SummaryThe mechanism of active stress generation in tension wood is still not fully understood. To characterize the functional interdependency between the G-layer and the secondary cell wall, nanostructural characterization and mechanical tests were performed on native tension wood tissues of poplar (Populus nigra  Populus deltoids) and on tissues in which the G-layer was removed by an enzymatic treatment. In addition to the wellknown axial orientation of the cellulose fibrils in the G-layer, it was shown that the microfibril angle of the S2-layer was very large (about 36°). The removal of the G-layer resulted in an axial extension and a tangential contraction of the tissues. The tensile stress-strain curves of native tension wood slices showed a jagged appearance after yield that could not be seen in the enzyme-treated samples. The behaviour of the native tissue was modelled by assuming that cells deform elastically up to a critical strain at which the G-layer slips, causing a drop in stress. The results suggest that tensile stresses in poplar are generated in the living plant by a lateral swelling of the G-layer which forces the surrounding secondary cell wall to contract in the axial direction.

Spruce wood was subjected to welldefined extraction treatments with sodium chlorite (NaClO 2 ) for delignification, as well as with sodium hydroxide (NaOH) at different concentrations for extraction of hemicelluloses. The corresponding changes of the macromolecular polymer assembly were investigated by small-angle X-ray scattering (SAXS). Measurements with Fourier-transform infrared (FTIR) spectroscopy and wide-angle scattering (WAXS) gave qualitative information about the effectiveness of the extraction process, while the scattering experiments provided information about the regularity and typical dimensions of the molecular structures. The scattering data indicated that delignification had only a moderate effect on the structural organisation of the cell wall, while further extraction with NaOH induced considerable nanostructural changes.

A growth ring of an adult Norway spruce (Picea abies [L] Karst.) was analyzed to a high resolution at the single cell level with respect to structural and mechanical changes during the growth period. For this purpose structural characterization was performed by means of light microscopy, scanning electron microscopy and wide angle X-ray diffraction for investigating the geometry of cells, their cell wall fractions and cellulose microfibril angles (MFA). The mechanical properties were determined in microtensile tests on individual tracheids which had been taken from sequentially cut tangential slices. The results revealed pronounced differences in tensile stiffness between earlywood and latewood cells but only minor differences in tensile stiffness between the cell walls of both tissue types. These comparatively small changes in cell wall stiffness across the growth ring were caused by slight changes in MFA. The findings suggest that trees mainly vary cell size to optimize water transport and mechanical stability during the growth period and that modification of the cell wall organisation plays a minor role.

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A possible relation between cellulose microfibril angle and lignin content in the S2 layer was investigated by X-ray diffraction and cellular UV microspectrophotometry on spruce tissues wPicea abiesx with different structural features and chemical composition. A strong correlation was not found, neither for the individual tissue types nor for the compiled data of all tissues. As the data did not confirm the findings in former studies, further examinations are necessary concerning a possible general interrelation between microfibril angle and lignin content.