Phyllotaxis
the spirals are counted, not admired
Concept

Plastochron — where it appears

The interval between one organ being placed and the next. It is the clock a stem's pattern is written on, and expressing a rate in plastochrons rather than in time is what makes a threshold comparable between specimens.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

A stem unrolled: 28 nodes at 137.51° with a rise of 0.090 circumferences. The counter is shown these coordinates and the circumference, and finds 2 parastichies one way and 3 the other. The faint strips left and right are the same stem: a family leaving one edge re-enters at the other.

A stem is a cylinder

The sunflower is the photograph, and it is the hard case. Nearly all real phyllotaxis happens on a stem, where the geometry is a lattice on a cylinder with two parameters — and where the spiral counts, which on a disc change with radius, are the same the whole way up.

cylinder · Stem lattice Three disturbances, three places to get in. The rule reads its neighbours, builds a profile of the energy at every azimuth, takes the least of it, and records a position. jostle noise enters at the neighbours; placement noise enters at the record. Two of the three are upstream of the choice and can change which minimum is taken; the third is downstream and never can.

A growing organ is part of the rule

Every model in the earlier essays places primordia on a surface and then treats the surface as furniture. But the surface grows between one placement and the next, and that growth reaches the rule through the only channel it has — where the neighbours are. What looks like a boundary condition turns out to be a term in the model.

mechanism · Mechanism claims

Named alongside it

The objects these essays reach for when they reach for this one.

AuxinBasinCylinderCylindrical latticeDescription versus mechanismDivergence angleElongationEpitheliumHonest limitsLadderLocal minimumMechanism

All concepts