Pollen tubes are tip growing plant cells that display oscillatory growth behavior. It hasbeen demonstrated experimentally that the reduction of the average pollen tube growth ratethrough elevated extracellular calcium or borate concentrations coincides with a greateramplitude of the growth rate oscillation and a lower oscillation frequency. We present asimple numerical model of pollen tube growth that reproduces these results, as well asanalytical calculations that suggest an underlying mechanism. These data show that thepollen tube oscillator is non-isochronous, and is different from harmonic oscillation.