Josh Payne, of Clinton Primary School, asks :-

When climbing a gum tree I found a knobbly wooden ball, about the size of an orange, which came away in my hand. What could this be?

Janice Lord, a botanist at the University of Otago, responded.

When you stab your finger on a prickle or get an insect bite, your body reacts to the perceived threat. The area may swell and scar tissue might form.

When a plant is damaged or attacked by an insect, fungus or other pathogen, the plant tissue may react in a similar way producing a deformity. Damage to the woody parts, such as roots, trunk or branches, of a tree often result in normally dormant buds growing into ball-like lumps called burrs or burls. These are made of very dense solid wood and usually are covered in bark, but also show spectacular swirls and patterns not seen in normally developed wood.

Because of these beautiful patterns burrs or burls are highly valued by woodworkers and cabinetmakers. Also because they are made of solid wood, they can be distinguished from other plant deformities such as galls, which are generally hollow and form around the attacking insect or fungus. The structure of the gall is, in part, controlled by chemicals produced by the attacker, meaning it is creating its own home while feeding on the plant. Galls can be so highly organised that it is possible to identify the origin attacker from the structure of the gall itself even when the insect or fungus is long gone.

In contrast, it is not possible to tell from a burr what damage originally triggered its formation, however unlike a gall, a burr can continue to grow long after the original injury has healed. The largest burrs known occur on Californian Redwood trees (Americans called them burls) and can measure more than 7m in diameter. Imagine making that into a fruit bowl!

Send questions to: Ask-A-Scientist, PO Box 31-035, Christchurch 8444 Or email: