe home. For a naturalist, they are lessons in gene expression and developmental chemistry.