  When pollen grains become exposed to the environment, they rapidly desiccate.
To protect themselves until rehydration, the grains undergo characteristic
infolding with the help of special structures in the grain
wall---apertures---where the otherwise thick exine shell is absent or reduced
in thickness. Recent theoretical studies have highlighted the importance of
apertures for the elastic response and the folding of the grain. Experimental
observations show that different pollen grains sharing the same number and type
of apertures can nonetheless fold in quite diverse fashion. Using thin-shell
theory of elasticity, we show how both the absolute elastic properties of the
pollen wall as well as the relative elastic differences between the exine wall
and the apertures play an important role in determining pollen folding upon
desiccation. Focusing primarily on colpate pollen, we delineate the regions of
pollen elastic parameters where desiccation leads to a regular, complete
closing of all apertures and thus to an infolding which protects the grain
against water loss. Phase diagrams of pollen folding pathways indicate that an
increase in the number of apertures leads to a reduction of the region of
elastic parameters where the apertures close in a regular fashion. The
infolding also depends on the details of the aperture shape and size, and our
study explains how the features of the mechanical design of apertures influence
the pollen folding patterns. Understanding the mechanical principles behind
pollen folding pathways should also prove useful for the design of the elastic
response of artificial inhomogeneous shells.
