Seed Plants and Gymnosperms

Jun 14, 2025

Overview

This lecture covers the evolutionary significance, adaptations, reproductive strategies, and classification of seed plants, focusing on gymnosperms and their life cycles.

Seed Plant Innovations

  • Seed plants transformed terrestrial ecosystems as primary producers.
  • Major adaptations include reduced gametophyte size, retention in sporophyte tissue, and origin of seeds.
  • Seeds protect the embryo with a seed coat and provide a food supply (endosperm).
  • Seeds replaced spores as the main dispersal method due to their hardiness.
  • Pollen evolution allowed fertilization without water; pollen grains carry sperm nuclei.
  • Seed plants are heterosporous (produce megaspores and microspores).

Importance of Sporophyte Dominance

  • The sporophyte (diploid) generation became dominant in seed plants.
  • Diploidy offers protection against mutations from increased terrestrial UV radiation.

Structure and Function of the Ovule and Seed

  • The ovule consists of the integument, megasporangium (nucellus), and megaspore.
  • Upon fertilization, the ovule becomes a seed with an embryo, food supply, and seed coat from integuments.
  • Female gametophyte provides nutritive tissue for the developing embryo.

Gymnosperms: Classification and Features

  • Gymnosperms have "naked seeds" not enclosed in an ovary.
  • Four main living groups: Cycadophyta (cycads), Ginkgophyta (ginkgo), Gnetophyta, and Coniferophyta (conifers).
  • Cycads and Ginkgo biloba have flagellated sperm; other gymnosperms and angiosperms do not.

Gymnosperm Groups and Examples

  • Cycadophyta: Palm-like, slow-growing, many endangered; Florida Coontie (Zamia integrifolia) is native.
  • Ginkgophyta: Only one species (Ginkgo biloba), tolerant to cold and pollution, distinctive fan-shaped leaves.
  • Gnetophyta: Includes Gnetum (trees/vines), Ephedra (produces ephedrine), and Welwitschia (desert plant).
  • Coniferophyta: Cone-bearing, largest group, includes pines, firs, spruce, cypress, redwoods; adapted to dry/cold climates.

Conifer Life Cycle

  • Conifers produce separate male (pollen/microsporangiate/staminate) and female (ovulate/megaspore/pistillate) cones.
  • Male cones produce microspores → pollen grains (male gametophyte).
  • Female cones contain ovules; fertilization occurs after pollination via pollen tube.
  • Life cycle takes nearly 3 years: pollination, then fertilization after over a year, leading to seed formation and dispersal.

Key Terms & Definitions

  • Sporophyte — Diploid, dominant life stage in seed plants.
  • Gametophyte — Haploid, produces gametes; greatly reduced in seed plants.
  • Ovule — Structure containing integument, megasporangium, and megaspore; develops into a seed after fertilization.
  • Integument — Outer layer that forms the seed coat.
  • Pollen grain — Male gametophyte that delivers sperm nuclei to ovule.
  • Heterosporous — Produces two types of spores: megaspores (female) and microspores (male).

Action Items / Next Steps

  • Review diagrams of the pine (Pinus) life cycle.
  • Learn examples and characteristics of each gymnosperm group.
  • Understand terminology for male and female cones.