When an alga and fungus physically intertwine in a very close symbiotic relationship, they actually form an entirely new growth called lichen. Lichen is a kind of primitive plant species that's nothing more than strands of alga linked with roots and branches of a fungus that together absorb minerals from the ground and conduct photosynthesis. They can grow almost anywhere, from moist bark to recently cooled lava to frozen rocks.
Unlike most parasitic or symbiotic relationships, when a fungus and alga grow together so tightly they can't be separated, they qualify as a different kind of material. Biologists have agreed that lichen straddles the monera and fungi kingdoms of living things since it is part one and part another. Its body, called a thallus, can be made of different types of fungi and blue-green algae, which will determine how much water it needs or to what it can attach.
The alga, called the physobiont, contains chlorophyll, so it can photosynthesize energy that it passes onto the fungus. The mycobiant, the fungus, has roots that leech minerals and water from rocks or plants that it, in turn, passes onto the alga. This allows the new growth to thrive in a greater diversity of climates than either algae or fungi do alone. It can even dry out during drought and reconstitute itself when the rains come.
Three major types of lichen grow into different shapes on various substrates. Crustose grows into unique flat plates that look like mushrooms. Foliose looks leafier, with small, green lobes, and grows in wetter climates beside moss. Lichen that resembles viney clumps hanging from branches is usually fruticose. It can be found in rainforests on partially rotting logs, in arctic regions on rocks, or cliffsides near salty ocean spray.
Lichen is not just a peculiar, uncategorizable kind of living matter; people use it for practical purposes, too. Since it's very sensitive to pollution, especially sulfur dioxide, environmentalists use it's presence as a gauge of the cleanliness of the air. Dyes and medicine can be extracted from them as well, and virtually every plant and animal relies on their slow erosion of wood and rock to replenish the topsoil with nutrients.