Question

What are the different types of lichens based on their external forms?

Answer 1

Hint: A lichen is an association between two different organisms. They are classified based on the organization of thallus.

Complete answer: Lichens are organisms that have a symbiotic relationship between algae and fungi. Their association is known as mutualism. Both the partners are benefited from this association. The algal component of the lichen is known as the phycobiont, whereas the fungal component of the lichen is called the mycobiont. The study of lichens is known as lichenology. Based on the external form or the thallus organization, the lichens are classified as follows:
a) Leprose lichen – They have the simplest thallus organization. The fungal hyphae envelopes a very small number of cells, sometimes one only. The thallus develops on the surface of the substratum. It provides a powdery appearance.
b) Crustose lichen – They have a crust-like appearance because they are very closely attached to the substratum. It is very difficult to detach them from their substratum. Their fruiting bodies are present on the upper surface of the body.
c) Foliose lichen – Their thallus is flat. It is leaf-like, well-branched. They have minute hair-like structures present on their lower surface known as rhizines. The rhizines help in attaching the thallus to the substratum.
d) Fruticose lichen – They have a very well-branched structure. They are generally erect and give a shrub-like appearance.
e) Filamentous lichen – They have a more developed algal partner. Thus, they dominate the algal partner.

Note: The fungal partner of the lichen is always more developed than the algal partner except for the filamentous lichens. The algal partners of the filamentous lichens are filamentous in form. They remain covered by only a few fungal hyphae. Due to this, the appearance of lichen is filamentous.