Plants use a network to communicate—it's made of fungal threads called mycelia. In addition to messages, resources can travel along the mycelia. And just like on the human-run internet, plants can use their network for sinister purposes. The BBC explains:
[S]ome plants steal from each other using the internet. There are plants that don't have chlorophyll, so unlike most plants they cannot produce their own energy through photosynthesis. Some of these plants, such as the phantom orchid, get the carbon they need from nearby trees, via the mycelia of fungi that both are connected to.
It's not all bad: some forms of communication can make plants stronger. Researchers at the University of Aberdeen have found that broad beans use the network to tell when a neighbor is attacked by aphids, the BBC explains:
Johnson found that broad bean seedlings that were not themselves under attack by aphids, but were connected to those that were via fungal mycelia, activated their anti-aphid chemical defenses. Those without mycelia did not.
"Some form of signalling was going on between these plants about herbivory by aphids, and those signals were being transported through mycorrhizal mycelial networks," says Johnson.
This could be viewed as an act of altruism on the part of the attacked plant, which sends some sort of signal out over the fungal network. Or it could more like a tapped phone line. Researchers have known for decades that plants communicate via airborne chemical signals, and some suspect that these wireless signals are consequences of other functions, rather than a targeted message from the sender plant.
[N]o experiment has yet demonstrated that volatile signaling between neighboring plants can benefit the emitting plant, prompting some researchers to suggest that “eavesdropping” is a more accurate description of what has been observed than “intentional” communication.