yep the single largest organism in the world is a honey fungus mycelium.
Edited: Italian chef Antonio Carluccio says it is delicious with spaghetti and red chilli. But to gardeners it is a menace that threatens their hedges, roses and rhododendrons.
The parasitic and apparently tasty honey fungus not only divides opinions; it is also widely seen as the largest living organism on Earth.
More precisely, a specific honey fungus measuring 2.4 miles (3.8 km) across in the Blue Mountains in Oregon is thought to be the largest living organism on Earth.
Fungi are not necessarly parasites. Many of them are symbiotic with trees.
Supplying nutrients to the tree via it's roots and presumably gaining some benefit in turn.
Truffles are in this category.
Fungi are also NOT plants. They don't photosynthesise and some species actually trap and consume worms.
They also are possibly the most important class of organism on the planet. Without fungi nothing would ever break down or rot away.
Mushrooms are awwesome
Now for homework go away and look up the total amazingness that are Slime Moulds !