Lichens are unusual creatures. A lichen is not a single organism the way most other living things are, but rather it is a combination of two organisms which live together intimately. Most of the lichen is composed of fungal filaments, but living among the filaments are algal cells, usually from a green alga or a cyanobacterium.
In many cases the fungus and the alga which together make the lichen may each be found living in nature without its partner, but many other lichens include a fungus which cannot survive on its own — it has become dependent on its algal partner for survival. In all cases though, the appearance of the fungus in the lichen is quite different from its morphology as a separately growing individual.
Lichens occur in one of four basic growth forms, as illustrated below:
- crustose – crust-like, growing tight against the substrate.
- squamulose – tightly clustered and slightly flattened pebble-like units.
- foliose – leaflike, with flat sheets of tissue not tightly bound.
- fruticose – free-standing branching tubes.
Despite the wide diversity of the basic growth forms, all lichens have a similar internal morphology. The bulk of the lichen’s body is formed from filaments of the fungal partner, and the relative density of these filaments defines the layers within the lichen.
At its outer surface, where it comes in contact with the environment, the filaments are packed tightly together to form the cortex. The dense cortex serves to keep out other organisms, and helps to reduce the intensity of light which may damage the alga cells.
The algal partner cells are distributed just below the cortex in a layer where the fungal filaments are not so dense. This is very similar to the arrangement in a plant leaf, where the photosynthetic cells are loosely packed to allow air circulation.
Below the algal layer is the medulla, a loosely woven layer of fungal filaments. In foliose lichens, there is a second cortex below the medulla, but in crustose and squamulose lichens, the medulla is in direct contact with the underlying substrate, to which the lichen is attached.
Introduction to Lichens