
Lichen growth forms

Lichens are grouped by common lichen growth forms. There are a wide range of growth forms (morphology). These groupings may not be consistent with lichen taxonomy, since lichens with similar growth forms are not necessarily related by ancestry. Groups based on growth may overlap and have borderline cases. A species usually has the same overall growth form wherever it is found, but not always.
Common groupings of lichens based on overall growth form include 1. bushy or shrubby (fruticose) 2. leafy (foliose) 3. crust-like (crustose) 4. scaly (squamulose) 5. stringy (filamentose), 6. whispy (Byssoid), 7. powdery (leprose), or 8. structureless (e.g., gelatinous). Ways of grouping by growth forms is usually based on the form of the part of lichen that is not involved in sexual reproduction, the "vegetative" part, which is called the thallus. This includes both the fungus (mycobiont) and the algae and/or cyanobacteria cells (photobiont) they surround, but does not include any part with sexual fruit bodies of the mycobiont. The thallus may or may not have a "skin", a surface layer of differentiated tissue that covers the undifferentiated middle tissue, the medulla. When part of a lichen has a cortex it is called corticate. If it lacks a cortex it is called ecorticate.
A simple way to group lichens that requires use of a microscope to see a lichen's surface structure (anatomy) is by the presence or absence of a "skin" (cortex). Fruticose lichens have one cortex encircling the "branches", even when the branches are flattened and look like "leaves". Foliose lichens have a separate upper cortex and lower cortex on each side of the "leaf". Crustose and squamulose lichens have an upper cortex, but do not have a lower cortex. Filimentous, byssoid, leprose, and other lichens do not have a cortex (are ecorticate).
The following are commonly used groupings for lichens:
Since growth forms may be variable, there are gray areas between these groups, and there is overlap, placement of lichens in these groups is not definite. Placement of a lichen in one of these groups may vary from author to author.
Sometimes, such as when the thallus grows "inside" solid rock, between the grains, only the fruiting part of a lichen is visible, and this may be visible in a dramatic way (e.g., "Caloplaca luteominea" subspecies "bolandri").