Theme 3-Topic 16

BIOL 108 Winter 2026 - Study Notes on Seed Plants and Gymnosperms

Introduction to Seed Plants

  • Seed plants originated approximately 360 million years ago (mya) and have significantly influenced plant evolution, establishing themselves as the dominant primary producers in terrestrial ecosystems.

  • Key features of seeds:

    • Seeds consist of an embryo and nutrients enveloped by a protective coat, facilitating effective long-distance dispersal.

  • The domestication of seed plants began around 8,000 years ago, allowing for permanent human settlement.

Diversity of Plant Species

  • Number of Known Species by Plant Category:

    • Nonvascular Plants (Bryophytes)

    • Phylum Hepatophyta (Liverworts): 9,000 species

    • Phylum Bryophyta (Mosses): 13,000 species

    • Phylum Anthocerophyta (Hornworts): 225 species

    • Vascular Plants

    • Seedless Vascular Plants

      • Phylum Lycophyta (Lycophytes): 1,200 species

      • Phylum Monilophyta (Monilophytes): 12,000 species

    • Seed Plants

      • Gymnosperms

      • Phylum Ginkgophyta (Ginkgo): 1 species

      • Phylum Cycadophyta (Cycads): 350 species

      • Phylum Gnetophyta (Gnetophytes): 75 species

      • Phylum Coniferophyta (Conifers): 600 species

      • Angiosperms

      • Phylum Anthophyta (Flowering plants): 290,000 species

Shared Derived Traits of Seed Plants

  1. Reduced Gametophytes

    • Gametophytes in seed plants develop within the protective spore walls retained in parental sporophyte tissues, offering protection from environmental stressors.

    • Comparison of gametophyte roles across plant groups:

      • Non-vascular plants (Bryophytes): Independent, dominant gametophyte; sporophyte is dependent on the gametophyte.

      • Seedless vascular plants: Dominant sporophyte; gametophytes exist independently.

      • Seed plants: Dominant, independent sporophyte; gametophytes are microscopic and depend on the sporophyte for nutrients.

  2. Heterospory

    • All seed plants are heterosporous, producing two types of spores:

      • Megaspores: Produced by megasporangia, which develop into female gametophytes.

      • Microspores: Produced by microsporangia, which develop into male gametophytes.

    • In seed plants, spores are not dispersed but retained within the sporophyte.

  3. Ovules

    • An ovule is composed of:

      • Megasporangium (2n)

      • Megaspore (1n)

      • One or more protective integuments (2n)

    • In gymnosperms, ovules contain one integument; in angiosperms, ovules usually have two integuments.

    • Fertilized ovules develop into seeds.

  4. Pollen

    • Microspores become male gametophytes within pollen grains, which contain cell walls coated with sporopollenin—making them resilient to desiccation, UV light, and physical damage.

    • Pollen grains transport male gametophytes to ovules, allowing for fertilization without the need for external humidity.

  5. Seeds

    • Seeds develop from fertilized ovules and are composed of:

      • An embryo

      • A food supply

      • An outer seed coat

    • The size of seeds varies greatly and is primarily determined by the quantity of storage reserves derived from the gametophyte.

      • Example: Orchid seeds weigh less than 1 microgram, while coco de mer palm seeds can weigh up to 30 kilograms.

Pollination

  • Importance of Pollination:

    • Pollination delivers pollen to the ovule for fertilization, changing the evolutionary method of fertilization.

    • In seedless plants, sperm require water to swim through short distances. However, pollen allows fertilization without water.

    • Male gametophytes are enclosed within pollen grains, enabling long-distance dispersal via wind or animals.

    • Upon germination, the pollen grain develops into a pollen tube, delivering sperm nuclei directly to the female gametophyte within the ovule.

Seed Dispersal

  • Seeds represent the dispersal stage for seed plants, in contrast to seedless plants where spores serve this role.

  • Advantages of seeds over spores:

    • Structural adaptations promote long-distance seed dispersal by air, water, or animals.

    • Larger and sturdier than spores, seeds possess protective coats and energy-dense storage that support early seedling growth.

    • Trade-off exists between seed size and number:

    • Large seeds: Fewer produced with ample nutrient reserves; advantageous for low-light germination (e.g., forest understory).

    • Small seeds: More produced but require optimal conditions for successful germination.

Evolutionary Change in Fertilization and Dispersal Conditions

  • Overview of fertilization and dispersal evolution across plant groups:

    • Nonvascular plants and seedless vascular plants rely on water for fertilization.

    • Seed plants utilize pollen for fertilization in aerial environments, moving away from water dependency. Dispersal is primarily achieved through seeds.

Origin and Diversification of Seed Plants

  • There are two living clades of seed plants:

    • Gymnosperms: Approximately 1,000 species, including conifers.

    • Angiosperms: Approximately 290,000 species, characterized as flowering plants.

Evolution of Gymnosperms

  • The term 'gymnosperm' translates to "naked seeds," as seeds in this group are exposed on structures known as sporophylls arranged in cones (strobili).

  • Gymnosperms emerged approximately 360 million years ago, with the earliest forms being 'seed ferns' that had fern-like leaves and produced seed-like structures.

  • Gymnosperms first appeared in fossil records during the Carboniferous Period (about 305 mya) and dominated terrestrial ecosystems throughout the Mesozoic era (251–65 mya).

  • Adaptations in gymnosperms supported survival under the drier conditions of the Mesozoic, further enhanced by their capability for water-independent fertilization and dispersal through seeds.

  • Angiosperms began to outcompete gymnosperms near the end of the Mesozoic period, resulting in their current dominance in terrestrial ecosystems.

Extant Gymnosperms

  • Gymnosperms consist of four phyla:

    • Cycadophyta: Approximately 350 species, once diverse and dominant during the Mesozoic; currently limited to small populations.

    • Ginkgophyta: Contains a single living species, Ginkgo biloba, notable for its bilobed leaves and wind-pollinated separate male and female sporophytes. All wild populations are extinct, existing only through human cultivation.

    • Gnetophyta: Comprising three genera (Gnetum, Ephedra, Welwitschia) with about 75 species, exhibiting morphological diversity and adaptation to various environments.

    • Coniferophyta: The most extensive phylum of gymnosperms (around 600 species), consisting primarily of woody shrubs and trees which include the largest and longest-lived species.

Life Cycle of Gymnosperms

  • Key features of gymnosperm life cycles include:

    • Dominant sporophyte generation (2n), with mature trees representing the sporophyte.

    • Development of seeds from fertilized ovules.

    • Male gametophytes delivered to ovules via pollen grains, whose resistant walls protect them during dispersal.

Specific Life Cycle of Conifers (Pinus)

  • Pine trees serve as the sporophyte (2n), generating sporangia on scale-like leaves within cones.

  • Heterospory is present, with megasporangia in female cones and microsporangia in male cones, leading to the development of male and female gametophytes.

  • Pollination is primarily facilitated by wind, with many conifer pollen grains exhibiting adaptations (like air bladders) for enhanced dispersal.

  • Conifer seeds possess ovuliferous scales that aid in wind dispersal, although fertilization and seed development timelines may extend up to three years.