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Between the xylem and the phloem is what may justly be called the most crucial part of the tree, the cambium. This layer is just one cell thick and shows as a bright green film when the outer ‘skin’ (the phloem) of a twig is scratched away. In spite of its thinness, the cambium is highly active. Throughout the growing season the cells are constantly dividing, producing new xylem cells on the inside and new phloem cells on the outside. When winter comes it slows almost to a standstill, while a new ring forms.

The cambium is able to adjust its work rate to the growth conditions of the tree. In situations in which a tree can’t get sufficient water or nutrients – or when the tree is confined to a pot – it slows down the rate of cell division.

When a tree is adequately fed and watered, the cambium speeds up, producing thicker annual rings. In bonsai, we are aware that life in a pot is bound to affect the vigour of a tree, so we must attempt to counterbalance this restriction by creating a very efficient root system and feeding it well.

If the cambium is kept as active as possible, the trunk thickens more rapidly, which helps the bark to mature, increasing the tree’s value. The cambium is enormously versatile, so much so that it is even able to alter the nature of new cells to perform any number of essential tasks. When grafting, it’s vital to get the two foreign cambium layers to meet exactly, because it is these that ‘fuse’ together. Once fusion has successfully taken place, the new xylem and phloem cells that it produces within the union are able to function as continuous pathways.

If you cut through a branch in summer, you will eventually find a ring of fresh buds crowding around the cut between the wood and bark. These have been developed by the cambium layer, which has modified its function in response to losing the supply of hormones produced by actively growing shoots and buds. Adventitious buds (ones which are produced at random) growing from the older branches and trunks of trees are also generated by the cambium in response to stress higher up the tree. When cuttings are taken, the cambium generates the new roots for the new plant. It also gives rise to new roots during the process known as air-layering.

Adventitious buds have been generated randomly, in odd positions on this bougainvillea’s trunk, in response to damage.

A Trident Maple (Acer buergeranium) forest in miniature without its characteristic foliage, as the buds are in their dormant period. Since the buds of all plants are as individual as their leaves, it is as easy to identify them in tight bud, in winter, as it is when they are in full leaf.