An examination of the mechanical interaction between elongating roots and rigid pores of precisely known size is reported. Sheets of steel mesh and glass capillaries were used as systems of rigid pores. The roots of grasses were found to be capable of penetrating pores much smaller than their nominal thickness, this capability being limited by the size of the root cap and the stele. Constricted root tips elongated at a slower rate but could grow down long capillaries if adequately aerated. The size of rigid pore critical to the growth of perennial ryegrass was 315 μm.