People can communicate by smell too - willy-nilly. Most often it's bacteria on human skin that detects changes - such as stress hormones - and respond in their own ways. Some people can smell stress in other people.
I'm an environmental studies major, so I have heard before that plants communicate with each other via chemicals and also apparently "scream" when injured (some pulse or other they measured - not something humans can hear). So I have long joked that if I were a vegetarian, I would probably favor a bumper sticker or t-shirt that said something like "I don't love animals, I just hate plants".
(Just trying to stay on-topic and not stray towards the topic lower down on the page, which may be a very bad motivation for commenting. <whistles innocently>)
Cleve Baxter did a lot of research on this, and his experiments were even featured on an episode of myth busters. His research indicated that plants can sense a humans intentions. He figured this out by attaching polygraphs to plants and then directing negative thoughts towards the plant (cutting, burning etc.). The polygraph then went haywire, but when he thought nice things about the plant the polygraph stayed normal.
