Is this a trick question? No. While the majority of plants are free-living autotrophs, some are parasites on other plants (think mistletoe, for example). And while the seeds of many of these parasitic plants won't germinate unless they are in contact with host plant tissue, this isn't true of dodder (Cuscuta species). Dodder actively seeks out its host (Pennisi, 2006).
Dodder is one of the top 10 noxious weeds in the US, covering its host plants with a carpet of yellow, thread-like tissues. In order to survive it must find a host plant as soon as possible after germinating. Recent research shows that dodder achieves this by sniffing out its host, detecting volatile chemicals released by other plants (Runyon et al. 2006). While herbivorous animals use scent to find their food, this is the first time such behaviour has been recorded in a plant.
Runyon and his co-workers carried out a series of elegant experiments to investigate dodder's host-seeking activity. In the first, dodder seeds (C. pentagona) were planted in vials set into a filter paper collar: when the plants germinated, the scientists could mark out their growth on the paper. They found that the majority of the seedlings grew towards neighbouring tomato plants. The next step was to determine whether this growth was in response to volatile chemicals released by the host.
To do this, new dodder seedlings, in their paper-collared vials, were put into simple choice chambers. These consisted of an open-air area, containing the dodder, linked to two other enclosed chambers by black PVC pipes (to eliminate light cues that might have affected growth). One of the chambers contained tomato plants, with artificial tomato plants in the other. Again, a significant majority of the dodder seedlings grew towards the real tomato plants.
The clinching experiment used the same choice-chamber set-up. But this time the choice was between volatile chemicals released by tomatoes, combined with a solvent, and the solvent alone. Most of the dodder seedlings grew towards the chamber containing tomato chemicals – this parasitic plant can indeed sniff out its host. What's more, it can distinguish between potential hosts, growing towards tomatoes in preference to wheat seedlings (wheat isn't normally parasitised by dodder).
These findings demonstrate that at least some plants can detect chemical cues sent out by other plants, and support earlier observations that plants seem able to signal to others when under attack by herbivores, triggering a response in the plants receiving that signal. And they open up a fertile area for future research, because while we now know that dodder can find its hosts using chemical cues, we still have no idea of the receptor systems involved.
E. Pennisi (2006) Parasitic weed uses chemical cues to find host plant. Science 313: 1867
J.B. Runyon, M.C. Mescher, C.M. De Moraes (2006) Volatile chemical cues guide host location and host selection by parasitic plants. Science 313: 1964-1967