BOTANY WITH BASICS OF HYDROBOTANY - 2010

9. LICHENS

The division includes 18-20 thousand species. Lichens are complex organisms based on a Symbiosis between a fungus and an alga. Through evolution, they have adapted to close coexistence, forming a qualitatively new Organism distinct from its components. The Fungi that make up lichens belong to ascomycetes, and more rarely, to Basidiomycetes.

The Algae belong to the divisions of cyanobacteria (blue-green algae) or green algae. Each lichen species includes a specific species of fungus and alga.

Fig. 9.1. Structure OF THE lichen Cladonia

Hygaeia: algal Cells (spherical), enveloped by fungal hyphae

The dual nature of lichens was established by Famintzin and Baranetsky in 1867. In 1936, Henkel and Yuzhakova discovered a third component of lichens — Bacteria capable of fixing free nitrogen from the air. All components are closely linked through development and METABOLISM. The fungus can form haustoria (absorbing Organs) that penetrate algal cells and absorb their contents. Research has established that this is a peculiar form of fungal parasitism on the alga. However, the fungal hyphae also protect the algal cells from desiccation and mechanical damage, supplying them with Water and mineral nutrients. When artificially separated, the alga is able to survive, whereas the fungus usually dies.

Photosynthesis and organic matter accumulation in lichens proceed very slowly: in crustose lichens, the annual growth is 1-8 mm, while in foliose and fruticose lichens, it is 1-35 mm.

According to The structure of the thallus, lichens are divided into three groups (Fig. 9.2). Crustose lichens resemble a crust or Skin that adheres tightly to the substrate (bark or stone) and often grows into it. Foliose lichens have the form of dichotomously lobed sheets that are loosely attached to the substrate. Fruticose lichens appear as branched stems loosely attached to the substrate.

Fig. 9.2. Structure of lichens: 1 - crustose (lungwort - Lobaria pulmonaria), 2 - foliose (dog lichen - Peltigera canina), 3 - fruticose (reindeer lichen, "reindeer moss" - Cladonia rangiferina)

Lichens reproduce mainly vegetatively: by fragments of the thallus or specialized organs such as soredia or isidia. Soredia are formed inside the thallus and consist of several algal cells enveloped by fungal hyphae. Through a rupture in the thallus, they fall out and are dispersed by the wind. Isidia also contain several algal cells enveloped by the fungus, but they are formed as outgrowths On the surface of the thallus. Each component is capable of reproducing independently. The fungus reproduces via spores, which germinate into mycelium. However, a lichen is formed only when the regenerated fungal mycelium "meets" a suitable alga in its path, which reproduces by Cell Division.

Lichens are divided into three unequal groups: 1 - Class Ascolichenes, formed by ascomycetous fungi; 2 - class Basidiolichenes, formed by basidiomycetous fungi; 3 - artificial group of "imperfect lichens" (Lichenes imperfecti), in which spore-bearing fruit bodies have not been discovered, and therefore their place in the lichen Classification remains unknown.

In terms of their water regime, lichens are poikilohydric plants lacking specialized organs for Water uptake and loss, and are practically devoid of physiological control over these processes. Water is absorbed by the entire surface of the thallus and retained in capillaries between hyphae and algal cells, as well as by the Swelling walls of certain hyphae; in gelatinous lichens, a significant amount of water is held by mucus. The ability of lichens to actively absorb water vapor not only from a saturated atmosphere, but also from an unsaturated one, is a major ecological advantage over other plants. Inorganic and organic nutrients dissolved in water are absorbed along with it. Lichens can withstand much more severe dehydration over prolonged periods compared to other plants.

Lichens have adapted to life in extreme environments where neither component can survive on its own. Therefore, lichens are the pioneer organisms that colonize barren areas (rocks, tree bark, etc.).

Significance of lichens. Lichens are widespread in the tundra and forest-tundra, where they serve as the primary food source for reindeer (various species of Cladonia, commonly known as reindeer moss).

Lichens cannot tolerate heavy shade and require clean air to develop, making them useful as bioindicators of pollution. They colonize the most resource-poor habitats.

Lichens are used as Dyes, and from them, litmus, fixatives for perfumery, and Antibiotics are produced. Some of them are suitable for human consumption.

Selection/41.html">Review Questions and tasks

1. Describe the structural and reproductive features of lichens.

2. What are the Specific characteristics of the geographic distribution and ecological spread of lichens?