We loved finding this plant by the roadside when we were kids, because despite it being called ‘touch-me-not’, I assure you we did precisely the opposite. Of course we all know that flowers bloom and fruits grow and such, but to see a plant actually close its leaves in front of your eyes in a matter of seconds when you touch them is pretty cool. And a reminder that plants are living things too, which people seem to forget, since humans just can’t empathise with a stressed plant as much as they can with, say, a panicking cow.
Found this rather lovely specimen of Mimosa pudica growing between two gaudy plastic barriers, much taller than usual in the field, where the plant likes to sprawl and spread instead. Quite the good photo model, I’d say, since I didn’t have to flatten myself onto the pavement for close-ups. Plus, I’ve always loved the specific shade of pink that the flowers are: a very nice pale pink, and looking like a fluffy pompom to boot.
Now, down to the cool science bits: the Mimosa pudica, which grows happily in fields and sunny areas along roadsides in Singapore, can also be found throughout Southeast Asia, and in Japan. It’s known for its ability to close its leaves when touched, and also closes its leaves at night, a process known as nyctinasty. This is controlled by environmental light levels and temperature. Light, in particular, is detected by a light receptor in the leaves known as phytochrome, which is also responsible for regulating processes such as seed germination, plant growth, and chlorophyll synthesis. Different forms of light are able to convert it from an inactive form to an active form, where it can act as a signalling molecule. When touched, the plant’s leaves generate an action potential (an electrical signal, just like in our nerves), which is transmitted down the leaves. Movement itself is mediated by cells in the pulvinus (a joint-like thickening) at the base of the leaves, which have aquaporin channels allowing for water movement in response to changes in potassium ion concentrations. What’s amazing is that it can respond differently depending on how hard it’s prodded, with a gentle prod causing only a few leaflets to close, and a violent prod causing the entire leaf, and even neighbouring leaves, to close. What’s even more amazing is that the plant is able to display habituation- meaning that when continuously dropped from a height they eventually ‘get used to it’, and stop closing their leaves when dropped. Even after a month, this persisted- meaning that these plants are able to learn, just like animals can (and maybe even better, since some people never seem to learn from their mistakes. We all know someone like that).