What phyllotaxis is for
Why leaves land at 137.5 degrees and what that buys the plant.
A fern does not decide where to put its next leaf. It puts it 137.5 degrees around from the last one, and that single rule is enough to stop any leaf sitting directly under another.
The angle is what you get when you divide a circle by the golden ratio. Nothing about the plant knows that. The rule is local and the pattern is global, which is the part worth sitting with.
The pattern is not the plan. The pattern is what is left when the rule has run long enough.
from the notebook