Seed plants -10

Seed Plants Overview

  • Seed plants include two major groups:

    • Gymnosperms

    • Angiosperms

  • First appearance of seed plants traced back to 305-465 million years ago.

  • Evolved from spore-bearing ancestors known as progymnosperms.

  • Angiosperms, or flowering plants, are considered the most recent evolutionary advancement in plant history.

Evolutionary Advantages of Seed Plants

  • The evolution of seeds marked a significant advancement:

    • Protection and Nutritional Support: Seeds protect the embryo and provide a food source.

    • Dormancy: Dormancy of seeds allows survival through harsh environmental conditions until favorable conditions resume.

    • Fruit Development: Later evolution of fruits improved seed dispersal mechanisms.

  • Extensive coevolution occurred between seed plants and animals during this period.

  • The advent of pollen allowed sperm to be dispersed without reliance on water.

Structure and Function of Seeds

  • Seed Composition:

    • Embryo: The developing plant.

    • Integument: The outer layer that hardens into a seed coat from sporophyte tissue.

    • Food Supply: Provided in the form of stored nutrients for the developing embryo.

Gametophyte Production in Seed Plants

  • Types of Gametophytes:

    • Male Gametophytes:

      • Microscopic structures within pollen grains.

      • Pollen is dispersed by wind or pollinators, and does not require water.

    • Female Gametophytes:

      • Develop within ovules and enclosed by diploid sporophyte tissue in angiosperms.

Five Phyla of Extant Seed Plants

  1. Coniferophyta (Conifers)

  2. Cycadophyta (Cycads)

  3. Gnetophyta (Gnetophytes)

  4. Ginkophyta (Ginkgos)

  5. Anthophyta (Angiosperms)

Gymnosperms Characteristics

  • Gymnosperms, known for having naked seeds, consist of four living groups:

    • Coniferophytes

    • Cycadophytes

    • Gnetophytes

    • Ginkgophytes

  • They lack flowers and fruits, with ovules exposed on scales.

Life Cycle of Conifers

  • Conifers are heterosporous, producing male (microspores) and female (megaspores) spores.

  • Male Cones:

    • Have microsporocytes that develop into pollen grains.

    • Each pollen grain contains a generative cell and a tube cell.

  • Female Cones:

    • Contain ovules that develop into female gametophytes and ultimately seeds.

  • Fertilization in conifers is achieved when the pollen tube penetrates the ovule.

Unique Characteristics of Specific Gymnosperms

  • Conifers:

    • Include species like pines, spruces, and the coastal redwood (the tallest vascular plant).

  • Cycads:

    • Grow in tropical/subtropical areas, resembling palm trees, with female cones weighing up to 45 kg.

  • Gnetophytes:

    • Unique gymnosperms with xylem vessels, including genera such as Welwitschia, Ephedra, and Gnetum.

  • Ginkgophytes:

    • Only one remaining species (Ginkgo biloba), characterized by flagellated sperm and dioecious reproductive structures.

Angiosperms Overview

  • Distinct Features:

    • Have flowers that develop seeds within protective chambers.

    • No longer reliant on moisture for reproduction.

  • Two types of gametophytes:

    • Microgametophytes (pollen)

    • Megagametophytes (ovules)

Flower Structure in Angiosperms

  • Main components include:

    • Stigma, Style, Carpel, Stamen (anther, filament)

    • Ovary, Petal, Sepal, Receptacle

  • Structures facilitate pollination and fertilization processes.

Angiosperm Reproductive Processes

  • Double Fertilization process unique to angiosperms:

    • One sperm unites with the egg (forming a zygote).

    • The other sperm fuses with polar nuclei to create triploid endosperm.

  • This process supports the developing embryo through nutritional delivery.

Fruit Development & Types

  • Fruit Functions:

    • Aids in seed dispersal and protection.

  • Types of Fruits:

    • Simple Fruits: Derived from a single ovary (e.g., nuts).

    • Aggregate Fruits: Formed from multiple carpels (e.g., raspberries).

    • Multiple Fruits: Result from clusters of flowers (e.g., pineapple).

    • Accessory Fruits: Developed from other plant parts (e.g., apples).

Monocots vs. Eudicots

  • Monocots:

    • One cotyledon, parallel leaf veins, scattered vascular bundles.

  • Eudicots:

    • Two cotyledons, network of leaf veins, vascular bundles arranged in a ring.

  • Key differences include flower parts and root systems.

Final Summary of Angiosperm Fertilization

  • Total of eight nuclei present during fertilization:

    • Includes three antipodal cells, two synergids, one egg cell, and two polar nuclei.

  • The fertilization process involves the formation of zygote and primary endosperm, creating a supportive environment for the future plant.