When wood is subjected to increasing mechanical stress its component parts ultimately undergo changes resulting in permanent deformation or failure. This paper deals with the minute microscopic changes upon which the permanent deformation of wood depends, and with the relationship between the microscopic structure of wood and some of its more important mechanical properties. The work has provided abundant material for a study of the structure of the walls of the cells of which wood is composed. Considerable light has been thrown on the structure and properties of the vegetable cell-wall in general, and it has been possible, from the facts for wood, to frame a hypothesis which seems to explain satisfactorily most of the properties of cell-walls. The examination of the mode of failure in timber, which has led to the work described in this paper, was begun at the request of Lt.-Col. Jenkin, R. A. F. I am deeply indebted to him for the original suggestion of this line of work and for his continued interest during its course. I have derived great advantage throughout the work by discussing various engineering questions, as they have arisen, with Major Robertson, R. A. F., and I desire to thank him for many helpful suggestions. I am further indebted to him for the supply of many fractured specimens of wood. The research has been carried out, in the course of more definitely applied work for the Air Ministry, in the Barker Cryptogamic Research Laboratory of the University of Manchester. I have to thank Prof. W. H. Lang, F. R. S., for encouraging me to attempt to relate the facts of the mechanical properties of wood to fundamental botanical questions bearing on the structure of cell-walls in plants.