Lichens are formed by a symbiotic association of a fungus and an alga. The fungus in most lichen species is an ascomycete, but in some lichens it is a basidiomycete. The alga is one of several distinct species of Chlorophyta or Cyanobacteria. The alga from this association can usually survive as a free-living organism, but the fungus cannot. The alga provides food for the fungus, and the fungus furnishes moisture, shelter, and minerals for the alga.

(A) Vegetative structure

Examine species of lichens that demonstrate the three general types of lichen growth forms: crustose, a prostrate, crust-like form; foliose, a more or less leaf-like appearance; and fruticose, or a branching, shrub-like form (Rust, fig. 20; V & C fig. 5.27). Do you find any evidence of algae? How do the lichens attach to their substrate?

There is a prepared slide of Physcia available for you to examine the arrangement of cells and internal structure of a lichen. Some lichens are also available for you to section. Make several thin sections and wet mount them. Stain the sections with KOH. This stain will make the algae stand out. In these and the prepared slides, you should be able to identify a developed upper cortex and lower cortex of tightly woven hyphae with a looser medulla or middle area. Are the alga cells distributed randomly throughout the lichen or are they associated with a particular area. What is the significance of the distribution of the algal partner in the lichen? Note the location and form of the algae. Are they blue-green or green algae? How can you tell? Are they unicellular or filamentous? Are any of the filaments of the fungus closely associated with the algae?

(B) Reproductive structures - Sexual reproduction of the fungal partner

On the specimens available, note whether there are any fruiting bodies associated with them. These are often brightly colored. Because most lichens are ascomycetes, these fruiting bodies are likely to be ascocarps, reproductive structures containing large numbers of asci arranged on apothecia. A prepared slide is available to show you a thin section of this area. How can you tell that the fungus is an ascomycete rather than a basidiomycete?

(C) Asexual reproduction of the lichen

Carefully examine the surface of the lichens for soredia or isidia. Survey the surface of the lichen until you find an area which seems to be covered with a light dust on it and is associated with cracks or openings through the upper lichen cortex. Examine these areas carefully and you should be able to observe soredia, which are specialized fragments of the lichen containing a few algal cells and surrounded by hyphae. Other lichens display isidia, which are tiny branches on the surface of the lichen. These branches are actually immature thalli, already with an upper and lower cortex and containing algal cells. By these means, lichens are able to propagate and actually maintain both members of the symbiosis. Can you think of a way both members can sexually reproduce and disperse their progeny while continuing to maintain a symbiotic relationship?