Contact forces are important in maintaining the twining habit of viny stems. A stem twining around a supporting pole puts itself into tension and uses a helical geometry to generate normal loads that are large relative to stem mass per unit length (Silk and Hubbard Journal of Biomechanics 24(7):599-606 1991). An electronic pressure-sensing device has been constructed to provide continuous in vivo measurements of the forces exerted by twining stems. The pressure-sensing element is based on a thin beam load cell that is sheared by a twining stem ascending a split pole. Preliminary results show that after morning glory stems begin to coil around a supporting pole the twining force increases in an oscillatory fashion over 3 or 4 d corresponding to positions at least 200 mm from the apex. The force-measuring device should reveal relationships between twining forces and developmental attributes or environmental factors.