INTEGRATIVE BIOLOGY 371

GENERAL MYCOLOGY

02 DECEMBER 2003

LICHENS

A lichen is an intimate biological association of a fungus (mycobiont) and an alga and/or cyanobacterium (photobiont). In lichen symbioses, the thallus components are so perfectly coordinated as to behave as a single organism. A lichen has a unique morphological entity with no obvious similarity to either mycobiont or phycobiont.

  1. Lichen Tissues (use a dissecting microscope). Lichens have a cortex on the upper and lower surfaces that acts as a protective covering. Observe this in Parmeliaconspersa. The medulla forms the bulk of the lichen thallus. It is composed of interwoven mycelium that is loose and cottony. Using a razor blade, make a section through the thallus (an edge cut). The medulla appears as the white cottony area. In Parmelia conspersa, the algal layer is a distinct layer confined between the upper cortex and the medulla. Place a drop of water on the cut surface of the lichen thallus. Make cross-sections of the thallus and observe the tissues. Note the green algal layer. In gelatinous lichens the algal layer is unstratified.
  1. Lichen Growth Forms. (observe with low-power dissecting microscope and visually).
  1. Crustose Lichens. These lichens lack a lower cortex. The thallus consists of little scales called areoles. The lower surface of the areoles grows faster than the upper surface and frequently forms a thin black spreading layer around it. This layer is termed the prothallus. In Rhizocarpon geographicum, observe the areoles (yellow) and the prothallus (black). Look at the other crustose lichens on rocks. Note the areoles and prothallus.
  1. Foliose Lichens. In this lichen type, the thallus is a leafy dorsiventral body with a distinct lower cortex and a rhizinal attachement to the substrate. Observe this in Parmelia conspersa and Parmelia carperata.
  1. Fruticose Lichens. The thalli of these lichens are hair-like, shrubby, finger-like, or strap-shaped. Observe the thallus structure of Cladoniadeformis, Cladoniasp., and Usnea strigosa. In the Usnea strigosa, pull on a string of the thallus until the outer part ruptures. Under the dissecting microscope, note the central cord.

  1. Vegetative Structures. Lichens have a variety of vegetative structures that are taxonomically important. Soredia are separable, noncorticated clumps of algal cells closely enveloped by hyphae. Observe the soredia in Ramulina farinaceae and Lepraria (a primitive lichen in which the entire thallus is a soredium). Make microscopic preparations of the soredia in both of these species and observe and illustrate the algal-lichen asscociation. Isidia are cylindrical or finger-like protuberances of the upper cortex in which the fungal and algal tissues are more or less incorporated. Under the dissecting microscope, observe the isidia of Parmelia conspersa. Cilia are hair-like thalline appendages along the margin of lobes. Observe these structures in Anaptychialeucomelaema under the dissecting microscope.
  1. Reproductive Structures. Lichen fungi are usually Ascomycetes but a few are Basidiomycetes. Sexual reproduction of ascolichens is basically similar to that in other Ascomycetes. Perithecia, pseudothecia and apothecia are found in ascolichens. In Pyrenula (Melanotheca) cruenta, the pseudothecia are seated on a stroma and both pseudothecia and stroma are embedded in the lichen thallus. Make cross sections through the ascomata with a razor blade. Observe the locule, asci and ascospores. In the lichens with discomycetous fungi, the apothecia can be cup-shaped or lirelline (elongated and sometime branched). Note the gross morphology of Zanthoria polycarpa. Examine Dermatocarpon miniatum. What type of ascomata does it have?
  1. Microchemical Tests. The color tests of K, C, and P (P=paraphenylenediamine is carcinogenic and will not be used in this lab.) have been employed by lichenologists for many years and have proved their usefulness in taxonomy. Apply KOH to the medulla of Parmelia conspersa. Note the color. Try this test with Xanthoria and Candelariaand note any color changes.

6.Draw and give the species name for each of the lichen growth forms.

7. What is a Podetium?

8.What biological roles do lichens play?

1