The Bushy Beard Lichen, Usnea strigosa on an oak branch. This lichen can be found over a wide area of eastern and south-central US west to eastern AR and TX and south to Gulf of Mexico. A lichen, or lichenized fungus, is actually two organisms functioning as a single, stable unit. Lichens comprise a fungus living in a symbiotic relationship with an alga or cyanobacterium (or both in some instances). There are about 17,000 species of lichen worldwide. A lichen is named for the species of fungus, regardless of which other organism is involved. The combined lichen has properties different from those of its component organisms. Lichens growing on trees are not parasites, but using the braches as a substrate to grow upon. Photo McCurtain Co., AR.