We consider natural and artificial hygromorphs, objects
that respond to environmental humidity by changing
their shape. Using the pine cone as an example that
opens when dried and closes when wet, we quantify
the geometry, mechanics and dynamics of closure and
opening at the cell, tissue and organ levels, building
on our prior structural knowledge. A simple scaling
theory allows us to quantify the hysteretic dynamics
of opening and closing. We also show how simple bilayer
hygromorphs of paper and polymer show similar behaviour that can be quantified via a theory which couples
fluid transport in a porous medium and evaporative
flux to mechanics and geometry. Our work unifies
varied observations of natural hygromorphs and
suggests interesting biomimetic analogues, which we
illustrate using an artificial flower with a controllable
blooming and closing response.