Lichens are composed of two organisms living in symbiotic mutualism: a fungus and an alga (Lichens, 2000). Most of the lichen is composed of fungal filaments (the mycobiont), with algal cells (the phycobiont), usually from either a green alga or a cyanobacterium (formerly called blue-green algae) living among them. In many cases the fungus and the alga making up the lichen are capable of independent life, but sometimes the fungus has become dependent on the algal partner for survival. The appearance of the fungus within the lichen is quite different from its morphology as a separate entity. Fungi are incapable of making their own food, and usually provide for themselves as parasites or decomposers. Lichen fungi cultivate partners that manufacture food by photosynthesis - the algae (The Special Biology, 2000).
Lichens reproduce in two basic ways: firstly, a lichen may produce soredia, a cluster of algal cells wrapped in fungal filaments which may disperse to form new lichens; secondly, the lichen may reproduce through isidia, which are like soredia except that they are enclosed within a layer of protective cortex tissue. An isidium is more like a miniature lichen. The fungus also produces spores from an area known as the apothecium. These spores are dispersed and germinate into new fungi, but not into new lichens. New lichens are only produced when the fungus and the alga disperse together (Lichens, 2000).
Lichens can grow in almost any location that is stable and has a reasonably well-lit surface. This includes on the soil, on rocks, and on the sides of trees. Lichens may absorb mineral nutrients from any of these substrates on which it grows, but is generally self-reliant in feeding itself through photosynthesis in the algal cells. This means that lichens growing on trees are not parasitic since they do not feed on the trees. Lichens growing on rocks may release chemicals which speed the degradation of the rock in...