Foliose Lichens

Although colorful Crustose Lichens are likely to be seen, they have a tendency to be overlooked because they seem to be part of the background, especially on the rocks where they are so common.

Foliose Lichens are more likely to be noticed. Abundant in many habitats and on a variety of substrates, they are often large and leafy enough to attract our attention. Their shapes and sizes vary too, from broad and flat to fine-lobed and delicate. Foliose means “leafy,” so perhaps it will help if you think of them as looking more like typical plants.

Parmelia sulcata, a common foliose lichen in Colorado. By James Lindsey. CC BY-SA 3.0

What distinguishes a foliose lichen from other types? Characteristic of foliose lichens is the fact that they have a lower cortex as well as an upper one, although the two generally appear different.

Stratified foliose lichens have a thallus, or lichen body, made up of distinct layers, including an algal layer that often shows as bright green in cross-section, as in the diagram below (courtesy of The Watcher). The medulla, often white, is a cottony mass of fungal hyphae and makes up most of the thallus thickness. If you cut through a lichen lobe with a razor blade, you can see these layers with a hand lens.


Cross-section of a foliose lichen. Diagram by Nefronus, CC BY-SA 4.0 via WikiPedia.

Compare this diagram, with upper (a) and lower (d) cortex, algal layer (b), and medulla (c). Thallus extensions (e), called rhizines, help anchor the lichen to the substrate. Foliose lichens are generally broadly attached to their substrates (but see Umbilicate Lichens for an exception).

See more examples of Foliose Lichens:
Peltigera, lichens of the forest floor
Umbilicaria, or Rock Tripe

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