Fyllotaxi

Fyllotaxi, or phyllotaxis, is the study of how leaves and other lateral organs are arranged around a plant stem or axis. It concerns the spatial pattern of organ initiation at the shoot apical meristem as a plant grows, and how these patterns influence light capture, reproduction, and mechanical stability. The term is used across botany to describe various arrangements, from simple alternate patterns to more complex spirals.

The most common pattern in flowering plants is a spiral arrangement, where successive leaves occupy positions at regular angular intervals around the stem. The divergence angle between consecutive leaves is often close to the golden angle of about 137.5 degrees, which tends to minimize overlap and shading. Other patterns include decussate arrangements, where successive leaves align in pairs at right angles, and whorled arrangements, where three or more leaves emerge at a single node. A plant may exhibit multiple visible spirals, or parastichies, corresponding to different Fibonacci or other numerical sequences.

Developmentally, phyllotaxis arises from the initiation of leaf primordia at the shoot apex and the subsequent growth of the meristem. Models emphasize factors such as the distribution of growth hormones like auxin, the plastochron (rate of organ initiation), and the geometry of the growing tip. These factors can produce robust, reproducible patterns even under varying conditions, and small changes can shift patterns from one regular arrangement to another.

Significance of phyllotaxis lies in its role in maximizing light interception, improving packing of seeds or florets, and revealing underlying developmental rules. Classic examples include sunflowers and pine cones, where visible spirals follow Fibonacci numbers, illustrating the link between biology and mathematics.