Lichen

Lichen is a compound organism, made up of two species. A fungus and a cyanobacteria or green algae live symbiotically, and both are benefited from this mutualistic relationship. In exchange for a safe habitat to live in, the cyanobacteria or green algae provide food to the fungus from their photosynthetic processes. (1) The body of a lichen is a thallus, or a plant body that is not differentiated into stems and leaves, and lacks roots and a vascular system.

Biology

Unlike plants, lichen do not have a vascular system. This means they do not have a xylem or phloem to move nutrients and water around their plant body. Lichen get their water and nutrients by absorbing them from their surroundings. (3)

Cortex

The outer layer of the lichen is called the cortex. The cells in the cortex are thicker and more closely packed, providing a small amount of protection for the organism. (3)

Medulla

Fungal filaments, or medulla, make up most of the lichen organism. Fungal cells are loosely packed in the middle of the lichen body, with thin cell walls and a threadlike structure. (3)

Attachments

Rhizines

Some lichen use rhizines to attach to their substrate. Rhizines are fungal filaments extending out from the medulla. Rhizines do not move water or help the lichen breathe. Their sole purpose is stabilizing the lichen down. (3)

Holdfast

Some lichen use holdfasts to fasten themselves down. This is a central peg that extends out from the lichen thallus. (3)

Pioneer Species

Lichen are considered pioneer species, or the first organism to appear in areas of primary succession. (2) They are able to colonize bare rocks, and an ecosystem is then able to begin developing on them.