The plant that learned to count

An experiment suggests that the sensitive plant is able to record the number and sequence of events, indicating that learning may be possible even without neurons.

Plants are much more capable than we think. They can perceive their surroundings, anticipate changes and even communicate with each other in ways that we are only beginning to understand. According to a new study published in Cognitive Science, they may also be able to count.

To determine whether plants can count, a group led by cognitive psychologist Peter Vishton of the College of William & Mary, in the United States, studied the behavior of the sensitive plant (Mimosa pudica), so called because it quickly closes its leaves when disturbed, as well as at night.

In the first phase of the experiment, the researchers exposed the plant to two normal days consisting of twelve hours of light and twelve of darkness, followed by one day of complete darkness. They repeated this three day cycle five times and found that the plant’s leaves began to move before the light period on the days when it was expected, but not on the days of complete darkness. “This seems to suggest that plants can ‘learn’, so to speak, this three day cycle and adjust their movements accordingly”, Vishton said.

Interestingly, the plant’s behavior follows a logarithmic curve, similar to those observed in animals that learn new things. “It is the same pattern we see all the time in animal learning”, Vishton said. “For example, if a rat is taught to perform a series of actions in a certain order, you expect to see a period in which it tries to understand the sequence and then a gradual increase in its ability to predict it”.

Of course, the 24 hour circadian rhythm of the sensitive plant is well known, and scientists had to make sure they had not simply conditioned the plant to a strange 72 hour cycle. So they shortened the length of the days to twenty hours, ten of light and ten of darkness. In that case too, the plants adapted quickly, opening their leaves before the light arrived, but not on the days of darkness.

Limited memory

To further test the plants and ensure they were responding to the number of stimuli rather than their timing, the researchers exposed them randomly to days of varying length from ten to 32 hours, finding that the pattern broke down when the days were shorter than twelve hours or longer than 24.

According to Vishton, this indicates that plants need a minimum threshold of exposure to process the sequence of light and darkness and a maximum memory limit beyond which they forget it. Within the range between twelve and 24 hours, however, they consistently showed the same predictive behavior. “The simplest explanation is that these plants record the number of events that occur”, Vishton said. “They are not just responding to duration”.

If the results are confirmed, Vishton believes they could reveal a new way of processing information that does not involve neurons at all.

“There are many cells in animals and humans besides neurons, and we assume they are not involved in learning”, Vishton said. “But maybe that is not the case. Maybe learning is present in every cell. It is just that we have never studied it”.

How these cells might learn, store and remember such information is a question that future studies will have to answer.

In the meantime, remember to water your plants: they might be keeping track of how many days have passed since the last time.

edited: Nicolas F. E.