Lichens are compounds resulting from the relationship between two organisms, alga (cyanobacteria) and fungi, making up its general structure in layers. There are about 17,000 species of lichen in the world. Lichens divide into three groups based on their growth: crustose, foliose, and fruticose. Crustose lichens are firmly attached to the substrate with their lower surface. They also form crusts in bright colors like yellow. Foliose lichens are distinguishable both on the top and bottom. They are flat, leafy like lettuce, or full of bumps and are partially attached to their substrate. Fruticose lichens have many round branches with a center core; some are hollow in the middle, while others are flat. These lichens are hair-like and stand out from the surface of the substrate.