The Telegraph Plant, Codariocalyx motorius moves rapidly in response to light and even sound, with leaves moving fast enough to view with the naked eye in real time. Darwin first described this plant in The Power of Movement in Plants and (in)famously said, "No one supposes that the rapid movements of the lateral leaflets of D. gyrans are of any use to the plant; and why they should behave in this manner is quite unknown." It turns out that Darwin's words might still ring true today! Although we know that the plant's motion is caused by the growing and shrinking of cells in the pulvinus of each leaf, little is known about the adaptive significance of this motion. As to why they move in response to sound, it has been shown that low frequency pulses disrupt the normal oscillation patterns of the leaves. The most common hypothesis to the telegraph's plant mysterious "dancing" is that its leaves are searching for the sunniest position. However, this has yet to be proven rigorously. 

--Gloria Marino

Engelmann, W, et al. Ultradian Rythms in Desmodium, 1998

Fostad, O., Johnsson, A., & Engelmann, W. (1997). Effects of Electrical Currents on 'Desmodium gyrans' Leaflet Movements Experiments Using a Current Clamp Technique. Biological Rhythm Research, 28(2), 244.

Lev-Yadun, S. The enigmatic fast leaflet rotation in Desmodium motorium, 2013