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

Lichen: Concept, Types, Structure, and Economic Importance

A lichen is a stable, self-supporting mutualistic symbiosis between a fungal partner, termed the mycobiont (mostly Ascomycetes), and a photosynthetic partner, termed the photobiont (green algae or cyanobacteria). Unlike casual physical aggregations, the two components form a distinct morphological and physiological entity where the fungus provides anchorage, moisture, and minerals, while the photobiont synthesises carbohydrates.

Morphologically, lichens are classified into three major types:

1. Crustose lichens: Form a flat, crust-like thallus tightly adpressed to the substratum without a lower cortex, such as Graphis and Lecanora.
2. Foliose lichens: Feature a flat, dorsiventral, leaf-like lobed thallus attached via rhizines, exemplified by Parmelia and Peltigera.
3. Fruticose lichens: Exhibit a shrubby, cylindrical, or strap-shaped, branched, erect or pendulous thallus, such as Usnea, Cladonia, and Ramalina.

`` [DIAGRAM: Heteromerous Foliose Thallus Structure] _______________________________ <-- Upper Cortex (dense fungal hyphae) | o o o o o o o | <-- Algal Layer / Photobiont zone | ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ | | ~ ~ ~ ~ ~ ~ ~ | <-- Medulla (loosely interwoven hyphae) |_______________________________| <-- Lower Cortex (compact hyphae) | | | <-- Rhizines (anchoring structures) ``

Internally, a heteromerous thallus displays four distinct zones: an upper cortex of compact, pseudoparenchymatous fungal hyphae offering protection; an algal layer containing active photobiont cells; a central medulla composed of loosely interwoven, air-spaced hyphae serving as a storage zone; and a lower cortex giving rise to rhizines that anchor the thallus to the substrate.

Lichens possess immense ecological and economic value. They act as pioneer organisms in lithosere succession by secreting organic acids that initiate soil formation through pedogenesis. Being highly sensitive to atmospheric sulphur dioxide (SO₂), they serve as biological indicators of air pollution, actively monitored by institutions like CSIR-National Botanical Research Institute (NBRI), Lucknow. Industrially, Roccella tinctoria yields the acid-base indicator litmus; Evernia prunastri and Pseudevernia are used as fixatives in perfumery; and Cladonia rangiferina (reindeer moss) serves as vital fodder in arctic tundras. Medicinally, Usnea species yield usnic acid, a potent broad-spectrum antibiotic.

Modes of Sexual Reproduction in Bacteria

Bacteria lack true eukaryotic syngamy and meiosis; instead, they undergo parasexual genetic recombination via three distinct processes:

Conjugation involves the direct, unidirectional transfer of genetic material from a donor (F⁺ or male) to a recipient (F⁻ or female) cell through a specialised sex pilus. Plasmids such as the fertility factor (F-factor) can replicate and transfer autonomously. When the F-factor integrates into the circular bacterial chromosome, it forms a High-Frequency Recombination (Hfr) strain, facilitating chromosomal transfer. Aberrant excision of the integrated factor produces an F' plasmid containing specific host genes.

Transformation involves the active uptake and integration of naked, extracellular DNA fragments from the surrounding medium by a competent recipient cell. First demonstrated by Frederick Griffith in Streptococcus pneumoniae and molecularly established by Avery, MacLeod, and McCarty, this process incorporates foreign genetic sequences into the recipient genome via homologous recombination.

Transduction is the bacteriophage-mediated transfer of bacterial DNA from one cell to another. In generalized transduction, mediated by lytic phages (e.g., P22 in Salmonella), random fragments of degraded bacterial host DNA are mistakenly packaged into the viral capsid and transferred. In specialized transduction, temperate phages (e.g., Lambda phage in Escherichia coli) integrate into a specific chromosomal locus as a prophage; imprecise excision during induction carries flanking, specific host genes to the recipient cell.

Progymnosperms: Characterisation and Phylogenetic Significance

Progymnosperms represent an extinct group of Mid-Devonian to Early Carboniferous vascular plants that bridge the structural gap between free-sporing pteridophytes and seed plants.

Morphologically and anatomically, progymnosperms are characterised by an arborescent habit exhibiting extensive secondary growth. They possessed a bifacial vascular cambium producing gymnospermous secondary xylem (pycnoxylic in Archaeopteris and manoxylic in Aneurophyton) with bordered pitted tracheids and xylem rays. Their foliage varied from simple, branching axes in the Aneurophytales to broad, fern-like photosynthetic fronds (sphenopterids and archaeopterids) in the Archaeopteridales. Despite advanced, gymnospermous vegetative anatomy, they reproduced exclusively via free-shedding sporangia lacking seeds, exhibiting both homospory (Protopteridium) and advanced heterospory (Archaeopteris).

Phylogenetically, progymnosperms are considered the direct evolutionary ancestors of gymnosperms. They resolve the macroevolutionary enigma of the "seed habit" by demonstrating that modern eustelic organization and bifacial secondary wood evolved prior to the origin of the seed. Furthermore, the transition from homospory to heterospory in taxa like Archaeopteris established the precursor developmental state required for the evolution of the integumented megasporangium (ovule), solidifying progymnosperms as the evolutionary link between trimerophyte pteridophytes and early pteridosperms (seed ferns).