The mechanism of growth stress in tension wood is still not fully understood. To characterize the functional contribution of polymers within the cell wall to the generation of growth stress, the ultra-structural arrangements of cell-wall polymers, as well as their orientation, were investigated by Fourier Transform Infrared microspectroscopy (FTIR) along a sequence of cell differentiation from the cambium to the mature wood in tension wood (TW) and normal wood (NW) of poplar.