The mechanical properties of young stems of Aristolochia macrophylla Lam. and Aristolochia brasiliensis Mart. et Zucc. were studied during elongation growth and primary differentiation. Data for the modulus of elasticity for the viscoelastic behaviour caused by longitudinal tension and for the shear modulus resulting from torsion around a longitudinal axis were related to the underlying structural changes by quantitative analysis of stem anatomy tissue distribution ultrastructure and cell wall biochemistry. The orientation of cellulose microfibrils was determined by light microscopy and small-angle X-ray diffraction and the lignin content was determined by thioglycolic acid derivatization and spectroscopic quantification. It was demonstrated that the increase in stability during early development is due to the complementary effects of increase in cell wall material lignification and cellulose microfibril alignment. A detailed micromechanical model considering internal prestresses is proposed to explain the characteristic biphasic stress-strain behaviour as well as the strain-hardening observed.