What is a Bonsai: How Trees Grow

What is a Bonsai?

Although the cultivation of bonsai requires some highly specialized techniques, bonsai themselves are first and foremost just trees, and as such grow in much the same way as the trees in woods and forests. So it is a good idea to remind yourself what a tree is, the various parts of its anatomy, how a broad-leaved tree differs from a conifer and a deciduous one from an evergreen, and above all, what conditions favour its development. All of these elements are indispensable to growing bonsai successfully.

Bonsai - how trees grow

What are Trees?

Of all plants, trees live the longest, thanks to a permanent structure of trunk and branches, which ensures that the leaves, flowers and fruits are nourished. This rigid permanent structure provides for the tree’s long life; it is made of that particularly resilient substance, wood.

A section cut across the trunk of a tree will show several zones. These are, starting from the outer ring and working towards the centre: the bark, the phloem, the cambium layer and the true heartwood. The cambium layer produces the wood fibres and the vessels which carry sap upwards to the aerial parts of the tree.

As the tree grows, these vessels become more numerous, with the new layers superimposed on the old, which gradually cease to play an active part in the life of the tree. In time, they harden to produce a substance which varies in its hardness according to the species: xylem, or wood. In this way, the annual life cycle of the tree produces successive layers of compressed xylem, clearly visible in a section of trunk. Each of these growth rings corresponds to one year in the life of a tree: so to ascertain the age of a tree, all you need do is count the rings.

section of trunk showing different layersIn a well kept bonsai, the production of cambium is continuous and regular, as long as the water supply is constant. But a tree growing in its natural surroundings may have to cope with dramatic variations in climate which can considerably influence its growth.

It is well known amongst bonsai collectors that a major preoccupation of the bonsai enthusiast is the thickening of the trunk. The art lies in allowing the trunk to thicken while miniaturizing the leaves and preserving the natural balance of the tree on a miniature scale. This can only be achieved by using artifice – wiring, and pruning the leaves, branches and roots. These methods can produce remarkable results.

Anatomy of a Tree

Whatever its species or variety, a tree has a particular anatomy, or structure, which is found in every example of its kind, regardless of age.

Underground parts

These consist of the roots of the tree which draw from the soil those sub-stances which are vital to the tree’s growth. A complex network of roots and rootlets ensures that the tree is at once firmly anchored and also fed. The capillary structure of the roots creates a continuous upward movement of water in the soil, making sure that the tree obtains all the substances it needs. The normal development of the aerial part of the tree (the focus of our interest) is linked to root development. If root growth is inhibited, the aerial part of the tree will stop growing and may even die, as a tree can only survive by growing. In bonsai, root growth is always restricted by the size of the container, which is why these trees must be regularly repotted.

Aerial parts

These consist of the trunk and the branch system. From the base of the trunk to where the first branches grow is known as the bole and the top of the tree, at the tip of the last branch, is the crown. The branch system consists of the branches and their sub-divisions, the most slender of which produce the leaves. In nature, the development of these elements is directly related to the physical environment, such as sunlight, temperature and wind. The art of bonsai lies in limiting the growth of the tree, while artificially maintaining the natural look of its trunk and branch system. This is done by precise cutting, pruning and wiring techniques.

A Eurya (Eurya emarginata) bonsai on display a...

Image via Wikipedia

Foliage

Leaves form an essential part of the tree and are directly involved in the tree’s growth processes. In fact, they make possible the trapping of the sun’s energy in the green chlorophyll they contain, a process vital to the development of all plant life. It is the leaves that absorb carbon dioxide from the atmosphere, a phenomenon which itself results from photosynthesis which takes place in sunlight. In darkness, particularly at night, the plant gives off the oxygen contained in the carbon dioxide. The carbohydrates necessary for growth are manufactured from water taken from the soil.

The shape of leaves varies from one species to another, but there is a fundamental difference between the leaves of pines and similar trees and the leaves of other trees. The former are generally evergreen (that is, the leaves do not fall all at once in autumn – or indeed at any time of year), the latter are deciduous, since the leaves drop from the trees in autumn. But one should be careful of such generalizations since there are many exceptions to the rule.

It is desirable to distinguish the needles of the conifers from the leaves of other types of tree. Leaves are made up of a petiole or leafstalk by which they are attached to the stem and a veined blade or lamina containing the cells which carry out photosynthesis.

Leaves may vary according to species and variety; they may be simple (the oak leaf, for example), multiple and compound (like the horse chestnut); they may be regular in shape, dentate or linear, ovate, lanceolate or cuneate, etc.

A conifer needle is very simple in structure. Its main characteristic is its narrow, elongated shape tapering to a point (hence the name). Unlike a leaf blade of a typical broad-leaved tree which is generally very thin, the needle is fleshy and thick, making it very resilient. But like leaves, needles also provide for photosynthesis.

The needle-like leaf is a good example of the way in which the leaf has adapted to a dry, cold or hot climate, by restricting transpiration. Because a needle lasts for several years, the tree is released from the burden of forming a complete set of new leaves every year. But it should be noted that the needle’s permanence is only relative, since they also eventually fall from the tree: a look under the nearest conifer will confirm this.

Once fallen, all leaves start to decompose and help to form humus. In this way, some of the minerals drawn up by the roots are returned to the soil.

Flowers and fruits

how bonsai grow

Almost all trees propagate themselves sexually from seeds, which develop as the result of the union of a male and a female cell. This union takes place within the flower, which is simply a leaf specially adapted for reproduction purposes. Seeds vary greatly in type and shape from tree to tree. For instance, they may take the form of a fleshy fruit whose pulp contains the seeds, or a protective nut, a berry, or an achene (dry, one-seeded fruit), or held within a cone in the case of a conifer.

Reproductive cells are produced in the flower by specialized organs: the stamens produce male cells (pollen), while the pistil produces the female cells (ovules). Where stamens and pistils are present in the same flower, the plant is said to be hermaphrodite; otherwise it is unisexual. In hermaphrodite plants, pollen falls directly on to the pistil sheltered by the petals, thereby fertilizing the ovules. In unisexual plants, pollen must be conveyed from flower to flower by the wind or honey-gathering insects.

The most beautiful flowers are frequently found on monoecious plants. Since the bright colours of the petals are designed to attract honey-gathering insects. The flowers of forest trees (as opposed to ornamental shrubs) are often lacking in colour, to the point of insignificance, hardly standing out from the leaves, since they are mainly adapted to wind pollination. Some plants bear flowers of one sex only, male or female (dioecious), others flowers of both sexes (monoecious), while some kinds have male, female and hermaphrodite flowers (polygamous).

Pollination produces seeds, which, after germinating in the earth will produce a new plant like its parent (unless the plant has been grafted). For successful germination, conditions, particularly humidity, heat and light must be favourable. The percentage of seeds which germinate in nature is very low, particularly in those trees whose seeds are small and without any food reserves.

I’m hoping now that you’ve absorbed all of this, that you know the answer to your question: What is a Bonsai?

23. February 2012 by Dave Pinkney
Categories: Bonsai | Tags: , , , , , | Comments Off on What is a Bonsai: How Trees Grow

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