Seed Plants: Structure, Diversity, and Reproduction

Seed Plants

Introduction to Seed Plants

Seed plants are a major group of vascular plants that reproduce using seeds rather than spores. The evolution of seeds was a significant adaptation, allowing plants to colonize a wide range of terrestrial environments.

• Spore Retention: In seed plants, the spore is retained within the sporophyte, where it develops into the gametophyte. Thus, the gametophyte develops inside the sporophyte, providing protection and resources.

• Comparison:

  • Spores: Single-celled, very tough reproductive units found in non-seed plants (e.g., ferns, mosses).

  • Seeds: Multicellular structures containing an embryo, a food supply, and a protective coat.

• Significance: Seeds provide enhanced survival, dispersal, and dormancy capabilities compared to spores.

Seed Structure and Development

Seeds develop from fertilized ovules and contain all the resources necessary for the early development of the plant embryo.

• Ovule: The structure in seed plants that develops into a seed after fertilization. It consists of integuments, the nucellus (megasporangium), and the megaspore.

• Seed Components:

  • Embryo: The young sporophyte plant.

  • Stored Food: Provided by the female gametophyte (in gymnosperms) or endosperm (in angiosperms).

  • Seed Coat: Derived from the integuments, protects the embryo and food supply.

• Development Process: After fertilization, the ovule matures into a seed, containing the embryo, food reserves, and a protective coat.

Reproductive Structures in Seed Plants

Seed plants have specialized structures for producing male and female gametophytes.

• Megasporangia: Produce megaspores, which develop into the female gametophyte containing the egg. The megasporangium is also called the nucellus.

• Microsporangia: Produce microspores, which develop into male gametophytes (pollen grains) that contain sperm.

• Integuments: Layers of sporophyte tissue that surround the megasporangium and develop into the seed coat.

• Ovule: The combination of integuments, nucellus, and megaspore. When a pollen grain lands on an ovule, it extends a pollen tube to the egg nucleus, enabling fertilization.

Classification of Seed Plants

Major Groups

Seed plants are divided into two major groups: Gymnosperms and Angiosperms.

• Gymnosperms: "Naked seeds" not enclosed in an ovary. There are three main phyla of gymnosperms.

• Angiosperms: "Seed in a vessel"; seeds are enclosed within a fruit (mature ovary).

Phylogeny of Modern Vascular Plants

The evolutionary relationships among vascular plants show the emergence of key innovations such as seeds, pollen, and flowers.

• Seedless Vascular Plants: Lycophytes and Pteridophytes (ferns and relatives).

• Seed Plants (Spermatophytes): Include gymnosperms and angiosperms.

• Critical Innovations: Supportive vascular tissue, dominant branched sporophyte generation, wood, ovules, seeds, pollen, flowers, fruits, and endosperm.

Gymnosperms

Phylum Cycadophyta (Cycads)

Cycads are ancient gymnosperms with distinctive features.

• Naked Seeds: Seeds are borne on special leaves (cones), not enclosed in fruits.

• Woody Plants: Most cycads are woody and have a palm-like appearance.

• Coralloid Roots: Specialized roots often associated with nitrogen-fixing cyanobacteria. Additional info: Coralloid roots help cycads survive in nutrient-poor soils.

Other Gymnosperm Phyla (Brief Overview)

• Phylum Ginkgophyta: Includes Ginkgo biloba, the only living species, known for its fan-shaped leaves and separate male and female trees.

• Phylum Coniferophyta: The conifers, which are cone-bearing plants with needle-shaped leaves and adaptations for drought resistance.

• Gnetales: An unusual group sometimes included within conifers, with unique reproductive features. Additional info: Gnetales include genera such as Ephedra, Gnetum, and Welwitschia.

Seed Plant Reproduction: Key Processes

Fertilization and Seed Formation

Fertilization in seed plants involves the transfer of sperm to the egg via pollen, followed by seed development.

• Pollen Grain: The male gametophyte, which delivers sperm to the ovule through a pollen tube.

• Ovule: Contains the female gametophyte and, after fertilization, develops into a seed.

• Seed Coat: Derived from the integuments, protects the developing embryo.

Comparison Table: Spores vs. Seeds

Feature	Spores	Seeds
Structure	Single-celled	Multicellular (embryo, food supply, coat)
Protection	Minimal	High (seed coat)
Dispersal	Wind, water	Wind, water, animals
Survival	Short-term	Long-term (dormancy possible)

Summary

Seed plants represent a major evolutionary advancement in the plant kingdom, with seeds providing protection, nourishment, and improved dispersal for the next generation. The division into gymnosperms and angiosperms reflects differences in seed enclosure and reproductive structures, with further diversity among the gymnosperms.