The Stack-and-Drag Model is a mathematic, not physiologic model for phyllotaxis. Growth is simulated by separately stacking and transforming. These stills are from a movie (1992).
(top left, dia 1) Two touching spheres are like cotyledons in an embryo (see apex/basics).
(2) Spheres are stacked on each other, with their centers on a cylinder with sphere radius. At a chosen moment, a hemisphere closes the cylinder.
(5) Stacking ends when the surface is covered.
(6) The bubble disappeared, transforming has begun.
(7-11, with changing viewpoint) The top of the structure expands, while the bottom parts are imploding. This is because flower (radially) and stem (axially) growth is too much to keep the model in size.
(12) A flower pattern results.