Reaction wood
Trees differ from other plants in their ability to grow tall and large which enables them to obtain more sunlight than their smaller neighbors.
As a tree enlarges, it develops sapwood and heartwood. The sapwood is sturdy and flexible, while the heartwood is strong and ridged. This woody structure helps to support the weight of the tree and allows it to expand to a large size.
The spreading root system also helps balance weight and keeps the tree anchored to the ground. Trees, like other plants, produce growth hormones which regulate the rate at which they grow. In most cases, a tree will grow only fast enough to support itself and carry out its normal functions. A straight tree has a better chance of staying erect than a tree that is twisted or leaning.
Large tree branches can also place a lot of force on the tree. Trees compensate for a lean and for heavy branches by producing reaction wood, forming in place of normal wood in response to gravity and the deposition of growth hormones. Reaction wood is much denser and stronger than normal wood, and its purpose is to balance the forces of gravity.