In his famous book, The Origin of Consciousness in the Breakdown of the Bicameral Mind, Julian Jaynes mentions his experiments on learning in mimosa plants. I found this fascinating and always wish he had provided more detail. Now a paper published in Nature points to evidence that plants are capable of associative learning. From the abstract:
Here we show that this type of learning occurs in the garden pea, Pisum sativum. By using a Y-maze task, we show that the position of a neutral cue, predicting the location of a light source, affected the direction of plant growth. This learned behaviour prevailed over innate phototropism. Notably, learning was successful only when it occurred during the subjective day, suggesting that behavioural performance is regulated by metabolic demands. Our results show that associative learning is an essential component of plant behaviour. We conclude that associative learning represents a universal adaptive mechanism shared by both animals and plants.