To grow straight, plants need a motor system that controls posture by generating forces to offset gravity. This motor function in trees was long thought to be only controlled by internal forces induced in wood. We grew artificially tilted seedlings and measured the change in curvature when removing the bark and evidenced a differential mechanical state of the bark on both side of the stem. In several species, bark contributes to the up righting of trees. Combined mechanical measurements and microstructural observations on adults trees enabled us to identify the mechanism responsible for the development of mechanical stress in the bark of plant stems. This mechanism does not result from cell wall maturation like in wood, or from the direct action of turgor pressure like in unlignified organs, but is the consequence of the interaction between wood radial pressure and a smartly organized trellis structure in the inner bark.