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Generally, the fungal mesh surrounds the algal or cyanobacterial cells, often enclosing them within complex fungal tissues that are unique to lichen associations.
The thallus may or may not have a protective "skin" of densely packed fungal filaments, often containing a second fungal species, which is called a cortex.
Fruticose lichens have one cortex layer wrapping around the "branches".
Foliose lichens have an upper cortex on the top side of the "leaf", and a separate lower cortex on the bottom side.
The filaments grow by branching then rejoining to create a mesh, which is called being "anastomose".
The mesh of fungal filaments may be dense or loose.
Common groupings of lichen thallus growth forms are: There are variations in growth types in a single lichen species, grey areas between the growth type descriptions, and overlapping between growth types, so some authors might describe lichens using different growth type descriptions.
When a crustose lichen gets old, the center may start to crack up like old-dried paint, old-broken asphalt paving, or like the polygonal "islands" of cracked-up mud in a dried lakebed.
The long life-span and slow and regular growth rate of some lichens can be used to date events (lichenometry).
Squamulose lichens may appear where the edges lift up. The thallus is not always the part of the lichen that is most visually noticeable.