Q.5. Differentiate between saxicolous, corticolous and terricolous lichen. (2014)
Related Question -
Q. Explain the term corticolous. (2016)
Q. Explain the term Saxicolous. (2016)
Ans. According to their habitats, the lichens are classified into three categories: Saxicolous Corticolous and Terricolous. The saxicolous lichens are predominantly rocklivers, growing on firm substrata in cold regions. The corticolous species are generally bark-livers. They are mostly confined to the tropics and subtropics where there is plenty of moisture. Here, they grow on the leaves and bark of tree as epiphytes, depending on their hosts for anchorage only. The terricolous species are terrestrial and therefore live in the soil.
Q.6. Describe reproduction in Parmellia. (2015)
Ans. Reproduction -
(A) Fragmentation - Either as a result of the death and decay of older portions or due to accidental injuries, the thallus breaks down into many fragments. Each fragment develops into a new thallus under favourable conditions provided the fragment contains both the components.
(B) Isidia - These are coral-like outgrowths on the upper surface of the lichens. These are composed of external cortical layer followed by an algal layer of the same kind as is present in the thallus. The isidia vary in shape from rod-like (e.g., Parmelia saxatilis), coralloid (Umblicarta Pystykata), cigar-shaped (Usnea composa), tiny coral-like buds (Peligerapra etextata) or scale-like (Collema crispurn). The isidia are believed to be primarily menat for increasing the photosynthetic surface of the lichen thallus. However, sometimes these are detached from the thallus and behave as reproductive bodies.
(C) Soredia - These are small rounded separable outgrowths of the lichen thallus which occur as a greyish white powdery mass in extensive patches usually over the upper surface of edges of the thalli, The soredia are composed of algal cells clasped and surrounded by fungal hyphae. Both the fungal and the algal components are the same as in the parent thallus. These serve as a means of propagation. Soredia are formed in many lichens e.g., Parmelia, Bryoria.
(D) Phyllidia - These are leaf-like or scale-like dorsiventral portions of the entire thallus of some foliose lichens e.g., Peltigera praetextata.
Asexual Reproduction
(A) Conidia - Many lichens produce conidia in flask-shaped pycnidia immersed in the thallus. The conidia, on being dispersed, germinate under favourable conditions by sending out hyphal branches in all directions, Any of these hyphal branches, on coming in contact with the appropriate algal cell, branches further to ultimately produce a lichen thallus. However, since their coming into contact with right algal partner is a remote possibility, the conidia play negligible role in the establishment of new lichen thalli.
(B) Oidia - The hyphae of some lichens break up into small bodies called oidia (Smith, 1921), which germinate into new hyphae.
3. Sexual Reproduction - This is similar to Ascomycetes because most of the lichens are ascolichens and henced fungi body represents the ascomycetations nature. Thus fruitification like apothecia and perithecia occur in lichens as the end products of sexual reproduction. The female reproductive organ is called carpogonium it is coiled and multicellular, the male organ is a flask shaped structure and is called as spermogonium. It produces non-motile spermatia. The male nucleus fuses with female nucleus. The fusion cell produces ascogenous hyphae within which develop 8 ascopores, and asci. The hymenium, is made up of asci and paraphysis. The hymenium is made up of asci and paraphysis. The fruiting body may be either as apothecia or perithecia. Their fruiting body may be either an apothyecia or perithecia.
Q.7. Give an illustrated account of the external and internal structure of lichens. (2009)
Ans. External Structutre: -
In size lichens vary from 1mm to some which are whole meters in diameter. In colour they may be of almost every conceivable shade: some are greyish, green others are white, orange, yellow grern brown or black. According to the form of 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.
Internal Structure: -
A vertical section of foliose licen like Physcia or Parmelia shows differentiation into the following four layers:
Upper Cortex: -
The upper layer is composed of more or less compactly interwoven fungal hyphae usually without intercellualar spaces. If present the intercellular spaces are filled with gelatinous material. The upper cortex is either fibrous or cellular in appearance egPeltigera.
The upper cortex is usually 10-15u thick with several layers of cells. In some gelatinous lichens the upper cortex is either lacking or one to few cells thick. The surface of the upper cortex may or may not be externally covered by an epidermis like layer of hyphae.
Algal Layer: -
Below the upper cortex lies the algal layer also known as gonidial layer. It consist of a tangled network loosely interwoven fungal hyphae intermeingled with algae. On the basis of distribution of algal cells and the nature of algal layer in the thallus, the lichens have been divided into two categories namely-
· Homiomerous – In the homiomerous thalli the algal cells are scattered uniformly in the loosely interwoven mass of fungal hyphae and
· Hetromerous lichens - In the heteromerous lichens the algal cells are restricted to a particular layer.
Among the blue gren algae present in the algal layer of the thallus the coomon are Nostoc and Rivularia. The common green algae present in the layer are Protococcus and Cladophora.
Medulla: -
The algal layer is followed by medulla which consists of loosely interwoven hyphae.
Lower Cortex: -
Beneath the medulla is the lower cortex which consists of closely packed dark coloured hayphae running parallel or perpendicular to the surface of the thallus. From the lower cortex arise root like structures or rhizinae which attach the thallus to the substratum.
Q.8. Write short note on Cephalodia. (2016)
Ans. Cephalodia: -
They occur as dark-coloured, peculiar gall-like swellings on the upper surface of the lichen thallus, e.g, Lecanora gelida and Peltigera aphthosa, or within the thallus, e.g, Solorina saccata. They are composed of the same fungal hyphae as in the lichen thallus but contain algae of a blue-green type, different from those found in thallus (Peltigera aphthosa).
Q.9. Write short note on Rhizine. (2016)
Ans. Rhizinae: -
Foliose 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. Rhizinae are groups of the fungal hyphae which anchor most of the foliose lichens to the substratum of rock, bark or soil. They do not appear to play an important role in transfer of water and mineral to the lichen.