Seed Plants: Evolution and Classifications

Overview of the Evolution of Seed Plants

• Definition and Classification:

  • Seed plants consist of two major groups: Gymnosperms and Angiosperms.

  • They evolved from spore-bearing plants known as progymnosperms.

• Timeline of Evolution:

  • Seed plants first appeared approximately $305 \text{ to } 465\,\text{MYA}$ (million years ago).

  • Angiosperms, or flowering plants, represent the most recently evolved group of plants.

• Key Evolutionary Success Factors:

  • The Seed: The evolution of the seed is central to the success of these plants. It protects the embryo and provides a dedicated food supply.

  • Dormancy: Seeds can enter a state of dormancy, effectively "stopping the clock." This allows the plant to survive harsh environmental periods before germinating when conditions improve.

  • Pollen: The evolution of pollen allowed for the dispersal of sperm without the requirement of water for transport.

  • Fruit Development: In later evolution, the development of fruits enhanced the dispersal of seeds.

  • Animal Coevolution: Significant levels of coevolution occurred between seed plants and various animal species.

Anatomy and Biology of the Seed

• Seed Structure:

  • Embryo: The young plant located inside the seed.

  • Integument: The embryo is protected by one or two extra layers of sporophyte tissue.

  • Seed Coat: The integument hardens to form the protective outer seed coat.

  • Food Supply: The seed contains a nutrient storage supply for the developing embryo.

  • Specific Example: The bean is a classic example of a dicotyledon seed.

Gametophyte Development in Seed Plants

• Male Gametophytes (Microgametophytes):

  • Contained within pollen grains.

  • Dispersed via wind or animal pollinators.

  • Eliminates the need for external water for fertilization.

• Female Gametophytes (Megagametophytes):

  • Develop within an structure called an ovule.

  • In angiosperms, the female gametophyte is completely enclosed within diploid sporophyte tissue.

The Five Phyla of Extant Seed Plants

• Gymnosperms: Consists of four phyla.

  1. Coniferophyta: Conifers.

  2. Cycadophyta: Cycads.

  3. Gnetophyta: Gnetophytes.

  4. Ginkgophyta: Ginkgophytes.

• Angiosperms: Consists of one phylum.

  1. Anthophyta: Flowering plants.

Characteristics and Classification of Gymnosperms

• General Characteristics:

  • The term "Gymnosperm" translates to "naked seeds."

  • They lack the flowers and fruits found in angiosperms.

  • The ovule is exposed on a scale (modified leaf).

• Conifers (Phylum Coniferophyta):

  • The most familiar and diverse gymnosperm phylum.

  • Includes pines, spruces, firs, and cedars.

  • Coastal Redwood: Known as the tallest living vascular plant.

  • Bristlecone Pine: Known as the oldest living tree.

• Cycads (Phylum Cycadophyta):

  • Slow-growing plants found in tropical and subtropical regions.

  • Sporophytes have a physical resemblance to palm trees.

  • Female cones are exceptionally large and can weigh up to $45\,kg$.

• Gnetophytes (Phylum Gnetophyta):

  • Unique among gymnosperms for possessing vessels in their xylem.

  • Composed of three unusual genera: Welwitschia, Ephedra, and Gnetum.

• Ginkgophytes (Phylum Ginkgophyta):

  • Only one living species remains: Ginkgo biloba.

  • Possess flagellated sperm.

  • They are dioecious, meaning male and female reproductive structures are located on different, separate trees.

Reproduction in Gymnosperms (Focus on Pines)

• Cone Distribution:

  • Male Cones: Typically grow in the lower branches. They contain microsporangia and produce pollen grains.

  • Female Cones: Grow in the upper branches. This positioning facilitates fertilization by pollen carried on the wind from the lower branches of other trees.

• Gametophyte Location:

  • Microgametophyte: Found in male cones, which are smaller and consist of modified leaves. These house the pollen grains (sperm cells).

  • Megagametophyte: Found in the larger female cones. The ovule (containing the egg) is positioned on the scales of these cones.

Anatomy and Diversity of Angiosperms

• Flower Structure:

  • Perianth: Composed of the Calyx (sepals) and the Corolla (petals).

  • Androecium: The male reproductive parts (stamens), consisting of the Anther and Filament.

  • Gynoecium: The female reproductive parts (carpels), consisting of the Stigma, Style, and Ovary (containing the ovule/megasporangium).

• Classification by Cotyledon:

  • Monocots: Plants with one cotyledon.

  • Eudicots (Dicots): Plants with two cotyledons.

Comparison of Monocots and Eudicots

• Structural Characteristics Table:

Angiosperm Life Cycle and Growth

• Reproductive Cycles:

  • Microgametophyte (Pollen): Contains a generative cell and a tube cell. It forms a pollen tube during germination to deliver sperm to the egg.

  • Megagametophyte (Embryo Sac): Contains the egg, polar nuclei, synergids, and antipodal cells.

  • Fertilization: Involves the meeting of sperm and egg within the ovule, leading to the formation of the embryo and the endosperm (food supply).

• Types of Plant Growth:

  • Primary Growth: Occurs at the apical meristems of both roots and shoots. Every plant experiences primary growth.

  • Secondary Growth: Deals with expanding the girth of the plant and the creation of wood. Wood is anatomically defined as proliferating xylem.

• Woody Plant Characteristics:

  • Monocots: Generally herbaceous and do not produce true wood. Examples include grasses and palms.

  • Eudicots: Capable of producing true wood through secondary growth.

  • Woody Cross-Section: Includes the pith (center), secondary xylem (wood), secondary phloem, and annual rings representing seasonal growth.