Typically a shoot has leaves attached to a stem. The apex of the stem has an actively growing apical bud. It produces additional nodes and internodes to add to the length of the shoot. Additional leaves appear at the nodes as they are produced.
In some plants, the lateral bud located in the axil of each leaf does not grow to form branches, especially at first. This condition is known as strong apical dominance.
Apical dominance is thought to be caused by the apical bud producing IAA (auxin) in abundance. This auxin is transported basipetally from the apical bud. The auxin causes the lateral buds to remain dormant. How could a lower concentration cause lateral buds to remain dormant and a higher concentration cause the apical bud to grow? This is explained in the following graph:
The difference in response between the two kinds of buds is explained in their sensitivity to the auxin concentration. Clearly the lateral buds are more sensitive to auxin than the apical bud. There is a concentration of auxin at which the apical bud is stimulated to grow while the lateral buds are inhibited. That concentration would be near the letter "C" of "Concentration" in the graph above.
When the apical bud is removed, the source of IAA is removed. Since the auxin concentration is much lower, the lateral buds can now grow. In fact their growth will be stimulated by a relativley small drop in auxin concentration (see graph). Thus, decapitating (pruning) a shoot will cause it to branch!
Such response is the basis for all pruning "to form." By pruning carefully, the branching pattern and form of the plant can be controlled. If pruning limits plant growth to two dimensions against a wall, we call it espalier. Three-dimensional pruning to form would be a kind of sculpture, called topiary. Perhaps you have observed topiary in the form of Mickey and Minnie Mouse at Disney World. If both shoot and root pruning is done frequently, the result can be a bonsai, the creation of a miniature tree. Frequently bonsai trees are not only dwarf in stature, but also respond to the severe pruning by producing dwarf leaves as well. Bonsai is an art form that involves miniature landscaping around the dwarf tree.
It might be worthy of note that an artist working in paint, marble, or bronze gets paid only once for her/his creative work. The plant artist working in espalier, topiary, or bonsai must be paid repeatedly to keep the work true to the original form as the work is alive and responsive! If the artist is an entrepreneur, the choice of material would be obvious.
To demonstrate how apical dominance works, we can decapitate a shoot and thus remove the natural source of IAA. But if we replace the apical bud with a dollop of lanolin (wool wax) with auxin dissolved in it, apical dominance is not diminished. The lateral buds remain dormant!
At this point you are left with the challenge to think of other ways you might manipulate the responses of plants to applied chemicals and surgical treatments. You should also be thinking of what would constitute appropriate controls for each demonstration.
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