Parastichy

Parastichy is a spiral line that connects successive elements in a plant’s phyllotactic arrangement, such as leaves around a stem or seeds in a flowering head. A parastichy traces a path through points that lie roughly along a spiral around the axis, and in many plants two main families of parastichies can be observed in roughly perpendicular directions, producing a characteristic grid-like pattern on the surface.

In many phyllotactic patterns the numbers of parastichies in these two families are consecutive Fibonacci numbers. For example, sunflowers commonly display 21 and 34 parastichies in the two directions, while pinecones may show 8 and 13 or 13 and 21. This Fibonacci structure is associated with the near golden-angle divergence between successive elements, about 137.5 degrees, which helps to maximize packing efficiency and minimize overlap.

Parastichies are typically not exact mathematical spirals but are well approximated by logarithmic spirals that trace through the lattice of points on a cylinder when viewed in projection. They provide a practical description of the geometric regularities in plant form and have been central to formulations in phyllotaxis and growth models.

The term parastichy derives from Greek para, meaning beside or alongside, and stichos, meaning row or line.