values of $s = 0,1,2,...,n$. Changing the curvature can have a real meaning, as for pine cones or pineapples where the physical substrate is not planar, but it can also be a way of taking into account local changes in the sizes of the cells, such as larger florets on the periphery of sunflowers.

Then, one associates to each point a Voronoi cell or tile (Dirichlet region) [@sadoc1; @sadoc2]. The position of each point $s$ is given by its polar coordinates $(\rho,\theta)$ for example, on the plane $$\begin{aligned}
\rho(s)=a \sqrt{s} \textrm{~and~}   \theta(s)=(2 \pi/ \tau) s\end{aligned}$$ (here, Fermat’s spiral $\rho \sim \sqrt{\theta}$ is the generative spiral) and the azimuthal angle between two successive points on the spiral is $2\pi/\tau$, where $\tau = (1+\sqrt{5})/2$ is the golden ratio [^1]. We take this azimuthal law as part of the model leading to the lowest anisotropy of the cells and to the densest packing under these conditions. (A hexagonal structure is the densest 2D packing solution for an infinite crystal or for finite crystals with fitting boundaries. Phyllotaxis is the densest packing for circular boundaries or issued from an isotropic seed. As the structure grows by increasing the number of points on the generative 