The excellent mechanical properties of wood arise from its cellular and cell wall structure. X-ray scattering, ultrasound, and mechanical testing is combined to study the effects of strain on crystalline cellulose in wood. Results for dry and re-moistened softwood samples are reviewed and new results are presented for native, never-dried samples of Silver birch. When softwood is stretched parallel to the cell axis, the mean microfibril angle diminishes significantly in compression wood, but only slightly in clear wood. The cellulose chains in the crystallites elongate and their distance diminishes. In the never-dried Silver birch samples, axial strain caused the mode of the microfibril angle distribution to slightly decrease from the initial value of 14 degrees to 12 degrees. Unlike in softwood, in never-dried birch crystalline cellulose showed auxetic tensile behaviour. The distance of the chains increased and the X-ray Poisson ratio νca was negative, -0.3 ± 0.2. Dehydration of never-dried Silver birch caused no difference to the microfibril angle distribution.