Lichens are a partnership between a fungus and an alga living together as one crusty organism.

The longer version

A lichen is not a single organism but a stable partnership between a fungus and a photosynthetic partner, usually a green alga or a cyanobacterium. The fungus supplies structure, holds water, and shields the partner from strong light and drying, while the partner produces sugar through photosynthesis.

This arrangement lets lichens live where neither partner could survive alone, from bare rock and roof tiles to frozen ground. Growth is extremely slow, and a patch may persist for many years. Because they take substances straight from the air, lichens are sensitive to airborne pollution. They come in crusty, leafy, and shrubby forms, and the same partnership can look quite different in each.

Where this entry sits

Plants and Trees gathers ordinary and surprising details about how green things live, from the tallest forest giants down to the moss on a garden wall. Expect notes on flowers, seeds, roots, leaves, fungi, and the quiet tricks plants use to spread and survive. The full list sits on the Plants and Trees page, where every entry is listed in order.