A
lichen is a composite organism that arises from
algae or
cyanobacteria (or both) living among
filaments of a
fungus in a
symbiotic relationship. The combined life form has properties that are very different from the properties of its component organisms. Lichens come in many colors, sizes, and forms. The properties are sometimes plant-like, but lichens are not plants. Lichens may have tiny, leafless branches (
fruticose), flat leaf-like structures (
foliose), flakes that lie on the surface like peeling paint (
crustose), or other growth forms. A
macrolichen is a lichen that is either bush-like or leafy; all other lichens are termed
microlichens. Here, "macro" and "micro" do not refer to size, but to the growth form.
Common names for lichens may contain the word "
moss" (e.g., "
Reindeer moss", "
Iceland moss"), and lichens may superficially look like and grow with mosses, but lichens are not related to mosses or any plant. Lichens do not have roots that absorb water and nutrients as plants do but like plants they produce their own food by photosynthesis using sunlight energy, from carbon dioxide, water and minerals in their environment. When they grow on plants, they do not live as
parasites and only use the plants as a substrate.
