Contact Stimuli in Plants

Plants are exceptionally sensitive creatures. Some are already hardened in respect to contact stimuli, but they still play a major roll in climbing with tendrils. Among other things, pumpkins, Virginia creepers, bindweeds or clematis’ are fitted with tendrils.

These tendrils, which are sensitive to differences, turn in a circle all day until they come into contact with an object. Thereupon, they change their growth and immediately start to twine around that object. The interesting thing is that the plants do not react to falling raindrops or gelatinous substances, but are able to distinguish them because of the slightly different pressure.

"Sensitive Like a Mimosa"

The mimosa behaves most outstandingly of all plants with regard to contact stimuli, which is indeed very sensitive as the saying already states. As soon as it is touched on the outermost pinnules, it quickly folds up the remaining leaflets from the outside to the inside and the leafstalk bends downwards. In the case of a very strong stimulus, that behaviour spreads to other leafstalks as well.

The mimosa remains that way for a few minutes and is unable to respond to further stimuli. Once it has 'recovered', it unfolds the leaves again and the stalks return to their original position as well.

Interesting Details

Additionally, the chemical transmission of messages between acacias is noteworthy. Scientists observed that giraffes only fed on a few leafs of an acacia and then covered long distances to carry on feeding on a different tree. They never started to feed on a neighbouring tree.

By studying the acacias, it was noticed that all the trees within a certain range suddenly produced bitter flavours in their leaves as soon as one tree was nibbled and therefore the giraffes refused those leafs. So the giraffes have to move a little before they find new acacias that weren’t yet gnawed. In this way, the acacias ensure their continuation, as they would soon disappear otherwise. How they exactly learn about the gnawing is still unclear, though.