A spiral pattern can be thought of as a special type of radial pattern. Like radial patterns, they converge to a single point. But spirals use iteration (repeating processes that are accumulative) to change the spacing of the concentric rings in a regular, mathematically predictable way. Similarly, the placement of modules on those rings changes in a repeatable specified way.
A special spiral artists call the equal-angular spiral (biologists call it phyllotaxy) can be discovered everywhere in nature-- in the structure of a nautilus shell, the growth of a pine cone, or the pattern of a sunflower's seeds.
