Lichenology

Step 1: Understanding the Concept:
The question asks to identify a genus of lichen that exhibits a fruticose growth form. Lichens are classified into three main morphological types based on their thallus structure.
Step 2: Detailed Explanation:
Let's define the three main growth forms of lichens and classify the given genera:

Crustose: A crust-like thallus that is tightly attached to the substrate and cannot be removed without damaging it. Examples: {Rhizocarpon} (map lichen) and {Graphis} (script lichen).
Foliose: A leaf-like, flattened thallus with a distinct upper and lower surface, loosely attached to the substrate. Example: {Parmelia} (rock shield lichen).
Fruticose: A shrub-like, hair-like, or finger-like thallus that is erect or pendulous (hanging). It is attached to the substrate only at its base. Example: {Cladonia} (which includes reindeer moss), {Usnea} (old man's beard), and {Evernia}.
Step 3: Final Answer:
Among the given options, {Cladonia} is the correct example of a fruticose lichen.