The structure of balsa wood has been characterized by scanning microscopy. The moduli and crushing strengths of the wood, in three orthogonal directions, have been related to the structure and its reponse to load: during deformation the cell walls bend elastically and collapse plastically or by fracture. It is found that the moduli, crushing strengths and anisotropy of balsa and of other wood are determined, in part, by the properties of the cell-wall material, and, in part, by the dimensions and shape of the cells themselves.