Abramson, Garrado, Lawson, Browne and Thomas (2002, pp. 173 - 185) describe an experiment of their own, designed to verify the existence of associative learning in plants, using a newly refined method. Different groups of Philodendron cordatum plants were exposed to a six-hour training period of light only, dark only, or alternating one-minute periods of light and dark. Following training, all plants were exposed to a ten-minute testing period in darkness, when their bioelectrical potentials were recorded, using EEG equipment. As expected, the different groups showed a difference in amplitudes, reflecting differences in their prior exposure to light: the plants were using some physiological mechanism to store information about their history of exposure (i.e. memory). Such an explanation is compatible with a non-mentalistic account - the plants could have been just retrieving stored information. However, if learning had occurred, the researchers reasoned, they would expect to see differences in the amplitudes for the group exposed to one-minute intervals of light and darkness, between minutes 1, 3, 5, 7 and 9 on the one hand, and minutes 2, 4, 6, 8 and 10 on the other, corresponding to the switching of the light in the training period. No such differences were found, although the group cautioned that one-minute intervals may have been too short to allow the plant to adjust its responses to changes in the light. The authors concluded:
At this point we cannot conclude it is possible for learning to occur in the absence of a nervous system. Further studies should examine the effect of longer intervals of light and dark exposure (2002, p. 184).
Another reason for caution regarding the above experiment is that plants may simply be capable of learning, but poor at timing. Temporal conditioning, where a single stimulus is presented at regular intervals that the subject learns to anticipate, has been demonstrated in vertebrate animals, but to date, attempts to demonstrate this phenomenon in invertebrates, including honey bees, crabs, earthworms and flatworms have failed (Abramson, personal email, 2 February 2003). Is it reasonable to expect plants to be able to do what most animals cannot?
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