Codariocalyx motorius, aka Desmodium gyrans (Fabaceae)
This tropical Asian shrub is widely distributed throughout South Asia including Sri Lanka, as well as in East and East-Southern regions of Asia. Fascination with the unusual motions in this Asian plant led to it being nicknamed, during the second half of the nineteenth century: the dancing plant, the semaphore or telegraph plant. Charles Darwin described it in his publication from the 1880s, The Power of Movement in Plants. He wrote of his bewilderment at its behavior, “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.” Later on, Sir Jagadis Chandra Bose (1858-1937), a physicist, extensively studied motion in Desmodium gyrans, a gyration of leaflets with a period ofabout four mn in winter and one mn in summer.
Leaflets at the basis of the leaves are the moving parts. They rotate rythmically. The plant has rotors in these leaflets. Why do they move? How does that engine work? To what purpose? Where does it get its energy?
Two lateral leaflets at the base of each leaf move in an elliptical trajectory. They appear to sample the intensity of sunlight and to direct the leaf for maximum exposure to it.
The motion is induced by the swelling or shrinking of motor cells at the base of the leaflets. Synchronous variation in the electric potential across the motor cells stems from the uptake or release of potassium ions and protons, in somewhat like manner to the firing of neurons in animals. The potassium ions influx and efflux are responsible for osmotic movement of water across the membrane of the motor cells, causing their swelling or shrinking.
Light collection optimization as the purpose for this device is a reasonable and probably sufficient hypothesis. Others were nevertheless formulated: could it be mimicry of an insect, a butterfly perhaps? A deceptive mimicry, to lure the butterfly into believing that another butterfly has already landed on those leaves in order to prevent it from doing likewise? To attact predators of butterflies or other insect herbivores? It might be both. Optimization of light harvesting and butterfly mimicry need not be mutually exclusive.
In any case, the animal-plant dichotomy is foreign to modern biology. That some plants are capable of motion, if it challenges a naive and conventional differentiation between the animal and plant kingdoms, has nothing fundamental or surprising about it.