Phyllotaxy is the arrangement of leaves on a stem. As a stem grows at its apex, new leaf buds form along the stem by a highly controlled developmental process. Depending on the species, the leaf origins on the stem may be opposite (in which leaves arise in pairs on opposite sides of the stem), whorled (three or more leaves arise from the same locus on the stem), or alternate (leaves are arranged in a helix along the stem).
Most species have alternate leaves. This pattern is often called spiral phyllotaxy because a spiral is formed when an imaginary line is drawn which connects progressively older leaf origins on the stem. The divergence angle of successive leaves determines the developmental spiral of leaves and homologous plant organs, such as the individual florets of a sunflower, and has been intensively studied by botanists and mathematicians since the mid-1800s. Interestingly, the angle between successive leaves on a stem is often about 137.5 degrees, known as the Fibonaci angle.
Two major theories have been proposed to explain spiral phyllotaxy: ava ble space theory and repulsion theory. Both propose that the siting of leaf nodes is determined by the position of existing leaf nodes. While the two are often portrayed as competing theories, they are not in fact mutually exclusive. Available space theory says that the physical space among existing leaf nodes determines where new leaves originate. Repulsion theory says that synthesis of a chemical growth inhibitor(s) at the apex of older leaf nodes determines the position of new leaves, and that new leaves form only where the concentration of the growth inhibitor(s) is below a certain threshold.