Apparently plants can smell.
Let me rephrase that.
Apparently plants are capable of detecting odors.
A recent article at UPI.com talks about a study that has shown that a lima bean plant will respond after exposure for an entire day, but will not after only 6 hours. A bit slower than us.
A 1983 study indicated that willow trees under attack by caterpillars emit pheromones to warn neighboring trees. The nearby trees then increased concentrations of chemicals that deter the caterpillars from also feeding upon them.
Dodder, a parasitic plant, has been shown to scent out its victims, and actually have a preference if given a choice. A more recent study seems to confirm this, narrowing down specifically the compounds that attract or repel the dodder.
A good article on this subject can be found here at Scientific American.