Spermatophyta - seed plants

Overview of Seed Plants

Seed plants are characterized by their ability to produce seeds, a significant evolutionary adaptation that allows them to thrive in various environments, including dry land. This adaptation first occurred during the Carboniferous period. Compared to non-seed vascular plants, seed plants can reproduce without needing to be in moist environments.

Adaptation and Structure of Seeds

The main advantage of seeds is that they protect the embryo, which is the most vulnerable life stage of the plant. Seeds encase the embryo along with a food supply, enabling plants to colonize areas away from water sources. This protective structure, along with a food source, allowed seed plants to adapt to harsher land conditions, including hot and dry environments.

Life Cycle: Alternation of Generations

Seed plants maintain a life cycle characterized by alternation of generations, involving two distinct phases: the sporophyte stage and the gametophyte stage. In seed plants, the sporophyte is the dominant form and is larger, while the gametophyte is reduced in size and relies on the sporophyte for nourishment and protection, particularly during reproduction.

Gymnosperms

Gymnosperms, which translate to "naked seeds," represent the first group of seed plants to evolve. They include conifers like pine trees, cycads, and others.

Female Gametophyte and Fertilization

The female reproductive structure of gymnosperms is characterized by the presence of megasporangia, which produce megaspores via meiosis. Instead of dispersing, the megaspore remains within the sporophyte, germinating into the female gametophyte that is nourished inside the sporophyte. This relationship provides further protection to the female gametophyte, which eventually produces egg cells.

  • Ovule: Contains the megasporangium tissue with the female gametophyte.
  • Archegonium: The structure in the female gametophyte where egg fertilization occurs, resulting in the formation of a zygote.
Male Gametophyte and Pollination

The male pine cone produces microsporangia that yield microspores through meiosis, which develop into pollen grains (the male gametophyte). The pollen discharges and lands on a female cone, where the male gametophyte germinates and creates a pollen tube to transport sperm cells to the egg cell in the ovule. Sperm needs to travel down this tube to fertilize the egg cell within the ovule.

Key Phyla of Gymnosperms

  1. Coniferophyta (Conifers): Includes pines and evergreens, mostly monoecious (male and female cones on the same plant).
  2. Cycadophyta (Cycads): Features cylindrical trunks and a rosette of leaves; they are dioecious (separate male and female plants).
    • Example: Genus Cycas.
  3. Ginkgophyta: Includes Ginkgo biloba (maidenhair tree), which is the only surviving species from its phylum.
  4. Gnetophyta: Includes organisms like Welwitschia, endemic to Africa, with unique adaptations to arid environments.

Angiosperms

Angiosperms are characterized by the presence of flowers and seeds contained within fruits, making them the most diverse group of seed plants.

Structural Components of Flowers

Flowers are the reproductive structures of angiosperms, consisting of several key parts:

  • Receptacle: The thickened part of the stem that supports the flower.
  • Sepals: Protective structures that encase the flower before it opens.
  • Petals: Brightly colored parts that attract pollinators.
  • Stamens: The male components of the flower, which consist of a filament and anthers that create microspores (pollen).
  • Carpals (Pistils): The female parts of the flower that include the stigma (sticky part that captures pollen), style (tube leading to the ovary), and ovary (contains ovules).
Pollination and Fertilization

Pollination is the transfer of pollen from an anther to a stigma. Once pollen lands on a compatible stigma, it germinates and grows a pollen tube to the ovule, where fertilization occurs. This process can involve:

  • Double Fertilization: One sperm fertilizes the egg, forming a zygote, while another sperm fuses with the central cell to form triploid endosperm, which serves as food for the developing embryo.
Seed Development and Dispersal

Once fertilization occurs, the ovary matures into a fruit that protects the seeds, aiding in their dispersal and survival until germination. Seeds have an outer seed coat, and the embryo inside has a food supply (typically triploid endosperm).

Classification of Angiosperms

Angiosperms are classified into two major groups based on their seed structures:

  1. Monocots: One cotyledon, parallel leaf venation, scattered vascular tissue in stems, and flower organs in multiples of three.
    • Example: Lilies.
  2. Dicots: Two cotyledons, net-like leaf venation, ringed vascular tissue, and flower organs in multiples of four or five.
    • Example: Rose bushes.

Overall, seeds have vastly increased the adaptability of plants, allowing them to occupy a wide range of ecological niches.