Drawing a tree means, up to a point, drawing its portrait, and the surest way to success is the" constructive," not the impressionistic, approach. First, there is the general line of the central trunk below the branches. As soon as a major limb branches off, however, the whole trunk usually changes its direction slightly. It bends away from the branch, just as the human body bends to adjust its equilibrium if the weight is put on one side.
This is the rule for all trees with alternating branches, apart from a few individual exceptions, or where conditions of growth force a deviation from the rule. The majority of trees in the European and American landscapes grow to this pattern. The zig-zag growth of the trunk with alternating branches is always seen in saplings and is repeated in the leaf arrangement. The twigs do not grow on a plane with the bough or trunk but are grouped spirally around it.
As the tree ages and becomes thicker, the zig-zag tendency becomes less noticeable. Twigs break off or grow irregularly, so that a static symmetry is no longer necessary. As the trunk thickens, the distance between the offsh,oots becomes smaller. Sometimes, too, the horizontal ground level develops a slope, and the trunk has to go with it; the upper part will tend towards the vertical again and give a general curve to the trunk. It may be that continuous washing away of soil around the roots makes the tree grow crooked from the beginning, and the weight of the trunk and branches prevents its righting itself to the vertical.
This condition is often seen beside water, particularly in willows, some species of which have an additional tendency to grow strange bends and curves in trunk and boughs. If the main trunk is broken off while the tree is young, two or more main stems generally form, and it looks as though several trees were growing from one thick trunk. Through such observations much can be learned of the history of a tree from its growth and deviations from the normal habitus; it has to grow in situ, whatever changes occur in its environment, and adapt itself to these changes.
Among the commoners trees in Europe, only the maple, horse chestnut, and oak have paired branches, and, of course, the conifers, which each year produce a new whorl of twigs at the top of the main stem. For obvious reasons these trees all grow straight, especially the conifers.
Their branches always remain relatively small compared to the main stem, whereas the oak, consistent with its compact shape and slower growth, forms branches hardly thinner than the trunk, and when the tree is full grown it is often impossible to distinguish the trunk amid the maze of branches at the top. Another reason for this is that the main stem of a young oak breaks very easily; this is also the case with fir trees. As a rule the oak does not grow so straight as it gets older; also the trunk gets thicker, and the branches tend to grow more strongly than the trunk.
A thick wood is the best environment for straight, regular growth. The need for light makes the stem grow as vertically as possible, and as the top grows higher the lower branches die off. They cannot spread out, and there is not light enough for their leaves. If one of these trees remains standing when a wood has been cut down, it looks most unnatural. Constant strong winds from the same direction can also make a tree look unnatural, with branches growing only to the leeward side. This happens mostly near or on the tops of mountains or where the wind has a long unbroken run.