Here's a study that suggests that a common species of weed engages in the altruistic phenomenon of kin selection. When growing with relatives, the plants don't try to take advantage of the situation and keep their roots to a minimum. But when growing next to unrelated versions, it's every weed for itself. Why does this matter? Because until now the phenomenon has only been demonstrated in animals. Likewise, it means that a plant (and a common, gutter-trash variety at that) is actually able to recognize its relatives versus it's non-relatives and respond accordingly.
Those of us who stay current on all things photosynthetic really shouldn't be surprised by this news. Afterall, plants are champs at sensing a variety of stimuli in their environment. Why shouldn't relatedness of neighbors fit this mold? Still, I just find it neat.