Question

The specialised structure present in the thallus of Lichen that help in nitrogen fixation is
1. Cyphellae
2. Isidia
3. Cephalodia
4. Soredia

Answer 1

Correct Answer - Option 3 : Cephalodia

Concept:

• The term ''Lichen'' was given by Theophrastus.
• Study of Lichens - Lichenology.
• Father of Lichenology - Erik acharius
• Lichen is a composite thalloid structure of algae & fungi.
• The algal part of Lichen is called phycobiont and the fungal part is called mycobiont. Both algae & fungi remain in symbiotic association i.e. they are beneficial for each other.

Explanation:

• In lichen Soredium, Cephalodum and isidium are modes of ''Asexual Reproduction''

Cephalodium:

• These are small wart-like structures formed on the surface of the thallus.
• It helps to retain moisture in lichens and its algal partner fixes nitrogen also​
• One of the characteristic features of cephalodium is that its algal and fungal components differ from that of the thallus.
• It is due to the fact that cephalodia develops on the younger parts of the thallus from the soredia of some other species. Hence, the cephalodium may be regarded as sterile thallus of some other lichen

 

Cyphella  

• These are the small pores or pits present in the lower cortex region of foliose lichens.
• Examples: Sticta sylvatica
• These perform the function of respiration in lichens

Soredium:

• Some small bud-like outgrowths, known as soredia, develop on the surface of the thallus.
• A soredium contains one or few algal cells closely enveloped by a weft of fungal hyphae.
• The soredia form a granular layer of greyish-white color on the surface of the thallus.
• They are detached from the thallus by the impact of wind or rain.
• Sometimes, as in Parmelia and Physcia, Soredia develop in an organized manner in special pustule-like areas. Then they are known as soralia.
• The soredia germinate in the suitable substratum and form new thalli.

​Isidium: 

• Isidim is small, stalked, greyish-black coral-like outgrowths that develop on the upper surface of the thallus.
• The isidium has an outer cortical layer enclosing the algal and fungal components.
• It is usually constricted at the base and is easily detachable from the parent thallus.
• It germinates under favorable conditions and forms a new thallus.