One of those science stories that sticks in the brain:
After plants are potted, roots branch out to suck up water and nutrients. But when several plants of the same species are potted together, things get a little nasty: Each plant flexes its muscles, so to speak, by extending its root growth to try and snatch up valuable resources.
Unless, that is, the plants are siblings—each having come from the same mother plant—in which case they become very accommodating, allowing each other ample root space.
Because the interactions between related and unrelated plants only happened when plants were in the same pot, where root space is limited, root interactions are likely what gives plants the cue that their neighbor is related.
(Photo: the European sea rocket, which was the object of the experiment.)
The Dish 