(a) Diagrammatically present the Geological Time Scale up to 'Epoch'. Write a note on the different types of fossils found in…

(a) Geological time scale up to epoch, and types of fossils

The geological time scale is a hierarchy of eon, era, period and epoch. Approximate ages are in million years ago (Ma).

• Hadean, Archean and Proterozoic eons (together Precambrian, about 4600–539 Ma).
• Phanerozoic eon (539 Ma–present), with three eras:
• Paleozoic (539–252 Ma): Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian.
• Mesozoic (252–66 Ma): Triassic, Jurassic, Cretaceous.
• Cenozoic (66 Ma–present): Paleogene (Paleocene, Eocene, Oligocene epochs), Neogene (Miocene, Pliocene) and Quaternary (Pleistocene, Holocene).

Types of fossils:

1. Petrifaction (permineralisation): cell spaces filled with silica or calcium carbonate, preserving internal anatomy, e.g. Rajmahal Hills plant fossils, Deccan intertrappean plants.
2. Compression: flattened organic film of a plant part between rock layers, e.g. Glossopteris leaves.
3. Impression: only the outline or surface markings remain in rock, without organic matter.
4. Mould and cast: a hollow left in rock by a decayed organism (mould) and its later infilling (cast).
5. Coal balls and amber: plant tissues preserved in carbonate nodules; insects and plant parts trapped in resin.
6. Trace fossils and coprolites: footprints, burrows and fossil dung.

(b) Red rot of sugarcane

Causal organism: Colletotrichum falcatum (asexual stage); sexual stage Glomerella tucumanensis.

Symptoms: Leaf midribs show red lesions with dark margins; upper leaves dry from the crown downwards. On splitting, the stalk shows red internal tissue crossed by white transverse patches, with a sour, alcoholic smell, and the cane becomes hollow and shrivelled.

Disease cycle: The fungus survives in diseased setts and crop debris. Planting infected setts gives the primary infection. Conidia from acervuli are spread by rain splash, irrigation water and wind, and enter through leaf midribs, nodes, root primordia and wounds. Infection is most severe in warm, humid weather, and it spreads again by ratoon crops and infected seed cane.

Control:

• Plant disease-free setts from healthy nurseries; avoid ratooning infected fields.
• Treat setts with hot water or moist hot air, or dip them in a fungicide such as carbendazim.
• Grow resistant varieties, renewed as new pathotypes overcome resistance.
• Rotate crops, rogue infected clumps, and remove debris.

(c) Stele and types of steles in pteridophytes

The stele is the central vascular cylinder, made of xylem and phloem with pericycle and pith if present, enclosed by the endodermis.

Protostele (no pith):

• Haplostele: solid xylem core surrounded by phloem, e.g. Selaginella.
• Actinostele: star-shaped xylem core, e.g. Psilotum, Lycopodium serratum.
• Plectostele: xylem plates alternating with phloem, e.g. Lycopodium clavatum.

Siphonostele (pith present):

• Ectophloic: phloem outside xylem only, e.g. Osmunda, Equisetum.
• Amphiphloic, solenostele: phloem on both sides of a continuous xylem ring, e.g. Marsilea, Adiantum pedatum.
• Dictyostele: ring broken by leaf gaps into separate meristeles, e.g. Dryopteris, Pteridium (polycyclic).

Evolution: Jeffrey's theory holds that the protostele is primitive, and the siphonostele arose from it; the dictyostele is the most advanced.