(a) What is lichen? With suitable diagrams, describe its types and structure. Write a note on the economic importance of lichens…

(a) Lichen: types, structure and economic importance

A lichen is a stable, self-supporting symbiotic association of a fungus (mycobiont, mostly an ascomycete) and an alga or cyanobacterium (photobiont). The fungus gives anchorage, protection and water and minerals, and the photobiont gives carbohydrate (and nitrogen if cyanobacterial).

Types by thallus:

1. Crustose: flat crust firmly attached to rock or bark, with no lower cortex, e.g. Graphis, Lecanora.
2. Foliose: leaf-like, flat and lobed thallus attached by rhizines, e.g. Parmelia, Peltigera.
3. Fruticose: shrub-like, upright or hanging branched thallus, e.g. Usnea, Cladonia, Ramalina.

Structure (heteromerous foliose thallus, in section): upper cortex of compact fungal hyphae; algal layer of photobiont cells among loose hyphae; medulla of loosely interwoven hyphae; lower cortex from which rhizines arise. In a homoiomerous thallus (e.g. Collema), algal cells are scattered through the thallus without layers. Reproduction is by soredia, isidia, fragmentation, and the fungus forms apothecia or perithecia.

Economic importance:

• Indicators of air pollution (SO₂), as many are very sensitive.
• Dyes and litmus: Roccella yields litmus and orchil.
• Perfumery: Evernia and Pseudevernia, used as fixatives.
• Food and fodder: reindeer moss (Cladonia rangiferina) in the tundra; Parmelia in Indian spices (dagad phool) and edible Umbilicaria (rock tripe).
• Medicine: usnic acid from Usnea as an antibiotic.
• Ecology: pioneers in rock weathering and soil formation, and nitrogen fixation by cyanolichens.

(b) Three modes of sexual reproduction in bacteria

Bacteria lack meiosis and gametes; gene transfer from donor to recipient is unidirectional and partial, by three means.

Conjugation: direct transfer of DNA through a sex pilus from a donor (F⁺, carrying the fertility plasmid) to a recipient (F⁻). An F⁺ cell transfers only the F plasmid, so the recipient becomes F⁺. When the F factor integrates into the chromosome (Hfr strain), chromosomal genes are transferred, starting at the origin, and rarely the whole chromosome moves. Faulty excision gives an F′ plasmid carrying host genes.

Transformation: uptake of naked DNA from the medium by competent cells and its integration by recombination. Griffith (1928) saw it in Streptococcus pneumoniae, and Avery, MacLeod and McCarty (1944) showed that DNA is the transforming agent.

Transduction: transfer of bacterial DNA by a bacteriophage. In generalised transduction (e.g. phage P1 in E. coli, P22 in Salmonella) a piece of host DNA is mistakenly packaged into the phage head during the lytic cycle. In specialised transduction (phage λ in E. coli), faulty excision of a prophage carries adjacent genes (gal, bio) to the next host.

(c) Progymnosperms: characters, examples and phylogenetic significance

Progymnosperms are an extinct group of the Middle Devonian to Early Carboniferous, combining pteridophyte-like reproduction with gymnosperm-like wood.

Characters:

• Large woody plants (trees or shrubs) with a bifacial vascular cambium producing secondary xylem of pitted tracheids and rays, and secondary phloem.
• Stele protostelic or eustelic; leaves either absent or branching, flattened, fern-like leaves.
• Reproduction by spores in sporangia, not seeds, with homospory (Aneurophyton, Protopteridium) or heterospory (Archaeopteris).

Examples: Aneurophyton, Proteokalon (Aneurophytales), Archaeopteris (Archaeopteridales), and Protopitys and others in other orders.

Phylogenetic significance: they link the free-sporing trimerophytes and the seed plants, showing that woody growth and the eustele came before the seed, and that heterospory evolved before the ovule. The aneurophytalean line is thought to lead to seed ferns, with Archaeopteris-type plants being related to early conifers.