Many species interactions are visible – a bee visits a flower, a lion chases a gazelle, a mistletoe perches atop a tree. However, a whole host of other interactions remain invisible. These interactions may include microbes or other organisms that are too small to see, or the interactions may occur belowground, hidden from sight. Plants engage in extensive belowground interactions with other plants, soil arthropods, microbes, and fungi. Plants also serve as a link between aboveground and belowground subsystems, an area that is currently receiving much attention in ecological research.