Phyllotactic

Phyllotaxis refers to the arrangement of leaves, seeds, or other plant organs around a stem or axis. When used as an adjective, phyllotactic describes patterns or structures produced by this arrangement. The study of phyllotaxis examines how plants initiate new organs at the growing tip and how subsequent organs position themselves around the axis in a regular pattern. These patterns are common across flowering plants and many conifers.

Common patterns include spiral phyllotaxis, where successive organs are offset by a divergence angle; parastichies are the visible spirals that run through the pattern. The divergence angle at which organs are initiated is often close to the golden angle of about 137.5 degrees, yielding parastichies that follow Fibonacci numbers. In some species, leaves appear in decussate patterns (two-ranked), described by phyllotactic fractions such as 2/5 or 3/8, where the arrangement repeats after a fixed number of steps around the stem.

Biologically, phyllotaxis arises from growth at the shoot apex, hormonal control (notably auxin), and mechanical constraints in the meristem. The pattern is thought to optimize light capture and resource distribution, though exact advantages vary by species. Patterns can change during development or under environmental influence, a phenomenon known as phyllotactic transitions.

Mathematical and computational models, including circle-packing and other growth rules, describe phyllotactic patterns as logarithmic or near-logarithmic spirals. The concept is observable in many plants, such as sunflower heads and pinecones, where seeds or bracts arrange in distinct spirals following Fibonacci parastichies. The term phyllotactic thus covers both the empirical patterns and the mathematical descriptions used to analyze them.