Lichens are a small group of composite plants consisting of two partners, a fungus and an alga or a cyanobacterium. The fungal partner is composed of filamentous cells, where every filament is called a hypha. The most common fungi in lichens are species of Ascomycetes or Basidiomycetes. The algal partner is either green algae Chlorophyta or Cyanophyceae family of blue-green bacteria. The non-fungal part is called a photobiont that consists of chlorophyll. Lichens exist in one of the following growth forms: crustose, foliose, squamulose, and fruticose. Lichens do not have a waxy cuticle like plants have on their leaves, nor do they have vascular tissue such as xylem and phloem to move nutrients and water around their thalli. The largest lichen can make a thallus up to 3ft long, although most of them are smaller than a few centimeters. They are colorful, ranging from yellow to greens and black hues. Mostly, lichens grow slowly. The one in which the phycobiont is a blue-green bacterium has the ability to convert nitrogen gas into ammonia. Some can reach the age of many centuries, mainly the one living in stressful environments such as arctic tundra or alpine.
