Asked by: Harriet Best, China
The leaves of the 'touch-me-not' fold up and droop each evening before reopening at dawn. They also do this more rapidly if they are touched or shaken. It is likely the responses evolved separately. Many plants close up at night, usually to protect pollen or reduce water loss while the leaves aren't photosynthesising. But the Mimosa genus is a creeping shrub and highly attractive to grazing animals. It seems that at some point in its evolution a Mimosa appeared that closed up when touched. Doing so reduced the leaf area presented to herbivores and made the plant look wilted. If this was enough to make grazers look for another plant, then the genes for touch sensitivity would have spread, eventually leading to a new species.