Lisa Pursey, of Waimairi School, asks :-

If you carved your initials into the bark of a tree and then visited the tree thirty years later, would the initials then be high up the tree?

Walter Fielding-Cotterell, an arboriculturist with the Christchurch City Council, responded.

Trees increase in height by way of cells produced by tissue contained in the tip of every shoot. This very small piece of cell dividing tissue is called the apical meristem. Cutting a cabbage sprout in half lengthways will give you a good idea of how an apical meristem is formed, the dome-like cluster of cells in the centre being the meristem itself. Radial growth is provided by the cambium, a thin cylinder of dividing cells extending from the top to the bottom of the tree. New conductive cells are formed on the inside and outside of the cambium layer increasing the diameter of the stem. Another layer of cambium, the cork cambium, forms the bark which protects the tree.

The function of the tip or apical meristem can be likened to chimney builders standing near the top of the chimney and constantly producing and laying bricks (cells) always rising with the height of the structure. Other builders (the cambium) continuously reinforce the chimney by building horizontally. Therefore, the brick or cell laid at 1.5 metres above the ground remains at that height. However the rapid production of cells beneath the bark causes it to rupture and repair itself (and the carving wounds). The new cells formed will fill and widen the initials until they eventually disappear.