We already know that the rhizosphere, the zone of soil around the roots of plants, is teeming with chemical signals from the plants themselves as well as from symbiotic bacteria, fungi, and other life. This chemical crosstalk is akin to the air-based communication accomplished through pheromones (in humans, other mammals, insects, etc...), and undoubtedly provides a rich, stimulating and ongoing dialogue for the plants.
In many cases, chemicals secreted from plant roots contribute to survival: witness allelopathy, the ability of some plants such as wormwood, goldenrod, or many cover crops to inhibit the growth of other species in their rhizospheres. It has always fascinated me that the plants can recognize members of their own species and selectively inhibit the growth of everything else -- but after all, they are different species, and a genetic resistance to a poison is not difficult to imagine.
Now, in a beautiful study published in Biology Letters, we learn that members of the same species alter their competitive behavior based on whether or not they are growing next to their siblings (plants grown from seed that came from a single parent). Seedlings of Cakile edentula, a variety of wild mustard, grow much more dense and aggressive root structures when next to members of their own species that come from different parents. This is a remarkable level of sensitivity to a very slight variation in genetic structure -- but should come as no surprise to herbalists who are quite familiar with plants' ability to sense, perceive, process, and alter their behavior in concert with their environments.