*** Appendix - Habituation in plants

According to Abramson, Garrado, Lawson, Browne and Thomas (2002, pp. 174-176), studies of behaviour modification in plants go back to 1873. Most studies of plant learning have examined habituation in Mimosa, a small shrub whose leaves are sensitive to stimulation. Mimosa leaflets close in response to touch, heat, electrical shock or puffs of air. In the absence of further stimulation, the leaflets re-open after about 15 minutes. Exposure to stimuli also makes the stem fall downward, but after the same interval of time, the stem rises again. In 1873, the researcher Pfeffer induced habituation in Mimosa leaflets: after repeated stimulation, the leaf-closing behaviour in response to stimulation was no longer observed. In 1906, another scientist, Bose, found that constant electrical or mechanical stimulation of the stem initially triggered a falling response, followed by a slow rise, after which the response could be triggered again. However, repeated stimulation led to a loss of response to the stimulus. Follow-up research in 1972 showed that increasing either the interval between trials or the intensity of the stimulus also increased the time required for habituation.

Habituation is also known tp occur in other plants: the carnivorous plant Drosera, known as the Sundew, and the Passion Flower Passionflora gracilis (Abramson, Garrado, Lawson, Browne and Thomas (2002, p. 175).

Aside from non-associative forms of habituation, there is experimental evidence that that more complex forms of habituation may be found in Mimosa. An experiment by Holmes and Gruenberg in 1965 showed that Mimosa could discriminate between different types of stimuli: it could be "trained" to stop closing in response to water droplets, but still retained its response to the touch of a finger, indicating that the change in response was not due to fatigue.

One could conclude from the results of the experiment that the plant had learnt to do something new: discriminate between two stimuli. This would imply that the habituation observed here was associative rather than non-associative in nature. Alternatively, there could be different response mechanisms within the plant for stimuli of different weights. The case remains intriguing but inconclusive.