Q.Give a general account on lichens.

Step 1. Define what a lichen is. A lichen is a symbiotic association between an alga and a fungus. The algal partner (the phycobiont or photobiont) is photosynthetic and supplies nutrition to the fungal partner, while the fungal partner (the mycobiont) supplies physical protection and anchors the whole thallus to its substrate using root-like structures called rhizinae.

Step 2. Describe how lichens reproduce. Asexually, the lichen thallus as a whole reproduces by fragmentation or by specialised propagules called soredia and isidia. Within that shared thallus, each partner can also reproduce independently: the phycobiont by akinetes, hormogonia or aplanospores, and the mycobiont through true sexual reproduction, producing ascocarps.

Step 3. Give the classification schemes. Lichens are grouped three separate ways - by habitat (corticolous on bark, lignicolous on wood, saxicolous on rock, terricolous on ground, marine or freshwater); by thallus morphology (leprose, crustose - crust-like, foliose - leaf-like, fruticose - branched shrub-like); and by algal distribution within the thallus (homoiomerous - algae spread evenly, heteromerous - algae confined to one distinct layer). A lichen is also called an ascolichen or basidiolichen depending on whether its fungal partner is an Ascomycete or a Basidiomycete. …

Cover definition, reproduction, classification and economic/ecological importance in turn, …

• Describing a lichen as simply 'a type of fungus' or 'a type of alga' instead of a two-partner symbiosis. …