Lichen

A lichen is not one organism pretending to be two; it is two organisms that function as one. The fungus obtains water and minerals from the environment and provides structure and protection for its partner, while the alga, or in some lichens a cyanobacterium, provides the carbohydrates and mineral nutrients the fungus needs to grow.

Reproduction reflects the partnership: lichens reproduce mainly by releasing microscopic bundles of all their components into the air, bundles called soredia, and wherever one lands on a suitable habitat it can begin a new lichen. The fungus, as the dominant partner, also produces spores, the reproductive route the spores glossary entry covers on its own.

A crustose lichen forms a crust over a surface such as a boulder, the soil, a car or roof shingles, and it is pressed against its substrate. UF/IFAS Extension's EDIS says crustose lichens exist as if painted onto the surface they grow on, and that it is nearly impossible to pry the organism from it. Lichens attach by different means, the U.S. Forest Service writes; rhizines are fungal filaments that move no water or nutrients the way roots do, and “they simply hold the lichen down to whatever it is sitting on.”

The U.S. Forest Service says just about anything that holds still long enough is a suitable substrate for a lichen, houses included, and that lichens can cause some damage to buildings and man-made structures, though it is a very slow process that does not endanger those substrates. The condition of a roof under a lichen colony is a licensed roofing contractor's call, not this page's. Whether lichen comes off a roof the way algae does, and why it can leave marks, are two separate answers: does lichen come off a roof the way algae does and why does lichen leave marks after cleaning.

The U.S. Forest Service describes a lichen as functionally on and off: when lichens are wet they turn on and start photosynthesizing and growing, and when dry they turn off, become brittle and go dormant. A dry lichen is usually gray or the color of its fungal layer; wet, the algal cells beneath show through, green algae giving a bright green color and cyanobacteria a dark green, brown or black one.

GAF's homeowner education page is the one roofing-maker sentence cited here that names lichen: dark stains on shingles may look like dirt but may be moss, lichen or blue-green algae, and this may not require a new roof. A green growth on a roof can be one of several things, which what is the green fuzz on my roof sorts out, and the alga inside a lichen belongs to the wider group covered at algae. Lichens also grow on tree bark, and what trees overhead add to a roof is taken up at do palm trees make roofs dirtier.