Question

Leaf of Mimosa droops down on touching because of
A. Water loss from leaflet bases.
B. Changes in water concentration.
C. Loss of water from cells to intercellular spaces in pulvinus and pulvinus
D. All the above.

Answer 1

If we touch Mimosa pudica (don't touch me), our touch serves as a stimulus to the plant and closes its leaves in response. When we contact the plant, certain chemicals are released from the stem.

Complete step by step answer: Nastic movements are the reactions of the plant parts to the stimulus. Seismonasty is the nastic reaction of a plant to a touch or vibration. Dropping down the leaves of Mimosa pudica upon contact is attributable to seismonasty. Due to the turgor strain, the leaves remain in the upright position. There is a loss of water and electrolyte from the leaflet bases when touched. This results in a loss of cell pressure and a decrease in the leaves.
Hence, the correct answer is option A, 'Water loss from leaflet bases'.

Additional information:
The Mimosa genus is a creeping shrub and very attractive to grazing animals. It seems at some stage in its evolution Mimosa tended to be closed when touched. Doing decreased the leaf area offered to herbivores and made the plant look wilted. If it was enough to make grazers search for another plant, the genes for touch sensitivity may have spread, potentially leading to a new species. They do this quicker, too, if they are touched or shaken. The responses are likely to have evolved independently. Many plants close up at night, normally to protect pollen or minimise water loss when the leaves are not photosynthesizing.

Note: In an experiment, scientist Monica Gagliano decided to study whether Mimosa plants in low light situations would have a higher learning potential than those grown in high light, because low light plants were already in low-energy environments and folding their leaves would be more energetically expensive for the plant.