Definition of Fungi:
Fungi are eukaryotic, spore-bearing, achlorophyllous organisms with absorptive nutrition. They reproduce sexually and/or asexually and have a body (thallus) made up of thread-like structures called hyphae, collectively forming a mycelium. Their cell walls are primarily composed of chitin and glucans.

General Characteristics of Fungi:

1. Eukaryotic:
   • Fungi have a true nucleus surrounded by a nuclear membrane and other membrane-bound organelles like mitochondria, Golgi bodies, and endoplasmic reticulum.
2. Achlorophyllous:
   • Fungi lack chlorophyll and cannot photosynthesize, relying on external sources of organic carbon.
3. Heterotrophic Nutrition:
   • They obtain nutrients through absorption. Nutrients are broken down externally by secreting digestive enzymes.
     Types of nutrition include:
     • Saprophytic: Decomposers of dead organic matter (e.g., Rhizopus).
     • Parasitic: Live on living organisms, often causing disease (e.g., Puccinia in wheat rust).
     • Symbiotic: Form mutualistic relationships (e.g., mycorrhiza, lichens).
4. Spores: Fungi produce spores for reproduction, which can be asexual (conidia, sporangiospores) or sexual (ascospores, basidiospores).
5. Cell Wall Composition: The cell wall contains chitin, a strong polysaccharide, and sometimes glucans or mannans, giving rigidity and protection.
6. Reproductive Strategies: Asexual reproduction occurs by spores, budding, or fragmentation. Sexual reproduction involves the fusion of compatible nuclei followed by meiosis.
7. Life Cycle and Phases: Fungi can exhibit a haploid, dikaryotic (n+n), or diploid phase during their life cycle.
8. Growth Conditions: They thrive in moist, warm environments but can also survive extreme conditions (e.g., sclerotia formation).
9. Role in Ecosystem: Decomposers recycle nutrients, pathogens regulate population dynamics, and symbionts contribute to ecosystem balance.

Somatic Structures of Fungi

The somatic (vegetative) body of fungi is primarily composed of mycelium and hyphae, with variations in structure and complexity.

1. Hyphae:

• Hyphae are the basic structural units of the fungal body. Types of hyphae:
• Septate Hyphae: Hyphae divided by cross-walls (septa), each containing pores for cytoplasmic continuity (e.g., Ascomycota and Basidiomycota).
• Aseptate (Coenocytic) Hyphae: Hyphae without septa, forming a multinucleate structure (e.g., Zygomycota).
• Modified Hyphae:
  • Rhizoids: Root-like structures for anchorage and absorption.
  • Appressoria: Swollen structures aiding host penetration.
  • Haustoria: Specialized structures for nutrient absorption from hosts.

2. Mycelium:

• A network of hyphae forms the mycelium, the vegetative part of the fungus.
• Types based on function:
  • Vegetative Mycelium: Absorbs nutrients.
  • Reproductive Mycelium: Produces fruiting bodies or spores.

Types of Fungal Thalli

Fungi exhibit diverse thallus types depending on their life stage, growth habits, and environmental adaptations.

1. Unicellular Thallus: Consists of single cells (e.g., yeast like Saccharomyces cerevisiae).
2. Filamentous Thallus: Composed of long, thread-like hyphae that form a mycelium (e.g., Rhizopus).
3. Plasmodial Thallus: A naked, multinucleate mass of protoplasm without a cell wall (e.g., Myxomycetes or slime molds).
4. Dimorphic Thallus: Exhibits both unicellular (yeast-like) and filamentous (hyphal) forms depending on environmental conditions (e.g., Candida albicans, Histoplasma).

Fungal Tissues Fungal tissues are aggregations of hyphae forming structures similar to plant tissues but without differentiation.

Plectenchyma:

• • Compact tissue-like aggregation of hyphae:
    • Prosenchyma: Loosely packed, parallel hyphae.
    • Pseudoparenchyma: Compactly arranged, resembling parenchyma

Specialized Structures:

• • Sclerotium: Hard, compact mass of hyphae that stores food and helps fungi survive adverse conditions.
  • Rhizomorphs: Root-like structures for water and nutrient transport.
  • Stroma: Dense, cushion-like structure containing reproductive bodies.

Modifications of the Thallus

Fungal thalli undergo modifications to adapt to their environment or perform specialized functions.

1. Vegetative Modifications:

• Rhizoids: Root-like hyphae for anchorage and nutrient absorption.
• Stolons: Horizontal hyphae that spread the fungus (e.g., Rhizopus).
• Sclerotia: Survival structures formed under stress (e.g., Claviceps purpurea).
• Rhizomorphs: Specialized structures for survival and infection.

2. Reproductive Modifications:

• Formation of specialized reproductive structures, such as:
  • Conidiophores: Bear conidia (asexual spores).
  • Sporangia: Enclose sporangiospores.
  • Fruiting Bodies: Complex structures that produce sexual spores (e.g., basidiocarps, ascocarps).

3. Specialized Hyphae:

• Appressoria: Flattened hyphal tips used to penetrate host surfaces.
• Haustoria: Intracellular hyphae that absorb nutrients from the host.
• Arbuscules: Tree-like structures in symbiotic fungi (e.g., arbuscular mycorrhizal fungi).