In the most recent study, conducted by a team at the Ben-Gurion University in Israel, pea plants have been found to alert each other to stressful situations. In the experiment the plants were placed close to each other, but not touching in any way. Some plants shared soil with those next to them, while others were completely separated. Next, a few of the plants were given drought like situations, while others were kept healthy and watered.
The plants kept in dry conditions responded to the situation by closing the pores on their leaves (which are called stoma) - a normal reaction for plants in drought. What was unexpected was that plants nearby (not kept in drought conditions but sharing the same dirt) did the same thing! Not only that, but they seemed to pass the message on to un-stressed plants even further away, which responded with stomatal closure as well. The plants who did not share any soil did not respond at all, meaning that the communication between the plants was done through root systems rather than the leaves.
Read about other experiments on plant communication at Environmental Graffiti. Link
(Image credit: Flickr user lobo235)