Fungi once were considered to be plants because they grow out of the soil and have rigid cell walls. Nowadays they are placed independently in their own kingdom of equal rank with the animals and plants and, in fact, are more closely related to animals than to plants.
Aren't fungi really plants?
Like the animals, they have chitin (the substance that gives hardness to the external skeletons of lobsters and insects) in their cell walls and store reserve food as glycogen (the form of glucose found in the liver and muscles). They lack chlorophyll and are heterotrophic (organisms which obtain carbon for synthesis from other organic matter or proteins). Familiar types of fungus include edible mushrooms, molds, mildews, yeasts, and the plant pathogens, smuts and rusts.
Believe it or not, the Earth's largest living organism may be a fungus in Oregon that lives three feet underground and is estimated to cover 2,200 acres.