Botanists do not quite know what processes regulate
the formation of primordia. The two leading models are of reaction-diffusion
(biochemical) and buckling (biomechanical).
Reaction-Diffusion involves chemical reaction
and diffusion of a mixture of chemicals which, under certain conditions,
result in non-homogeneous spatial concentrations of the different
chemicals - and hence patterns. Reinhardt
et. al. (2000) showed that the hormone auxin has a role in the
formation and positioning of primordia, an evidence pointing to
chemical regulation of the pattern formation. [That reaction-diffusion
can lead to non-homogeneous concentration was first proven mathematically
in 1952 by the mathematician/computer scientist Turing
using Partial Differential Equations. It was later confirmed in
several actual chemical reactions, notably the so-called BZ Reaction.]
Buckling involves two layers of cells at the tip
of a plant: the corpus, or center of the stem, and the tunica, or
outer layer. The botanist J. Green proposed that the tunica, as
it grows faster than the corpus, buckles (similarly to the skin
on fingers after a prolonged bath). As with reaction-diffusion,
the mathematical modeling of this phenomenon involves systems of
partial differential equations. Using stress analysis of incisions
of growing heads of sunflower, Dumais
and Steele (2000) give some supporting evidence for buckling.
Experiments were apparently less conclusive in some other plants
(Snow and R. Snow, 1951).
One can also imagine a combination of both mechanisms: auxin is
known to make cell walls more flexible. This localized flexibility
in turn may facilitate the buckling in specific places (R. Meicenheimer,
How Plants Grow - Electron
Micrographs of Meristems - How
Do Primordia Form ?