Q.3. Write short note on the following -
(a) Growth forms of lichens (2006)
(b) Economic importance of lichens (2007, 14, 16)
(c) Homiomerous lichens or heteromerous lichens (2008, 11, 12)
(d) Phycobiont of lichens (2007)
(e) Soredia (2008, 16)
(f) Isidia (2005, 12)
Ans. (a) Growth Forms of Lichens: -
· Crustose Lichens having the form of a crust, as in the crust on a bread. Many marine algae grow closely appressed to a surface, and are referred to as crustose algae. Many lichens also grow close to the surface of rocks, tree trunks and other substrata, and are referred to as crustose lichens. Coralline algae are often crustose in habit. Crustose are often times found on the ground, on buildings, ships, plants, the ground. Crustose Lichens are the base of all life and begin growing after organic life is destroyed, such as, after a volcanoe, or earthquake.
· Leprose lichens are of the simplest form. The fungal and algal components grow as powdery patches. (It is difficult to produce a good photograph of this type.)
· Foliose lichens the structure of foliose lichen shows a sandwich of fungal layer with algal mat in the middle. Small rootlets called rhizines attach the lichen to the substrate. The top and bottom layers different.
· Fructicose lichens a scalelike broken base produces a number of erect hallow standards that terminate into bright red fruiting bodies. It occurs on the ground or on dead wood. It has a common name of “British Soldiers” referring possibly to the red “uniform”. Algal host is Pleurococcus.
(b) Economic Importance of Lichens: -
(1) Pioneers of Vegetation: -
Lichens are well known as pioneers on bare rock surfaces. Many lichens disintegrate rocks into soil particles by secreting acids. As the rock becomes broken up into soil particles thus initiating the substratum in which mosses and eventually higher plants become established.
(2) As Food: -
Numerous small cratures eg.mites,slugs,snails,caterpillars and termites are known to feed upon lichens like Aspicilla calcarea.Some lichens eg,Cladonia rangiferina the well known reindeer moss is an important source of food for reindeer ,caribou and other animals of the arctic and subarctic regions.Their plentiful storage of a starch like carbohydrate also makes some of them useful for human food.though most are highly unpalatable. In Iceland the cetaria islandica, the well known Iceland moss is used as a source of food for sheep and cattle as well as for human beings.For use as food the lichen acids,dried and then powdered. The powder when boiled in water yields a jelly which is eaten with milk.Parmelia, perlata has been used as curry by the natives in the Bellary district of Madras.
(3) Medicinal Use: -
Several species of lichens were of use in earlier times in the treatment of various diseases, but today they have been replaced by more effective drugs. Lobaria pulmonaria was used for the treatment of lung diseases. Xanthoria parietina was used in cases of jaundice, whereas Peltigera canina, the dog lichen was used in hydrophobia.Parmelia saxatilis was considered an excellent drug for epilepsy. Cladonia pyxidata was used in whooping cough. Several lichens contain usnic acid and other lichen acids which have been shown to have an antibacterial properties.
(4) Use In perfumery: -
Several species are used in the preparation of dhup, hawan samgris and other perfumes.Evernia prunastri yields an excellent perfume.
(5) In thePreparation of Dyes: -
The use of certain lichens as sources of dyes of various colours, has diminished with the chemical advances of recent years. But some dyes such as litmus are still widely obtained from them. Rocella, Parmelia, Evernia and Ochrolechia are some dye yielding lichens.
(6) In Tanning: -
Some lichens contain tannins and are used for tanning animal hides in France and other European countries.
(c) Homiomerous Lichens: -
A vertical section of foliose lichens shows differentiation into the upper cortex, algal layer, medulla and lower cortex. Below the upper cortex lies the algal layer which is also known as gonidial layer. It consists of a tangled network of loosely interwoven fungal hyphae intermingled with algae. On the basis of the distribution of algal cells and the nature of algal layer in the thallus the lichens have been classified into two main categoriesnamely homiomerous lichens and heteromerous lichens. In the homiomerous lichens the algal cells are scattered uniformly in the loosely interwoven mass of funagal hyphae. In the heteromerous thalli the algal cells are restricted to a particular layer.
(d) Phycobiont of Lichens: -
Lichen is the association of fungi and algae. The fungal member is known as mycobiont and the algal member is known as phycobiont. The phycobiont mainly occurs in the upper part of the thallus and are intermingled with fungus hyphae. In the formation of lichens the fungus usually forms a tough often leathery investment with the algal cells being interspersed most typically forming a layer near the upper surface. In a lichen, the fungus lives parasitically on the alga which manufactures food by virtue of the chlorophyll present in its cells while the alga enjoys the protection afforded by the funagal envelope to live in very dry conditions.
The algae in the lichens belong to one of the two groups, the cyanophyceae or to chlorophyceae.The alga may be unicellular,or filamentous. Twelve genera of blue green and twenty one genera of green alga have been described as lichen symbionts.The most common blue green alga genera are nostoc, rivularia etc and the common green alga genera involved are-Protococcus,Cladophora etc.
(e) Soredia: -
Many lichens produce special small, rounded granules or bud like outgrowths which contain both the components of the thallus. These structures are called soredia. They are common in many lichens, eg., Physcia sp, Ramalina sp, Evernia sp. Soredia are usually found in the form of greyish white or greyish green powder on the upper surface or margin of the lichen thallus. They are disseminated by wind or rain and on reaching a suitable substratum germinate to produce new tahlli.In some lichens the soredia are found within definite, pustule like bodies known as soredia.
(f) Isidia: -
These are coral like outgrowths on the upper surface or the margins of the lobes of many lichens. They contain both algae and fungal components. The internal structure consists of an external cortical layer of fungal hyphae with an internal algal layer. Their function is usually supposed to be that of increasing the photosynthetic area of the thallus. They also serve as organs of vegetative reproduction.
Q.4 Describe different forms of lichens giving illustrations and examples. (2008)
Ans. According to the form of the plant body, the lichens have been classified into three groups: -
(i) Crustose or crustaceous lichens, forming a thin crust over the rock or other material on which they grow. These lichens occur as incrustations on rocks, soils and trees. They are partly or wholly adherent to the substratum. The common examples are-Graphis, Lecanora, Lecidea and Hematomma. The crustose thallus may be granular or smooth and if thick may become divided into small, marked off, angular pieces or elongate lobes.
(ii) Foliose or foliaceous lichens with leaf like thalli being more or less prostrate and flat resembling the thalli. These lichens occur as flat and leaf like lobed thalli, attached to the rocks and twigs by means of rhizoid like outgrowths called rhizinae. These rhizinae develop from the lower surface of the thalli. The common representatives are Physcia, Parmelia, Peltigera and Collema etc. It is one of the most conspicuous type of growth of lichens. Sometimes this type of lichen forms smaller lobes as in Physcia or Parmelia and sometimes forming hugeplates attached to the centre eg,Umbilicaria.
(iii) Fructicose lichens - Fructicose lichens have much branched, cylindrical or ribbon like thallus which is either upright (Evernia, Cladonia) or pendulous (Usnea). These are attached to the substratum by their basal portions composed of strands of densely packed hyphae. The thalli show no differentiation into an upper and lower surface. The common examples are Usnea, Cladonia and Ramalina etc.
