Water Uptake

• Lichens absorb water directly through the thallus.
• They can desiccate completely and later revive upon rehydration — a key adaptation for survival on bark and rock.

Photosynthesis

• Algal partners use sunlight to produce sugars.
• Cyanobacteria can additionally fix atmospheric nitrogen, enriching the habitat.

Gas Exchange

• Occurs freely across the thallus; lichens lack stomata.

Growth Rate

• Extremely slow (typically 0.5–5 mm per year), depending on species and climate.

Chemical Production
Lichens produce complex secondary metabolites used for:

• UV protection
• Anti-bacterial and anti-fungal defense
• Herbivore deterrence
• Coloration (yellows, greens, oranges, whites)