The trunk and branches
Like the roots, trunks and branches have three functions. They, too, store sugars in the dormant period. They act as highways, transporting water and nutrients from the roots to the leaves, and energy-rich sugars from the leaves to other parts of the tree. And they ensure that the leaves have a high and wide distribution, in order to gain as much exposure to sunlight as possible. If you look at a cross-section of a trunk or branch, you’ll see the familiar annual rings, of which each ring indicates a year’s growth. Fast-growing trees have thick annual rings and slow-growing trees demonstrate thin ones. A 100-year-old bonsai with a 5cm (2in) trunk will have annual rings that are no more than 0.25mm(0.09in) wide!
In the centre of the trunk, the wood is darker and harder. This is the heartwood, which is dead, it neither stores sugars nor transports water. Heartwood is just structural timber, a prop to add physical strength to the trunk to enable it to support branches. This is why trees which become hollow can still thrive, though, naturally, a few do fall down.
The fine roots growing from the thicker ones absorb moisture and nutrients from the soil. Roots of intermediate size are unnecessary on a bonsai tree.
The paler wood is sapwood. This, too, is structural, but the outer rings are also involved in the transport of water and nutrients from the roots to the growing parts of the tree: the shoots, the leaves, flowers and fruit.
The rings which do the transporting are called the xylem (pronounced ‘zylem’). As xylem ages, it becomes less efficient until it ceases to have any function at all and becomes heartwood. In winter, the sugars are largely stored in vessels called medullary rays. These rays will radiate through the sapwood from the outer edge of the heartwood.
On the outside of the trunk are two darker, softer layers. The innermost layer is the phloem (pronounced ‘flo-em’), which distributes sugars from the leaves to the other parts of the tree, giving them energy to grow. If you ringbark a tree – cut a strip of bark away round the trunk – the tree will die. Not because the crown of the tree is starved of water, but because the roots don’t get the sugars they need to survive. When you damage the phloem by deep-pruning a branch or allowing training wire to crush the bark, it interrupts the flow of essential sugars. This may lead to the death of the roots below the damaged area. Each year a new layer of phloem is produced, but this doesn’t normally lead to the formation of such clearly visible rings as the xylem.
Enveloping the phloem is bark, which varies in thickness and texture according to species. Bark is made up of an accumulation of old, spent phloem, and has a variety of practical purposes. Bark is waterproof, so it prevents moisture from leaking out of the phloem. It is also home to small structures, called lenticels, which permit the trunk and the branches to ‘breathe’. Another function that bark performs is to protect the phloem from impact, abrasions and attack by a variety of insects or fungal infection.
These crowded pine candles have arisen in response to consistent pruning. They were generated by the activity of the cambium layer.
The cambium
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.
CROSS-SECTION THROUGH A LEAF
THIS SECTION THROUGH A LEAF SHOWS THE STOMATA AND THE SAUSAGE-SHAPED GUARD CELLS THAT ARE RESPONSIBLE FOR OPENING AND CLOSING THEM.
The foliage
The leaves are the food factories of a tree. They take water supplied by the roots and carbon dioxide from the atmosphere and convert them to complex sugars, making use of sunlight to energize the reaction, in a process known as photosynthesis. The sunlight is captured by the green chlorophyll, which acts as a catalyst for the chemical reaction that takes place. Even red-leaved species feature green chlorophyll, but this may be masked by the red pigmentation which is present in greater abundance.
During the day, the leaves take in oxygen and carbon dioxide through pores, called stomata, which are usually found on the underside of each leaf. At night they expel carbon dioxide by respiration. The stomata are able to open and close in response to the ambient temperature and humidity, thus controlling the rate at which water evaporates from the leaf. Some water has to, of necessity, evaporate all the time in order that a fresh supply