A common method for measuring the cell-wall extension activity of expansins. (a) Cell-wall specimens are excised from the growing region of a young seedling that has been grown in the dark (etiolated). The specimens are frozen and thawed in order to destroy the cells but leave the cell walls intact (the cuticle is abraded to facilitate penetration of proteins). The specimens are heat-treated to inactivate endogenous expansins and then clamped under constant tension in an extensometer. The extensometer measures the change in length of the sample, with or without the addition of exogenous expansins. Walls may be collected in parallel from other seedlings and extracted to obtain fractions with expansin activity, assayed as an increase in cell-wall length. (b) Photograph of a typical cell wall sample, placed on an index finger for scale, prior to clamping in the extensometer. (c) Time course for irreversible wall extension (creep) of heat-treated walls with and without the addition of expansin.