Mimosa pudica is a legume native to the Americas. Today, its small pompoms of pink flowers, emerging from creeping, prickly stems covered in feather-like leaves and heads of prickly pods are familiar in grasslands, forest margins and waste places across the tropics and subtropics, as well as a horticultural novelty.
A surprising property of Mimosa pudica was described by Reverend John Layfield in 1598 after he visited the West Indies; at night, or when touched, the plant's leaflets fold upwards and its leaves fold down. These nyctinastic and seismonastic responses confounded the prelate's preconceptions that plants were passive when confronted by their environment. Such sensitive responses elicited waves of speculation among early modern natural philosophers, which ultimately led to hypotheses that could be tested experimentally.
The race was on to introduce mimosas into British gardens. By 1648, Jacob Bobart the Elder was growing two American mimosas, 'Herba humilis' and 'Herba sensibilis', in the Oxford Botanic Garden; presumably in primitive glasshouses 'for Shelter from our barb'rous Sky'. In 1654 Bobart even demonstrated these plants' curious properties to the diarist John Evelyn. In addition to scientific interest, Mimosa pudica attracted the attention of Romantic poets such as Erasmus Darwin and Percy Bysshe Shelley.
The movement of Mimosa pudica leaves reveals the power of water to transform plant shape. Mechanically, a plant cell is like a water-filled balloon inside a cardboard box. As long as the balloon is full of water the box will remain firm, remove some of the water and the box will become soft; the plant starts to wilt.
At the base of each Mimosa leaf and leaflet is a specialised organ called a pulvinus. When pulvinus cells are full of water leaves unfold but when the cells lose water the leaves fold up. Folding is mediated by an electrical signal, regulated by calcium ions, that passes across the leaf telling pulvinus cells to pump potassium ions in or out. When Mimosa leaves are open, pulvinus cells contains large concentrations of potassium ions, so water moves into the cells across an osmotic gradient. However, when a Mimosa leaf is touched, cells in the lower part of the pulvinus expel potassium ions. This reverses the osmotic gradient and water is temporarily drawn into the surrounding tissue, the pulvinus cells shrink and leaves fold. The evolutionary significance of leaf movements appears associated with protection against water loss, temperature change and herbivore damage.
Allen RD 1969. Mechanism of the seismonastic reaction in Mimosa pudica. Plant Physiology 44: 1101-1107.
Webster C 1966. The recognition of plant sensitivity by English botanists in the seventeenth century. Isis 57: 5-23.