Plant of the month (©Pierre Laszlo, all rights reserved)
Mimosa pudica (L). (Fabaceae)
There is so much we can learn from the natural world. Often it tries to teach us by challenging our naive preconceptions. For instance, common sense and its near-synonym, conventional wisdom, tell us that plants differ from animals in not having motion. Which is nonsense, since many plants move in many ways.
A celebrated counter-example known already in the eighteenth century is Mimosa pudica, which Linnaeus thus named because it recoils upon being touched — pudica in Latin means bashful.
This plant undergoes several types of motion. The leaves change orientation between daylight and night time, a feature which quite a few other plants share. Photosynthetic light collection implies maximum exposure of the foliage to sunlight. When not needed, the foliage closes down, just like the awning on a shop. This first type of motion is known as nyctinastic, movements associated with diurnal light and temperature changes and controlled by the circadian clock and the light receptor phytochrome.
Another motion is named seismonastic or thigmonastic movements: when touched, the leaves close and then reopen after a few minutes. This rapid movement of leaflets is thought to be an adaptative defense against herbivores.
What about the sensing mechanism, by which the plant detects having been touched? It involves propagating waves of calcium ions, analogous to the sodium/potassium waves along a neuron, in the nervous system of mammals.
And how is motion effected in the leaflets of the plant? Taking advantage of fast loss of water from swollen specialized cells, known as motor cells. Water efflux from these motor cells is controlled by protein molecules, aquaporins, that effect selective permeability to water molecules across cell membranes.
It is quite a sophisticated defense mechanism. It is associated with electrical activity — technically known as a resistor with memory, a memristor for short.