more concerned by 2D foams, even if some points can be generalized. Then, on a hexagonal reference order, dislocations are dipoles formed by a pentagonal and a heptagonal cell. A $T1$ transformation in a hexagonal structure generates a quadrupole of two such dipoles .

An array of dislocations constitutes a grain boundary, with its own, remarkable dynamics. Grain boundaries are easily seen in the bubble rafts of Bragg and Nye [@mott], separating crystalline hexagonal grains of different orientations. We shall see that grain boundaries occur in phyllotaxis (Fig.\[fg2\]) where they play an essential part in the growth of the structure.

The representation of a foam as a space-filling cellular network is a simplification of the complex, biphasic material where an interface is a concrete physical object (the continuous fluid phase). But it is also a natural extension when the interfaces have no concrete existence per se, as in granular packings or in phyllotaxis where the constituents are individual grains or florets packed together.

Phyllotaxis (leaf arrangement in botanics - e.g. Fig.\[fg2\]) is a space-filling structure with specific symmetries different from the translation invariance of a crystal, the inflation-deflation symmetry of a quasicrystal and the global uniformity of a random foam. 