Plants are able to pre-stress their tissues in order to actuate their organs. Here, we demonstrate with two tissue types of the secondary xylem of conifers (normal wood and compression wood of spruce (Picea abies)) that either tensile or compressive stresses can develop in the longitudinal direction during the swelling of the cell wall. This dramatic difference appears to be due mostly to differences in cell geometry and cellulose fibril orientation. A mechanical model was developed to demonstrate swelling experiments with the help of sodium iodide experiments. The reversal of longitudinal extension can be predicted, based on the orientation of the (nearly inextensible) cellulose fibrils and the shape of the cell.