Apical dominance is a phenomenon seen in plants in which a central stem becomes dominant, growing faster than other stems and secreting hormones which inhibit stem growth below the terminal bud at the end of the apical stem. A number of plants and trees exhibit apical dominance, with fir trees being a classic example. The distinctive triangular shape of the fir tree is the result of apical dominance, with the trunk of the tree being the apical stem.
There are a number of reasons why plants adopt this process. By pouring energy into a central stem, plants can achieve height quickly, which increases their access to nutrients and also helps the plant block competition. This phenomenon is also exhibited in rooting systems, where apical dominance results in a long, very strong taproot which keeps the plant firmly anchored in place, unlike a fibrous root system, which can make the plant unstable in some soils.
The terminal bud on the apical stem secretes the hormone auxin, which blocks growth in lower stems. If it is removed or the circulation is inhibited, other stems below will start to shoot up, and one may in turn become apical. Apical dominance can also be seen in branches; branches which grow from the main stem will in turn inhibit the growth of smaller branches from stems along their length.
Gardeners can manipulate plant shape by removing terminal buds. Doing this promotes a bushy, shrublike growth habit with a number of spreading branches, none of which becomes dominant. This look may be desired for some trees and plants, and gardeners can use pruning and other manipulative techniques to force plants to grow in a particular way. It is important to identify the terminal bud for removal if the goal is to create a shrubby growth.
In trees and plants which lack apical dominance, growth tends to take on a more chaotic and less controlled appearance. The branches will sprawl, and many will be the same length or very similar in length because no stem is dominant. Tomatoes are an example of a plant which does not exhibit this phenomenon, with all of the stems growing at more or less the same rate. These plants tend to grow out rather than up, sometimes requiring support for their spreading branches, as they are designed to eliminate competition by spreading out to create a cleared area all their own.