Lichen
Q.17. What are lichens? Explain the biotic relationship between the components of lichen thallus. (2011, 12)
Ans. Lichen are peculiar dual organisms or composite organisms produced by the intimate association of two plants, a fungus and an algae. The two plants live in close association with each other so much, so that they appear to be a single plant. The fungal member is known as mycobiont and the algae partner is called phycobiont. This type of relationship is called symbiosis. The fungi in lichens are slightly parasitic upon algal cells from which they obtain food and in turn they help algae in absorption and retention of water. Components of lichens are cyanophyta (blue-green algae) and chlorophyta (green algae). The fungi are chiefly ascomycetes rarely basidiomycetes.
Biotic relationship between the components of lichen thallus: -
The two partners in the lichen thallus derive mutual benefit from their partnership. There are advantages on both sides. They hold that the fungus with its rhizoids absorbs water and minerals in solution from the substratum. It also absorbs moisture from the fog and moist air. The water and minerals thus absorbed are passed on to the algae. The fungal hyphae are also somewhat gelatinous. They absorb water repidly and hold it tenaciously. In this way they afford protection to the algal partner from fatal drying when the air humidity is very low. The algae is thus able to live in exposed places where it would have been impossible for it to grow in the free state. In addition the fungal hyphae, which form the body of the lichen, provide shelter to the algae.
They thus protect it form intense light, drought, and other adverse weather conditions. The duty of algae is to synthesize the necessary carbohydrates with the help of its green chloroplasts. The algae, therefore provides food to the fungus.
Q.18. Describe the criteria for the classification of lichens (2007, 08)
Related Questions -
Q. Write short note on classfication of Lichen. (2013)
Q. Differentiate between crustose and foliose lichens. (2006, 09, 12)
Q. Describe types of lichens on the basis of external morphology. (2008, 10)
Ans. Occurence: - Lichen are widely distrbuted and grow in different habitats. They occur in abundance on the shady side of trees, trunks, rocks and logs of wood. In India lichens occur all over himalayas and higher hills of peninsular India.
Structure: - The plant body of the lichen is thallus. It is gray and grayish green irregular shaped structure. Some species are yellow, orange, brown or red in colour.
Many scientists classify the lichens on the basis of (i) the nature of fungal element and (ii) the kind of the fructification.
According to G.M. Smith (1955), lichens has been divided into two sub-classes :
1. Ascolichens: - The lichens of this sub-class are found in almost all the parts of the world. These lichens are made up of a fungus of class-Ascomycetes and an algae of class Cyanophyceae.
2. Basidiolichens: - The lichens of this sub-class are made up of a fungus of class-Basidiomycetes and an algae of class-cyanophyceae or Chlorophyceae.
On the basis of presence of fungi, Zahlvrukner has been classified lichens as :
1. Ascolichens: - The fungi present in these lichens belong to the class Ascomycetes. Ascolichens are further divided into :
(i) Gymnocarpae: - The ascocarp of these lichens is of apothecium type.
(ii) Pyrenocarpae: - The fungi of these lichens is of perthecium type.
2.Basidiolichens: - The fungi of these lichens belong to the class Basidiomycetes.
Hue (1899) divided lichens into three types on the basis of the external structure of the thallus.
(1) Crustose lichens: - These occur as incrustations on rocks, soil trees. They are partly or wholly adherent to the substratum. The examples are Graphis, Haematomma puniceum, Lecanora, Lecidea. The thallus may be granular or smooth.
(2) Foliose lichens: - These occur flat, broad, much lobed and leaf like. They grow more or less free on the substratum but close to it. The foliose thallus is attached to rocks and twigs by rhizoid like outgrowths called rhizinae. The common example of foliose lichens are Xanthoria physicia, Peltigera, Parmelia.
3. Fructicose lichens: - The thallus of these lichens is cylindrical and much branched. The thallus grows either erect or hanging from rock of areas, foliage and branches of tree . They are attached to the substratum by basal disc only, e.g., Usnea, Cladonia, Ramalina etc.
