Foliose

Foliose describes a growth form of lichens characterized by a flat, leaf-like thallus that is loosely attached to its substrate. The thallus is typically divided into a distinct upper surface and a lower surface, with lobed margins that give a leafy appearance. Unlike crustose lichens, foliose lichens can usually be pried away from the substrate without breaking, and unlike fruticose lichens, they are not greatly branched or erect.

The lower surface often bears rhizines, small root-like structures that anchor the thallus. The upper surface may be cortex-rich and colored in greens, grays, or yellows, while the lower surface tends to be lighter. Foliose lichens can grow on rocks (saxicolous), on tree bark (corticoles), or on the ground (terricolous), displaying a range of color patterns that help distinguish species.

Reproduction and development occur through sexual and asexual means. Apothecia on the upper surface release spores in sexual reproduction, while asexual strategies include the production of soredia or isidia on the thallus margins that disperse and form new thalli.

Ecology and significance: Foliose lichens are important components of many ecosystems, contributing to nutrient cycling and providing habitat for microfauna. They are also sensitive indicators of air quality, often used in biomonitoring programs because many species respond to atmospheric pollutants. Notable examples include Parmelia sulcata, Flavoparmelia caperata, Hypogymnia physodes, and Peltigera canina. As a growth form, foliose lichens display considerable diversity across temperate and boreal regions.