The fungi kingdom is vast and varied, encompassing everything from single-celled baking yeast to what might be the world's largest organism —a 2,200-acre mycelium in Oregon. Fungi are everywhere, from the roots of most plants to the stomach of nearly every animal. Mycologists estimate that there are more than 1.5 million species of fungi, but fewer than 7 percent have been identified. Of those, about 20 percent are mushrooms. Below, an abridged family tree.