Chemo-mechanical protein aggregates in plants, e.g. the forisomes of legumes, transform the chemical free energy of their reaction with calcium ions into mechanical energy. Light and scanning electron microscopy analyses demonstrated that the spindle-shaped bodies with lengths between 25 and 40 μm consist of fiber bundle units which change their length and diameter during a reversible switching reaction induced by either calcium, barium, strontium ions or by changes of the pH value. For the determination of forces generated during the switching reaction a force measurement in vitro system based on the bending of thin glass fiber was developed. By using this set-up, dynamical forces of forisomes, in the presence of different alkaline ions, in the range up to 180 nN were measured.