The mechanism by which the stems of trees recover after being bent or tilted out of the vertical is considered. The mechanical consequences of the tendency for the main stem apex to reerect itself and grow vertically or to be replaced by a vertically growing lateral branch, and of the asymmetrical radial growth of displaced stems, are pointed out. The asymmetry usually develops on the upper side of a bent or tilted hardwood stem and on the under side of a softwood stem. Since such asymmetry can usually be accounted for on the basis of accentuated development of wood rather than bark, the question of the possible functional significance of the wood arises. Experiments are cited to demonstrate that the wood (reaction wood) is effective as a means for assisting displaced stems to recover, contracting longitudinally on the upper side of a hardwood stem and expanding on the under side of a softwood stem. Consideration is then given to the structure, differentiation, and chemical composition of the reaction wood formed by hardwoods and softwoods with a view to locating the force active in aiding recovery and determining its origin. The consequences of associating the support of a still-growing stem displaced from the vertical with the idiosyncracies of secondary wall formation of reaction wood cells, and the reerection of such a stem with lignification of these walls, are explored. On this basis, the hypothesis is advanced that the recovery force is located in the region of differentiating reaction wood cells undergoing lignification, the active force arising from the swelling of cell walls as a result of deposition in them of lignin. Objections to the hypothesis are mentioned.
