  Ratcheting surfaces are a common motif in nature and appear in plant awns and
grasses. They are known to profer selective advantages for seed dispersion and
burial. In two simple model experiments we show that these anisotropically
toothed surfaces naturally serve as motion rectifiers and generically move in a
unidirectional manner when subjected to temporally and spatially symmetric
excitations of various origins. Using a combination of theory and experiment we
show that a linear relation between awn length and ratchet efficiency holds
under biologically relevant conditions. Thus, grass awns efficiently transform
non-equilibrium environmental stresses into useful work and directed motion
using their length as a fluctuation amplifier, yielding a selective advantage
to these organelles in many plant species.
