Mimosa is a plant that closes its leaves when you touch it. Scientists in Australia dropped pots of mimosa again and again. At first, the leaves closed.

But after many drops, the plants stopped closing their leaves. When the pots were shaken, the leaves still closed. So the plants were probably not just tired. About a month later, many plants still did not close their leaves when dropped.

Mimosa pudica folds its leaves when touched, probably to protect itself. However, closing leaves has a cost, because the plant cannot carry out photosynthesis while they are closed. In 2014, Monica Gagliano and her team dropped potted mimosa plants 15 centimeters, 60 times in a session. The plants gradually stopped closing their leaves.

When the pots were shaken instead, the leaves closed again, which suggested the plants were not simply tired. They had become used to dropping, a process called habituation. About a month later, many plants still ignored the drop. Plants grown in low light stopped responding sooner and kept the change longer, perhaps because closing their leaves was more costly when light was scarce.

Touch a Mimosa pudica and its leaves fold shut. The move protects the plant, but it has a cost, because closed leaves cannot photosynthesize. In 2014, Monica Gagliano and colleagues at the University of Western Australia dropped potted mimosas 15 centimeters, 60 times per session. The plants gradually stopped folding their leaves when dropped, but they still folded them when shaken.

The researchers took this as habituation, not motor fatigue. About a month later, many plants still ignored the drop. Those grown in low light habituated faster and retained the change longer, which fits the idea that plants learn faster and forget more slowly where it matters.

Habituation in mimosa had been reported since the 1960s, so the new part was how long it lasted. Critics were not convinced. In 2018, Biegler argued in the same journal that the study had not ruled out motor fatigue, which could explain most of the data.

A related claim from Gagliano's team, that pea plants could learn to associate wind with light, failed to replicate in 2020. Part of the argument is simply about words. Whether a plant's response counts as learning or memory depends on how those terms are defined.