the structure, capable (through $T1$ flips at grain boundaries) of responding easily to (shear) stress, whether local or global.

Summary
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In summary, cells are organized in grains, concentric blocks separated by grain boundaries, surrounding a central core: A succession of large grains of hexagonal cells that are concentric circular annuli, bounded and separated by circular grain boundaries $[f_{u-1}, f_{u-2}, f_{u-1}]$ made of $f_{u-1}$ heptagonal cells, $f_{u-2}$ hexagonal cells, and $f_{u-1}$ pentagonal cells.

Growth of the structure (i.e. addition of further cells $n+1,...$) can be achieved by addition of a first cell, under which the sequence $c(s)$ is invariant. The spiral lattice includes defects and grain boundaries that are responsive to local shear stress. Local chirality in the pattern gives the sense of shear and makes the response unambiguous.

Using foam terminology, a grain boundary in phyllotaxis constitutes one layer, with successive Fibonacci numbers of cells, depending on whether one counts outwards or inwards [@oguey], with the difference - another Fibonacci number - made of (pentagonal) defect inclusions. In phyllotaxis (and not in foam), it is at a critical point and has a definite chirality, ready to flip under shear.

The grain boundaries serve as natural boundaries for optimal packing 