phyllotaxy

Phyllotaxy, also spelled phyllotaxis, is the pattern of leaf arrangement on a plant stem and the organization of leaf primordia in the shoot apex. It describes how successive leaves are positioned relative to the stem and to each other. Patterns include alternate (one leaf per node), opposite (two leaves per node, often decussate with successive nodes rotated), whorled (three or more at a node), and various spiral arrangements.

Many spiral phyllotaxes show a constant divergence angle between successive leaves, often near 137.5 degrees, the golden angle. This arrangement can maximize light capture and minimize shading, and it yields parastichies—visible spirals that commonly follow Fibonacci numbers.

Phyllotaxy results from the interaction of leaf initiation at the apical meristem, local auxin distribution, and mechanical constraints within the growing shoot. The plastochron, the interval between successive leaf initiations, influences the pattern, as does the connection of leaf traces to vascular bundles in the stem.

While largely species-specific, phyllotaxy can be affected by environmental factors such as light direction and nutrient availability in some plants. It is a central topic in botany, with implications for plant form, physiology, and taxonomy. Notable examples include sunflowers and many conifers, where Fibonacci-related parastichies appear in the arrangement of seeds, scales, or leaves.