Plants respond to mechanical stress by adaptive changes in growth. Although this phenomenon is well established, the mechanism of the perception of mechanical forces by plant cells is not yet known. We provide evidence that the cortical microtubules subadjacent to the growth-controlling outer epidermal cell wall of maize coleoptiles respond to mechanical extension and compression by rapidly reorientating perpendicular to the direction of the effective force change. These findings shed new light on many seemingly unrelated observations on microtubule reorientation by growth factors such as light or phytohormones. Moreover, our results suggest that microtubules associated with the plasma membrane are causally involved in sensing vectorial forces and provide vectorial information to the cell that can be utilized in the orientation of plant organ expansion.