Green Algae

Lichen

Lichens are living partnership of a fungus and an algae, but they are in the plant catogory. The fungus component is called the mycobiont and is composed of intertwined, threadlike fibers called hyphae that are tightly packed into a tissuelike sheet. The fungus uses these hyphae to absorb food from its surroundings. The algal component, called the photobiont, makes its own food through photosynthisis and grows as a mass of green cells dispersed among the fungal hyphae. Lichens survive in a wide variety of environments, either forming small, circular crusts or leaflike structures attached to bark, rocks, or soil, or as hairlike structures hanging from tree branches.

    The relationship between the fungus and the alga in a lichen is an example of mutualism, in which both partners benefit from the partnership. This relationship enables each to tolerate harsh conditions where neither could survive alone. In this partnership, the fungus furnishes the alga with water, prevents overexposure to sunlight, and provides simple mineral nutrients, while the photosynthesizing alga supplies food to the fungus even if no other organic material is available. In dry, barren areas where plants have a hard time growing, such as polar tundra, deserts, rocky outcrops, or high mountains, lichens are the primary photosynthesizers. Some remarkable species even grow inside porous rocks, just below the surface where some light can still reach the algal cells.

    Their ability to grow in severe conditions often makes lichens pioneers in plant succession, the process in which plants colonize bare rock or soil. Lichens release acids that break down inhospitable rock, permitting soil-trapping mosses and grasses to grab hold. In areas where soil gradually accumulates, such as a forest floor, the pioneering lichens are eventually replaced by plants and trees, although other lichens may grow on these plants and trees.

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