Fig. 1.

External and internal mechanical forces cause deformation (strain) of plant cells. When a plant organ is bent (top), cells on the convex side are stretched (experience positive strain) while cells on the opposite, concave side are compressed (negative strain). During rapid turgor-driven cell expansion (bottom), local positive strain rates as high as 50–70% h^–1 have been measured in the elongation zone of maize and Arabidopsis roots (Ishikawa and Evans, 1993; G. Monshausen and N. Miller, unpublished data).