chapter 16

Overview of the Fungi Kingdom: Ecology and Classification

• Definition and General Characteristics of Fungi

  • Fungi are eukaryotic organisms.

  • Cellularity: Most are multicellular; however, some species are unicellular.

  • Cell Wall Composition: Fungal cell walls are composed of chitin.

  • Nutritional Strategy: Fungi are heterotrophs that acquire nutrients through absorption.

  • Phylogenetic Position: Fungi are more closely related to animals than to plants. They are classified within the Unikonta group, specifically under the Opisthokonts, along with animals, choanoflagellates, and nucleariids.

• Ecological Roles of Fungi

  • Principal Decomposers: Fungi are the primary decomposers of the biosphere.

  • Mechanism: They possess powerful enzymes capable of breaking down organic matter, allowing them to absorb nutrients directly.

  • Nutrient Recycling: Their decomposition activities recycle essential elements such as Carbon, nitrogenous compounds, and phosphates back into the environment.

  • Trophic Levels in Food Webs:

    • Level I: Primary producers (e.g., Oak tree, acorns, Pine tree).

    • Level II: Primary consumers (e.g., Pine borer, Mouse).

    • Level III: Secondary/Tertiary consumers (e.g., Red-tailed hawk, Golden-crowned kinglet, Snake, Salamander).

    • Decomposers: Bacteria and Fungi.

• Classification of Major Fungal Groups

  • Chytridiomycota (Chytrids): Characterized by swimming spores.

  • Zygomycota (Zygomycetes): Characterized by the production of zygospores.

  • Glomeromycota (Glomeromycetes): Known for producing large asexual spores and forming arbuscular mycorrhizae.

  • Ascomycota (Ascomycetes): Also known as sac fungi; spores are Produced in sacs (asci).

  • Basidiomycota (Basidiomycetes): Also known as club fungi; spores are produced on club-shaped structures (basidia).

Biological Structure and Specialized Mechanisms of Fungi

• Fungal Anatomy and Morphological Structures

  • Hyphae: Tubular filaments that allow for rapid growth.

    • (a) Septate Hypha: Contains a septum with pores that allow nuclei and other organelles to move between cells.

    • (b) Coenocytic Hypha: Lacks septa; consists of a continuous cytoplasmic mass with multiple nuclei.

  • Mycelium: An interwoven mass of hyphae that constitutes the vegetative growth and performs feeding functions.

  • Fruiting Bodies: Structures consisting of tightly packed hyphae specialized for reproduction (e.g., the visible mushroom structure).

  • Scale of Structures: Many fungal structures are microscopic; for example, spore-producing structures may be roughly $20\,\mu m$ in size.

• Predaceous Fungi

  • Exemple: Arthrobotrys anchonia.

  • Mechanism: These fungi adapt their hyphae to produce loops that trap, kill, and digest prey such as nematodes. The loops are approximately $25\,\mu m$ in diameter.

• Specific Fungal Species and Case Studies

  • Armillaria solidipes (Honey Fungus): Noted as a significant fungal organism.

  • Amanita muscaria (Fly Agaric): A well-known fruiting body example with distinct white spots.

  • Saccharomyces sp.: Yeasts used in brewing and baking.

  • Penicillium sp.: Fungi used in cheese production and medicine.

Ecological Symbiosis and Mutualistic Relationships

• Mycorrhizae

  • This is a symbiotic relationship between fungi and the roots of vascular plants.

  • Benefit to Plant: The fungal hyphae significantly increase the surface area for the absorption of water and minerals.

  • Benefit to Fungus: The fungus gains access to sugars (food) produced by the plant through photosynthesis.

• Lichens

  • A mutualistic partnership between a fungus and a photosynthesizer (either an alga or a cyanobacterium).

  • Structure: The fungus provides the physical structure, captures minerals, and maintains hydration.

  • Nutrition: The alga or cyanobacterium provides food (carbon compounds) for both organisms through photosynthesis.

  • Ecological Importance: Lichens are widely distributed and act as excellent colonizers. However, they are highly sensitive to airborne pollutants because they derive most of their minerals from dust and the air.

  • Morphological Forms: Lichens can be Fruticose (shrub-like), Crustose (encrusting), or Foliose (leaf-like).

Detailed Classification and Impact: Zygomycetes and Glomeromycetes

• Zygomycetes (Zygote Fungi)

  • Characteristics: Multicellular decomposers or parasites. They include fast-growing molds.

  • Examples:

    • Rhizopus stolonifer (Black bread mold).

    • Species causing fruit rot in apples.

    • Rhizopus species are also used industrially for making tempeh, sake, meat tenderizer, margarine coloring, and industrial alcohol.

  • Structure: Includes the sporangium with spores, sporangiophores, and zygosporangia.

  • Pilobolus: A unique zygomycete that uses grazing animals for seed dispersal.

• Glomeromycetes (Arbuscular Mycorrhizal Fungi - AMF)

  • Ecological Importance: All known species exist in symbiotic associations with plants. They cannot grow independently of their host plant.

  • Arbuscules: Specialized hyphae used for exchange within plant root cells (observed at $50\,\mu m$ scale).

Detailed Classification and Impact: Ascomycetes and Basidiomycetes

• Ascomycetes (Sac Fungi)

  • Unicellular Examples: Yeasts used for fermentation (Saccharomyces) or those that can be parasitic (Candida). Reproduction occurs via budding.

  • Multicellular Decomposers: Include morels, truffles, and cup fungi.

  • Multicellular Parasites and Pathogens: Include black mold (Stachybotrys chartarum), green mold, Penicillium, powdery mildew, and Pneumocystis jirovecii.

  • Noteworthy Species:

    • Aspergillus: Produces aflatoxin, recognized as one of the most carcinogenic substances known.

    • Chestnut Blight: A fungus that decimated mature American chestnut trees.

    • Ergot: Linked to "St. Anthony's Fire," the Salem witch trials, and possibly influenced cultural elements like the Beatles.

• Basidiomycetes (Club Fungi)

  • General Features: Multicellular organisms with a basidium spore-producing structure. They produce sexual spores called basidiospores.

  • Ecological Role: They play a central role in the decomposition of plant litter.

  • Examples: Mushrooms, toadstools, stinkhorns (Phallus impudicus), puffballs, and shelf fungi.

  • Hallucinogens: Species such as Psilocybe.

  • Agricultural Impact: Many are parasites, such as rusts and smuts (e.g., Wheat rust, Corn smut), which cause billions of dollars in annual crop losses.

Introduction to the Plant Kingdom and Land Adaptations

• What is a Plant?

  • Autotrophic eukaryotes that are typically attached to a substrate.

  • Cell Wall: Composed of cellulose.

  • Storage: Use starch as a primary storage compound.

  • Growth: Characterized by indefinite growth.

• Evolution from Water to Land

  • Plants evolved from an ancestral green alga (likely related to modern Charophytes).

  • Evolutionary Timeline:

    • Origin of Land Plants: $475\,mya$.

    • Origin of Vascular Plants: $425\,mya$.

    • Origin of Seed Plants: $360\,mya$.

• Specific Land Adaptations

  • Cuticle: A waxy layer that reduces water loss.

  • Stomata: Pores that allow for gas exchange while regulating water loss.

  • Root System: Anchors the plant and absorbs water and minerals using vascular tissue (xylem).

  • Shoot System: Includes stems for support, leaves for photosynthesis, and flowers for reproduction. Uses vascular tissue (phloem) to transport sugars.

  • Lignin: Polysaccharide that hardens cell walls for structural support.

  • Dispersal: Use of spores or pollen to reproduce on land without requiring external water for sperm to swim.

Plant Anatomy: Systems and Organs

• The Shoot System

  • Consists of reproductive shoots (flowers), apical buds (sites of growth), nodes, internodes, and axillary buds.

  • Leaves: Comprise a blade and a petiole. Specialized leaf types include:

    • Tendrils: For climbing.

    • Spines: For protection (e.g., cacti).

    • Storage leaves: For nutrients/water (e.g., onion).

    • Reproductive leaves: For producing plantlets.

    • Bracts: Leaf-like structures, often colored (e.g., poinsettia).

• The Root System

  • Main types: Taproots and lateral branch roots.

  • Specialized Roots:

    • Storage Roots: (e.g., carrots).

    • Prop Roots: Support tall, top-heavy plants.

    • Pneumatophores: "Air roots" for obtaining oxygen in wet environments.

    • Buttress Roots: Support for large rainforest trees.

    • "Strangling" Aerial Roots: (e.g., strangler figs).

• Vascular Tissue

  • Xylem: Conducts water and dissolved minerals upward from the roots to the shoots.

  • Phloem: Transports sugars and organic nutrients from leaves to the rest of the plant.

Plant Life Cycles: Alternation of Generations

• Process of Alternation

  • Plants alternate between a multicellular diploid stage and a multicellular haploid stage.

  • Sporophyte Phase ($2n$): The multicellular diploid plant. It produces haploid spores through the process of meiosis.

  • Gametophyte Phase ($n$): The multicellular haploid plant. It produces haploid gametes (sperm and egg) through the process of mitosis.

  • Cycle Steps:

    1. Sporophyte ($2n$) undergoes Meiosis.

    2. Spores ($n$) are produced.

    3. Spores undergo Mitosis to become the Gametophyte ($n$).

    4. Gametophyte produces Gametes ($n$) via Mitosis.

    5. Fertilization of gametes creates a Zygote ($2n$).

    6. Zygote undergoes Mitosis to grow back into a Sporophyte ($2n$).

Plant Classification and Diversity

• Bryophytes (Nonvascular Plants)

  • Examples: Mosses, liverworts, hornworts.

  • Key Features: No vascular tissue; primitive roots (no support); require a moist environment for flagellated sperm to reach the egg.

  • Dominance: Gametophyte phase is dominant over the sporophyte stage.

  • Human Interest: Peat (harvested for fuel) and "Tollund Man" (a bog mummy preserved in moss).

• Pterophytes (Seedless Vascular Plants)

  • Examples: Ferns, whisk ferns (Psilotum nudum), horsetails (Equisetum).

  • Key Features: Possess vascular tissue (xylem/phloem) and true roots. Still require a moist environment for flagellated sperm.

  • Dominance: Sporophyte phase is dominant over the gametophyte stage.

  • Dispersal: Via spores. Sori are clusters of sporangia on the underside of fern leaves.

• Gymnosperms ("Naked Seed")

  • Definition: A seed is a dormant plant embryo packaged with a food supply (the "lunchbox") and a protective seed coat.

  • Examples: Pines, blue spruce, redwoods, cycads, ginkgos, lone cypress, bristlecone pine.

  • Key Features: Vascular tissue, lignin, and roots are present. Fertilization does not require water; dispersal occurs via pollen and cones.

  • Seeds: The ovule ripens into a seed; they have no fruit coating.

• Angiosperms (Flowering Plants)

  • Examples: Most trees, grasses, flowers, and food-producing plants like tomatoes and grains.

  • Key Features: Possess flowers (specialized reproductive structures) and fruits.

  • Reproduction: Pollen grains (male gametophytes) develop in anthers. Eggs (female gametophytes) develop in ovules within the ovary.

  • Flower Anatomy:

    • Stamen: Male part (consists of Anther and Filament).

    • Carpel: Female part (consists of Stigma, Style, and Ovary).

    • Supporting parts: Petal, Sepal, Receptacle, Ovule.

  • Fruits: A mature ovary that aids in the protection and dispersal of seeds.

  • Dispersal Methods: Wind (e.g., dandelion, maple), water (e.g., coconut, lotus), animals (e.g., burrs, berries), bursting/mechanical (e.g., Jewelweed, witch hazel), and human intervention (e.g., wheat, beans).

Seed Dispersal Catalog

• By Wind: Milkweed, Cattail, Dandelion, Maple.

• By Animals: Beggar-ticks, Sandbur, Blackberry, Cherry.

• By Water: Lotus, Coconut.

• By Bursting: Violet, Jewelweed, Witch Hazel, Bean.

• By Human Action: Wheat, Bean, Cherry.

• Specific Examples: Tomatoes, Ruby grapefruit, Nectarines, Hazelnuts, Milkweed, and the Red silky oak tree.