Abstract text: Progress in understanding the molecular architecture of plant secondary cell walls has enabled the use of computational methods, and molecular modelling in particular, to investigate their nanostructure and macromolecular interactions. We report modelling studies that address especially the structure of softwood secondary cell walls at different length scales and levels of approximation. We first develop molecular models of cellulose microfibrils both as individual nano-objects and in aggregated states mimicking their cell wall environment. We carry out simulations of their response to changing water content, and compare the predictions to X-ray and neutron scattering experiments on Norway spruce wood samples undergoing similar moisture changes. The models reproduce detailed features of the experimental data, as shown by computed scattering, thus supporting the underlying structural hypotheses. Our findings provide insight into the structure and stability of microfibril interfaces, their role in stress transfer and in controlling water diffusivity. We further develop a three-phase model of the cell wall structure in the sub-100 nm scale, and use it as a basis for a scattering analysis tool for wood samples. Our research contributes to the understanding of the properties of wood tissues and how they stem from the fine architecture of the secondary cell walls.