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When pollen grains become exposed to the environment, they rapidly desiccate.To protect themselves until rehydration, the grains undergo characteristicinfolding with the help of special structures in the grainwall---apertures---where the otherwise thick exine shell is absent or reducedin thickness. Recent theoretical studies have highlighted the importance ofapertures for the elastic response and the folding of the grain. Experimentalobservations show that different pollen grains sharing the same number and typeof apertures can nonetheless fold in quite diverse fashion. Using thin-shelltheory of elasticity, we show how both the absolute elastic properties of thepollen wall as well as the relative elastic differences between the exine walland the apertures play an important role in determining pollen folding upondesiccation. Focusing primarily on colpate pollen, we delineate the regions ofpollen elastic parameters where desiccation leads to a regular, completeclosing of all apertures and thus to an infolding which protects the grainagainst water loss. Phase diagrams of pollen folding pathways indicate that anincrease in the number of apertures leads to a reduction of the region ofelastic parameters where the apertures close in a regular fashion. Theinfolding also depends on the details of the aperture shape and size, and ourstudy explains how the features of the mechanical design of apertures influencethe pollen folding patterns. Understanding the mechanical principles behindpollen folding pathways should also prove useful for the design of the elasticresponse of artificial inhomogeneous shells.

Mechanical design of apertures and the infolding of pollen grain