The Evolution of Plants and Fungi

Overview of Plant and Fungal Evolution

The Scope of Study: The study of terrestrial life evolution encompasses several key groups and mechanisms: * The evolution of plants and fungi. * Terrestrial adaptations specifically developed by plants for land survival. * The biological phenomenon of Alternation of Generations. * Clades of plants: Bryophytes, Seedless Vascular Plants, Gymnosperms, and Angiosperms. * The biology and ecology of Fungi.

Characteristics and Adaptations of Plants

General Features: * Cellularity: Plants are eukaryotic, multicellular organisms. * Embryophytes: Defined by the formation of a multicelled embryo that remains on and is nourished by the parental body. * Metabolism: They are photoautotrophs, utilizing photosynthesis to convert light energy into chemical energy. * Reproduction: Plants utilize both sexual and asexual (vegetative) modes of reproduction.
Transition to Land: * Timeline: Adaptations for life on dry land have accumulated over approximately $475$ million years, based on the oldest known plant fossils. * Key Terrestrial Adaptations: * Structural Support: Mechanisms to support the body against gravity in the absence of water's buoyancy. * Nutrient Acquisition: Specialized structures for obtaining nutrients from soil and air. * Reproduction and Dispersal: Strategies to facilitate fertilization and the spread of offspring without a constant aquatic medium.

Alternation of Generations *1N Gametophyte (Haploid) | Sporophyte 2N

Definition: A life cycle characteristic of all plants involving two distinct multicelled phases.
Diploid ( $2n$ ) Phase: * Begins with a Zygote produced via fertilization. * The zygote undergoes Mitosis to become a multicelled Sporophyte. * The sporophyte produces Spores through the process of Meiosis.
Haploid ( $n$ ) Phase: * Spores ( $n$ ) undergo Mitosis to develop into a multicelled Gametophyte. * The gametophyte produce Gametes (sperm and eggs) via Mitosis. * Fertilization of gametes restores the diploid zygote state.

Major Plant Divisions

Nonvascular Plants (Bryophytes): * Anatomy: Lack xylem and phloem (vascular tissues). * Support: No lignin in cell walls; generally low-growing. * Dominance: The Gametophyte is the predominant generation. * Fertilization: Requires water for sperm to swim to the egg. * Examples: Mosses, liverworts, and hornworts.
Seedless Vascular Plants: * Anatomy: Possess vascular tissue (xylem/phloem), allowing upright growth. * Dominance: The Sporophyte is the predominant generation. * Fertilization: Still requires external water. * Examples: Ferns, club mosses, spike mosses, whisk ferns, and horsetails.
Gymnosperms: * Seeds: Produce "naked" seeds not enclosed in specialized chambers (ovaries). * Dominance: Sporophyte predominant. * Fertilization: Utilize pollen grains; water is not required. * Examples: Conifers, cycads, ginkgos, and gnetophytes.
Angiosperms (Flowering Plants): * Seeds: Seeds form within a floral ovary, which matures into a fruit. * Dominance: Sporophyte predominant. * Fertilization: Utilize pollen; water is not required. * Examples: Monocots (lilies, grasses) and Eudicots (tomatoes, roses).

Detailed Life Cycles: Mosses and Ferns

Moss Life Cycle (Bryophyte): 1. The Gametophyte ( $n$ ) is the mat-like, photosynthetic form with leaf-like parts and Rhizoids for anchoring. 2. The Sporophyte ( $2n$ ) is non-photosynthetic, consisting of a stalk and a capsule, remaining attached to the female gametophyte. 3. Within the capsule, spores are formed via Meiosis. 4. Spores drift in the wind and develop into male or female gametophytes. 5. Sperm must swim through water from male gametangia to fertilize the egg in female gametangia. 6. Fertilization produces the zygote, which grows into the new sporophyte.
Fern Life Cycle (Seedless Vascular): * The mature sporophyte ( $2n$ ) features Fronds with Sori (clusters of sporangia) on the underside. * Meiosis occurs in sporangia to produce spores. * The gametophyte is a small, heart-shaped structure (underside houses gametangia). * Water is essential for fertilization; the zygote develops into a young sporophyte emerging from the gametophyte.

Gymnosperms and Conifers

General Characteristics: High resistance to drought and cold; often possess needle-like or scale-like leaves.
Conifers: * Produce non-motile sperm. * Ovules form on the surface of woody cones. * Examples: Pine, redwood, cedar.
Diverse Gymnosperm Representatives: * Cycads: Feature fleshy seeds and palm-like appearances. * Ginkgo biloba: Characterized by fan-shaped leaves and fleshy seeds. * Ephedra: A gnetophyte known as "Mormon tea" with pollen cones. * Welwitschia mirabilis: A gnetophyte from the Namib Naukluft Desert, Namibia; examples can live to be over $2,000$ years old and possess two long, wide leaves.

Angiosperms: Flowers, Fruits, and Diversity

Flower Anatomy: * Stamen (Male): Consists of the Anther (pollen production) and Filament. * Carpel (Female): Consists of the Stigma (receptive tip), Style (stalk), and Ovary (containing Ovules). *The ovary becomes the fruit and the ovule becomes the seed.1 * Other Parts: Petals (attractants) and Sepals.
Life Cycle Details: * Pollen grains (male gametophyte) land on the stigma. * The pollen tube grows down the style to the ovary. * Fertilization: Produces a zygote and Endosperm (nutrient tissue). * The ovule becomes a seed; the ovary matures into a fruit.
Fruit and Dispersal: * Fruits are ripened ovaries housing seeds. * Dispersal Modes: Wind, hitch-hikers (clinging to fur/clothing), and digestive processing (animals eating fruit).
Pollination: * Approximately $90\%$ of angiosperms are pollinated by animals (birds, bats, insects). * Attractant: Nectar, a high-energy fluid.
Major Lineages: * Monocots: Orchids, palms, lilies, grasses. * Eudicots (Dicots): Tomatoes, cabbages, roses, cacti, and most flowering trees.

Fungal Biology and Structure

Metabolism: * Fungi are Absorptive Feeders and the main decomposers in ecosystems. * They secrete powerful Hydrolytic Digestive Enzymes to break down complex molecules like cellulose, lignin, and keratin.
Ecological Roles: * Saprobes: Feed on dead organic material. * Parasitic: Feed on living hosts. * Mutualistic: Exchange nutrients with hosts for mutual benefit.
Anatomy: * Hyphae: Threadlike filaments (may be septate or aseptate). * Chitin: A nitrogen-containing polysaccharide that makes up the fungal cell wall. * Mycelium: A mesh formed by interwoven hyphae. * Fruiting Body: The visible reproductive structure (e.g., a mushroom).

Fungal Reproduction and Diversity

Reproductive Mechanisms: * Sexual Cycle: Involves the fusion of cytoplasm (Dikaryotic stage, $n+n$ ), followed by the fusion of nuclei to form a zygote ( $2n$ ), which then undergoes meiosis to produce spores ( $n$ ). * Asexual Cycle: Mycelium produces spore-producing structures ( $n$ ) to release spores.
Diversity Examples: * Bracket Fungi: Decomposers found on forest floors. * Budding Yeast: Unicellular fungi that reproduce via budding. * Molds: Rapidly growing fungi; some produce chains of spores. * Mushrooms & Fairy Rings: A ring of mushrooms at the edge of an expanding underground mycelium. * Puffballs: Can release trillions of spores. * Corn Smut (huitlacoche): A parasitic fungus on corn, considered a delicacy.

Symbiosis and Practical Uses of Fungi

Beneficial Partnerships: * Mycorrhiza: A symbiotic relationship between fungal hyphae and plant roots; enhances nutrient uptake for the plant. * Lichen: A symbiosis between a fungus and either green algae or cyanobacteria.
Parasitism: * $30\%$ of the $100,000$ known fungal species are parasites. * About $500$ species infect animals/humans (e.g., ringworm, athlete's foot, yeast infections). * The majority of fungal parasites target plants.
Practical Applications: * Medicine: Production of antibiotics like Penicillin. * Research: Used in molecular biology, biotechnology, and proteomics. * Biofuels: Ability to digest cellulose and lignin for fuel production. * Culinary: Used in making cheese, bread, and beer; direct consumption of mushrooms, truffles, and morels.