“An alga took a ‘likin’ to a fungus.” This is a simple way to remember the components of lichens, an interesting group of organisms frequently overlooked by the average New Orleanian. Lichens are a mutualistic association between an alga and a fungus, meaning that they both benefit from their relationship. The fungus receives food from the photosynthetic activity of the alga and the fungus provides a moist resting site for the alga.
There are 25,000 species of lichens world-wide and, interestingly, each species as recognized is composed of distinct species of algae and fungi. Lichens may be placed in one of three categories depending on their form: crustose (crusty and flat), foliose (leafy), or fructicose (shrubby). All of these types may be found in New Orleans where they usually grow on trees, telephone wires, and buildings. One form, “old man’s beard” (Usnea), looks so much like Spanish Moss (a bromeliad) that the latter was given the scientific name Tillansia usneoides (usneoides means “looks like Usnea”).
The simplest lichens consist of fungal threads (hyphae) interwoven among scattered algae. Higher lichens have a more complex organization forming a thallus which has a characteristic growth pattern. The algae are usually layered just below the surface of the thallus.
Lichens are very important as early invaders of rugged terrain because they can enter a state of suspended animation if they get too dry, hot, or cold. As they dry, the upper surface thickens and photosynthesis stops. Upon contact with water, these processes rapidly resume and a high rate of metabolism reappears.
Lichens have three principal means of reproducing. The simplest is for a piece of thallus to break off and relocate. Another method is for soredia (small clusters of fungal hyphae and algae) to erupt from fissures (cracks) in the thallus and form a new lichen if they land in a suitable habitat. The last method is to form a cup-shaped ascocarp that produces fungal spores. These spores begin to grow purely as a fungus until they encounter the right alga, then the lichen is produced.
Begin to look closely at these marvels of nature. You just might take a likin’ to them!