Botany - B.Ye. Yakubenko 2017

Part Two. The System of the Organic World
Chapter V. The System of the Organic World
Laboratory Class Topic 10.29. Lichens, Lichenized Fungi (Lichenes)

General Remarks. Lichens are multicellular symbiotic organisms whose vegetative body, the thallus, is formed by Fungi and Algae. The fungal component of a lichen thallus predominantly belongs to the ascomycetes, with only about 10 species belonging to Basidiomycetes, while the algal partners are mostly green and blue-green algae. Thus, lichens are dual organisms consisting of an autotrophic photobiont and a heterotrophic mycobiont. Through Photosynthesis, algae produce organic nutrients that also feed the fungi. In turn, fungi absorb Water and mineral nutrients from the environment and supply them to the algae, while their tightly interwoven hyphae protect the thallus.

Based on their morphological Structure, lichens are divided into three main groups: crustose (crust-like), foliose (leaf-like), and fruticose (shrub-like).

In crustose lichens, the thallus is tightly fused with the substrate, appearing as crusts, patches, or cracks on rocks and tree bark. Foliose lichens have the appearance of dorsiventral scales, plates, or small leaves, attached to the substrate at the center of the thallus by rhizinae—bundles of hyphae—with their edges lifting above the substrate. Fruticose lichens resemble dichotomously branched little shrubs, attached to the substrate by rhizinae only at the Base of the thallus.

Microscopically, Two Types of thallus structure are distinguished: homoiomerous and heteromerous. Externally, the lichen thallus is covered by a cortex (upper cortex) made of tightly interwoven, modified fungal hyphae. In homoiomerous lichens, algae are distributed evenly among the fungal hyphae throughout the entire thickness of the thallus. Heteromerous lichens have a stratified thallus. Directly beneath the upper cortex lies a loose gonidial (algal) layer composed of fungal hyphae and algae. Below the gonidial layer is the medulla, formed exclusively by loosely arranged fungal hyphae. Underneath the medulla lies the lower cortex, consisting of tightly interwoven hyphae from which absorbing rhizoidal hyphae extend.

Lichens reproduce via spores—produced exclusively by the fungus either sexually or asexually—and vegetatively through fragments of the thallus, soredia, or isidia. Soredia are formed within the gonidial layer and appear as tiny powdery propagules consisting of several algal Cells wrapped in fungal hyphae. Escaping through cracks in the thallus, soredia are dispersed by the wind. Isidia are small, wart-like or leaflike outgrowths on the thallus surface that also contain both algae and fungal hyphae. Under favorable conditions, both soredia and isidia develop into a new thallus.

Lichens grow extremely slowly and live for 50–100 years. They occur on tree bark, rocks, soil surfaces, leaves of trees and shrubs, and even on moss cushions.

Due to their carbohydrate content, in some Regions of the world, such as Northern Europe, they serve as valuable forage plants. For example, lichens of the genus Cladonia (reindeer moss) provide staple food for deer, and certain species are consumed by humans in Japan. Furthermore, they produce lichen acids, which are used in pharmacology. In the past, litmus was extracted from lichens. Lichens from the genera Usnea, Cetraria, and Parmelia exhibit notable antibiotic and antitumor properties, while oakmoss (Evernia prunastri) is widely used in perfumery as a fixative.

In the biosphere, lichens play a crucial role as pioneer organisms initiating The process of soil formation.

Objects:

1. Script lichen (Graphis scripta (L.) Ach.)

2. Rim lichen (Lecanora carpinea (L.) Vainio)

3. Common sunburst lichen (Xanthoria parietina (L.) Th.Fr.)

4. Gray reindeer lichen (Cladonia sylvatica (L.) Hoffm.)

5. Oakmoss (Evernia prunastri (L.) Ach.)

Tasks:

1. Examine the morphological types of lichens: crustose, foliose, and fruticose.

2. Study the Structural Features of lichen fruiting bodies.

3. Investigate the specific reproductive Organs of lichens.

4. Sketch the morphological and Anatomical Features of the thallus and apothecium.

Equipment and Materials: MBR-1 or Biolam microscopes, scalpels, blades, razors, prepared slides, charts, handout materials.

Macroscopic Study of Lichens. Select lichens representing different morphological groups: crustose, foliose, and fruticose. Carefully observe them and sketch the general appearance and structural features of each. The thallus of a crustose lichen (Graphis) cannot be separated from its substrate. The foliose lichen (Xanthoria) features a golden-yellow thallus that is only partially attached, with most of its body elevated above the substrate surface. Brightly colored, open fruiting bodies—apothecia of various sizes—stand out on its surface. Fruticose lichens possess a heavily branched thallus, the greater part of which rises above the substrate. The thallus takes the form of a small shrub, hence the name of this morphological group. Pay special attention to the branching pattern of the thallus and the remnants of the gomphus—a dense web of mycelial filaments used by the lichen to attach to the substrate (Fig. 82).

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Microscopic Study of Lichens. Using a prepared slide, first under low and then under high magnification, examine the Cytology/practical/54.html">Longitudinal section of the apothecium in the common sunburst lichen, focusing on the hymenial layer. Familiar asci containing eight ascospores and filamentous paraphyses are clearly visible. Above the hymenium, a protective brown layer of spherical cells—the epithecium—stands out, formed by the slightly expanded tips of the paraphyses. The base of the hymenium is underlain by a robust layer, the hypothecium, formed by numerous colorless, densely interwoven hyphae. Along the margin, the amphithecium containing green or blue-green algae is distinct. The central region consists of loosely interwoven hyphae interspersed with occasional unicellular algae. The layer containing algae is termed the gonidial or algal layer. The region of the thallus lacking algae forms the medulla. The thallus is covered by an upper cortex on the dorsal side and a lower cortex on the ventral side (Fig. 83).

The distinct, well-defined hymenium is easily distinguishable at the top. It contains asci with ascospores alongside filamentous paraphyses. Above the hymenium, a protective brown layer of spherical cells—the epithecium—stands out, formed by the slightly expanded tips of the paraphyses. The base of the hymenium is underlain by a robust layer, the hypothecium, formed by numerous colorless, densely interwoven hyphae. Along the margin, the amphithecium with green or blue-green algae is clearly visible.

The central portion contains loosely interwoven hyphae interspersed with scattered unicellular algae. The layer incorporating the algae is called the gonidial or algal layer. The algae-free layer of the thallus is designated as the medulla. The thallus is enclosed by upper and lower cortical layers.

Next, let us examine the homoiomerous thallus type. It features upper and lower cortical layers formed by a dense mesh of hyphae. Between them lies the algal layer, where unicellular algae are evenly distributed among the fungal hyphae throughout the entire thallus.

In the heteromerous thallus type, the upper and lower cortical layers of densely interwoven fungal hyphae are also clearly visible. Two additional layers can be distinguished between the cortical layers. Take a closer look: you will notice that adjacent to the morphologically upper cortical layer is a layer containing algal cells—namely, the gonidial layer—while below it lies the medulla (or medullary layer), which lacks algae and appears gray or colorless (see Fig. 84).

Examine soredia and isidia separately, as these are the structures through which lichens reproduce. For this, take an oak moss or *Evernia* thallus. A white coating of tiny granules is noticeable along its margins.

Place them in a drop of water previously applied to a Glass Microscope slide, then cover with a coverslip. Gently tap the slide preparation with a dissecting needle. Examine it under low and high magnification of the microscope. You will clearly see one or more algal cells tightly encircled by colorless fungal hyphae. These are soredia. Make a drawing and label the above-mentioned structural components in your sketch.

Unlike soredia, isidia are rod-like or variably shaped and sized outgrowths along the margins of the thallus. They easily break off from the thallus and, upon reaching favorable conditions, give rise to a new lichen individual. Their distinctive feature is that the algae are always covered by an outer cortical layer.

Conclusion. Lichens, or fungus-algae composites, are multicellular symbiotic organisms whose thallus consists of a phototrophic alga and a heterotrophic fungus. Based on the MORPHOLOGICAL STRUCTURE OF the thallus, lichens are classified as crustose, foliose, or fruticose, while anatomically they are categorized as homoiomerous or heteromerous. Reproduction in lichens is predominantly vegetative.

Self-Assessment Questions

1. What components make up the lichen thallus?

2. Why are lichens classified as symbiotic organisms?

3. What are the morphological types of lichen thalli?

4. What are the anatomical types of lichen thalli?

5. Which components comprise the thallus of a heteromerous lichen?

6. Which components constitute the thallus of a homoiomerous lichen?

7. What are the modes of reproduction in lichens?