Lecture+Topic+20-Seed+Plants+Gymnosperms+and+Angiosperms

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I. Overview: Transforming the World

  • The evolution of true seeds significantly influenced plant evolution, enabling their dominance in terrestrial ecosystems.

  • Seeds and pollen grains are crucial adaptations that allow plants to thrive in terrestrial environments.

  • A seed is made up of an embryo (the future plant), nutrients, and a protective outer coat.

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D. Common Characteristics Among Seed Plants

  1. Heterospory

    • Seed plants are heterosporous, producing two distinct spore types:

      • Microspores: develop into male gametophytes.

      • Megaspores: develop into female gametophytes.

    • Megasporangia (2N) produce megaspores (N), while microsporangia (2N) yield microspores (N).

    • Diagram: Megasporangium (2n) → Megaspore (n) as unfertilized ovule, integument, spore wall, immature female cone.

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  1. Reduced Gametophytes

    • These structures are virtually microscopic.

    • Diagram: Megasporangium (2n) → Megaspore (n) as unfertilized ovule, integument, spore wall, immature female cone.

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  1. Reduced Gametophytes

    • Provides protection as gametophytes develop within the spore's walls, which are retained in the parent sporophyte's tissues.

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  1. Ovules

    • Comprised of integument, megasporangium, and megaspore or egg.

    • Gymnosperm megasporangia have one integument, whereas angiosperm megasporangia typically have two.

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  1. Pollen

    • Microspores develop into pollen grains, which contain male gametophytes.

    • Pollination is the transfer of pollen to the part of seed plants containing ovules, negating the need for water for fertilization.

    • Pollen can be dispersed over large distances by wind or animals.

    • If germinated, pollen forms a pollen tube that delivers sperm to the female gametophyte within the ovule.

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E. The Evolutionary Advantage of Seeds

  1. Seeds originate from ovules.

  2. A seed is a sporophyte embryo with a nutrient supply encased in a protective coat.

  3. Advantages of seeds over spores include:

    • Dormancy: Seeds can remain dormant for extended periods until conditions are right for germination.

    • Dispersal: Seeds can travel long distances via wind or animals.

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  1. Seed life stages include:

    • Embryo attached to the mother plant.

    • Vascular tissues formed for nutrient transport.

    • Seed stage: embryo with a food supply in a protective coat.

  2. Greater complexity over time.

  3. Examples of Monilophytes, Gymnosperms, and Angiosperms found in Botanic Gardens.

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II. Gymnosperms

  • Define gymnosperms as seed producers with "naked" seeds, generally found on cones.

  • Four main phyla of gymnosperms:

    1. Cycadophyta (cycads)

    2. Gingkophyta (e.g., Ginkgo biloba, one extant species)

    3. Gnetophyta (e.g., Gnetum, Ephedra, Welwitschia)

    4. Coniferophyta (conifers such as pines, firs, and redwoods).

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B. Gymnosperm Diversity

  1. Ginkgophyta:

    • Only one extant species: Ginkgo or maidenhair tree.

    • Notable for air pollution resistance, commonly used as an ornamental tree.

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  1. Cycadophyta:

    • Approximately 200 species; resemble palms (but are not).

    • Produce seeds in large cones and thrived during the Mesozoic era; few species remain today.

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  1. Gnetophyta:

    • Exhibit diverse growth forms with few species.

    • Includes plants found in various habitats, e.g., tropical and desert.

    • Example: Welwitschia mirabilis, potentially over 1,000 years old.

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  1. Coniferophyta:

    • Comprises about 550 species, crucial ecologically and economically.

    • Features needle-shaped leaves and is mostly evergreen.

    • The largest forest types are predominantly conifers (e.g., Siberia and North American taiga).

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  • Largest organism: Giant Sequoia, Sequoiadendron giganteum (up to 2,000 tons).

  • Tallest organism: Coast Redwood, Sequoia sempervirens (current record at 112.3 meters).

  • Oldest non-clonal organism: Bristlecone Pine, Pinus longaeva (> 4,800 years).

  • All noted species currently found in California.

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C. The Life Cycle of a Pine: A Closer Look

  1. Key features:

    • Dominance of the sporophyte generation.

    • Development of seeds from fertilized ovules.

    • Sperm transfer to ovules via pollen.

  2. The typical pine tree represents the sporophyte and produces sporangia in its male and female cones.

  3. Small cones yield microspores, developing into male gametophytes.

  4. Larger cones house ovules, which produce megaspores leading to female gametophytes.

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  • Illustrated life cycle process:

    • Microsporangium (2n) → Microsporocytes (2n) → Pollen grains (n).

    • Pollen will germinate and fertilization occurs, leading to seed development.

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  1. Embryonic connection to the mother plant initiates growth.

  2. Vascular tissues form for nutrient transport.

  3. Seed development: embryo, food supply, and protective coat.

  4. Transition towards flowers and decreasing water reliance.

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III. Angiosperms

  • Defined as seed plants with reproductive adaptations that include flowers and fruits, the most diverse group of plants.

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A. Characteristics of Angiosperms

  1. Classified in the phylum Anthophyta (Greek for flower).

  2. Flowers: specialized structures tailored for sexual reproduction, enabling diverse pollination strategies.

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  • Flower structure parts:

    1. Sepals: (calyx) protect immature flower buds.

    2. Petals: (corolla) brightly colored to attract pollinators; often small in wind-pollinated species.

    3. Stamens: microsporophylls that produce pollen.

    4. Carpels: megasporophylls producing megaspores and female gametophytes (seeds).

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C. Fruits

  1. Typically consist of a mature ovary and may include additional flower parts.

  2. Fruits serve to protect seeds and facilitate their dispersal.

  3. Can be fleshy or dry at maturity.

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  1. Various adaptations for seed dispersal include mechanisms for wind, water, or animal transportation.

    • Examples: barbs, wings, and seeds encased in fleshy fruits.

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D. The Angiosperm Life Cycle

  1. Flowers of the sporophyte contain both male and female reproductive structures.

  2. Male gametophytes form within pollen grains from microsporangia of anthers.

  3. Female gametophytes develop within ovules in an ovary attached to a stigma.

  4. Many flowers bring about cross-pollination mechanisms to enhance genetic diversity.

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  • Life cycle diagram showing the process from gametophyte formation to fertilization and seed development.

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  • Angiosperms: approximately 250,000 species, compared to fewer than 1,000 for gymnosperms.

Reasons for Angiosperm Success

  1. Rapid life cycles (weeks vs. years for gymnosperms).

  2. Cooperative relationships (mutualism and co-evolution) enhance pollination and dispersal.

  3. More effective vascular systems (vessel elements and sieve tube members) support diverse growth.

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IV. Angiosperm Diversity

  • Two main groups:

    • Monocots: one cotyledon.

    • Eudicots: two cotyledons.

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A. Monocots

  • Represent more than one-quarter of angiosperm species (65,000 species).a. Single cotyledon.b. Leaf venation: parallel.c. Stem vascular bundles: scattered.d. Generally fibrous root systems.e. Floral organs typically in multiples of three.

    • Examples: lilies, orchids, palms, grasses, and major grains.

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B. Eudicots

  • Comprising over two-thirds of angiosperm species (165,000 species).a. Two cotyledons.b. Leaf venation: net-like.c. Vascular bundles arranged in a ring.d. Central taproot prevalent.e. Floral organs usually in multiples of four or five.

    • Examples: legumes, trees (oaks, maples), roses, and other human crops.

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Study Points

  1. Importance of pollen grains for land reproduction.

  2. Life histories of pine and flower plants, including gametophyte and sporophyte structures.

  3. Differentiation of haploid and diploid structures in gymnosperms and angiosperms.

  4. Key flower structure functions: sepals, petals, stamens, carpels, filament, anther, stigma, style, ovary, ovule.

  5. Adaptations of fruits for seed dispersal.

  6. Evolutionary changes in land plants relative to water dependency.