Can <i>Mimosa pudica</i> Plants Enumerate Light Exposure Events?

Plants sense and respond to information present in their surrounding environment. Recent work has sought to characterize the limits of these information processing abilities. Here, we present evidence that the movements of Mimosa pudica plants are mediated by the number of illumination events to which they have been exposed. The plants were repeatedly presented with 2 days in which light was provided for half of the day, followed by a third day in which light was not provided. The nyctinastic movements of the plants shifted to follow this light-light-dark pattern. During early, dark hours prior to light onset, the plants moved more on days in which light was likely to be provided and less on days in which light was unlikely. This movement tendency was not present during the initial weeks of the study. The plants altered their movement patterns over 15 days in a fashion that is well fit by a logarithmic function. To test whether plant movement was based on temporal factors, rather than event enumeration, we altered the lengths of day-night cycles in the second and third phases of the study. After accommodating their motion to follow a 3 × 24-h light-light-dark cycle, the plants immediately generalized their performance after an abrupt shift to a 3 × 20-h cycle. In Study Phase 3, the day length was randomly varied between 10 and 32 h after every light-light-dark cycle. The plants exhibited key movement patterns when randomly selected day durations were between 12 and 24 h. Although higher levels of variability were apparent, the movement levels of the plants seemed to be modulated by the number of light exposure events. The results provide evidence that plants, and perhaps other non-neuronal tissues, may be capable of processing enumeration-related information, although replication with additional controls is needed.