(a) Describe the structure and functions of cyphellae and cephalodia in lichens. (b) What are prions? How are these different…

(a) Cyphellae and Cephalodia in Lichens Cyphellae are small, round, cup-like pores in the lower cortex of foliose lichens such as Sticta. Each has a raised cortical rim around an opening where the cortex is broken, exposing the loose medullary hyphae and algal cells. They differ from pseudocyphellae, which are mere breaks in the cortex without a rim, and from soredia, which are propagules. Function: aeration, gas exchange (CO₂ intake, O₂ release) between the medulla and air, like stomata or lenticels.

Cephalodia are small gall-like or wart-like outgrowths, internal (in the medulla) or external (on the upper or lower surface), found in lichens whose main photobiont is a green alga (Coccomyxa), such as Peltigera aphthosa, Stereocaulon and Lobaria pulmonaria. They are colonies of cyanobacteria (Nostoc, Scytonema) enclosed by fungal hyphae. Function: the cyanobacterial heterocysts fix atmospheric nitrogen, supplying fixed nitrogen to the thallus, while the green alga carries out most photosynthesis.

(b) Prions, Their Difference from Viroids, and Transmission Prions are infectious proteins with no detectable nucleic acid. A prion is an abnormally folded, β-sheet-rich form (PrPˢᶜ) of a normal host protein (PrPᶜ, encoded by the PRNP gene). PrPˢᶜ converts PrPᶜ into more PrPˢᶜ by templating, forming protease-resistant amyloid aggregates that cause transmissible spongiform encephalopathies: scrapie, BSE, kuru and Creutzfeldt-Jakob disease (CJD).

Difference from viroids: viroids are naked, small, circular, single-stranded RNA molecules, with no protein coat and no protein coded, and they infect plants (potato spindle tuber viroid). They replicate through host RNA polymerase. Prions are protein-only, infect mammals (yeast prions are non-infectious), and propagate by conformational conversion without nucleic acid replication.

Transmission: eating infected tissue (kuru from ritual cannibalism; variant CJD from BSE-infected beef); iatrogenic spread through contaminated surgical instruments, dura mater or corneal grafts and cadaver-derived growth hormone; inherited PRNP mutations; and sporadic spontaneous misfolding. Prions resist heat, formalin and UV.

**(c) Sporangiophore and Sporangium Dehiscence in *Rhizopus*** The sporangiophores are erect, unbranched, aseptate hyphae that arise in tufts of 3-5 from the nodes of the horizontal stolon, opposite the tufts of rhizoids. The tip swells into a globose sporangium, separated from the stalk by a septum that bulges into the sporangium as a dome-shaped, sterile columella, with a small apophysis below. The sporangium contains many multinucleate sporangiospores around the columella, and is covered by a thin wall encrusted with calcium oxalate crystals.

Dehiscence: as the sporangium matures it dries, and the brittle wall breaks into pieces, leaving a collar at the base of the columella. The columella collapses like an umbrella. The dry, powdery spores are exposed and dispersed by wind (unlike Mucor, whose wall deliquesces and spores are released in a slimy mass).

(d) Merits and Demerits of Numerical Taxonomy Numerical taxonomy (Sokal and Sneath, based on Adansonian principles) classifies Operational Taxonomic Units (OTUs) by overall phenetic similarity, calculated from many equally weighted characters.

Merits: it is objective, quantitative and repeatable, with less personal bias; it handles many characters, including anatomy, chemistry and molecular data, by computers (cluster analysis, ordination); it shows relationships in phenograms; and new data can be added to refine groups.

Demerits: equal weighting of all characters ignores their differing taxonomic and adaptive value; similarity from convergence or parallelism misleads, as it ignores phylogeny, so phenetic groups may not be natural; choice and coding of characters and OTUs remains subjective; results vary with the coefficient and clustering method; and it needs large data sets and computing.

(e) Distribution of Gymnosperms in India India has about 70 native gymnosperm species, mostly in the Himalaya, with fewer in the Peninsula and Northeast.

Western Himalaya: Pinus roxburghii (chir) in sub-montane zones; Cedrus deodara, Pinus wallichiana, Picea smithiana and Abies pindrow in temperate zones; Taxus wallichiana (yew, source of taxol) and Cupressus; and Juniperus and Ephedra gerardiana in dry alpine and cold-desert tracts.

Eastern Himalaya and North-East: Abies densa, Larix griffithii (a deciduous conifer), Tsuga dumosa, Picea spinulosa, Podocarpus and Cephalotaxus griffithii; Gnetum in moist forests of Assam and Arunachal.

Peninsular India: Cycas circinalis and Gnetum ula in the Western Ghats, Nageia wallichiana in the Western Ghats, and Cycas beddomei, endemic to the Eastern Ghats of Andhra Pradesh and endangered. Ginkgo is only cultivated.

Several species are threatened by overharvesting and habitat loss.