Gymnosperms

Seed plants evolved during the Devonian period.
Fossils reveal fern-like plants that were heterosporous: producing two types of spores, microspores and megaspores.
- Microspores: Small spores that typically develop into male gametophytes.
- Megaspores: Larger spores that develop into female gametophytes, retained within the parent sporophyte.
Fertilization occurs within the sporophyte, reducing dependence on surface water.
Seeds emerged as a more effective means of dispersal, as they are dormant and protect the embryo sporophyte.

Heterospory led to the evolution of seeds, as it minimized dependence on water for fertilization and increased reproductive efficiency.
The concepts of r- and k-strategies relate to the reproductive strategies of plants:
- r-strategy: High number of offspring with minimal parental care (e.g., spores).
- k-strategy: Fewer offspring invested with more resources (e.g., seeds).
Key differences between gymnosperms and angiosperms include:
- Structure of reproductive organs (cones vs. flowers).
- Life cycles including phenomena like double fertilization in angiosperms.

The evolutionary shift to heterospory involved a division of labor in spore production:
- Numerous small microspores to maximize chances of fertilization.
- Fewer large megaspores rich in nutrients for embryo support.
This led to forms of gametophytes that developed separately, increasing genetic diversity and efficiency in reproduction.

Secondary growth allows for plants to grow larger and respond to competition for light and resources.
With larger size, the challenge became producing enough viable offspring, leading to the development of seeds.
The seed encapsulates the entire reproductive cycle: spermatophyte, megagametophyte, and sometimes endosperm within the ovule.

Pollination allows for sperm delivery to egg cells without requiring water:
- Open fertilization: Involves direct connection of gametes in some gymnosperms.
- Siphonogamy: Use of pollen tubes for sperm delivery in higher seed plants.

Two main lineages of seed plants:
- Gymnosperms: Seeds exposed on cones, do not form flowers or fruits.
- Angiosperms: Seeds enclosed within fruits, often more complex reproductive strategies including double fertilization.
Gymnosperms evolved in response to stress such as dry conditions, leading to structural adaptations in leaves (xerophytic features) and reproductive strategies (pollination adaptation).

Gymnosperms typically have:
- Thick cuticles to resist desiccation.
- Secondary growth allowing for increased size and stability.
- Different reproductive structures: microstrobili (male cones) and megastrobili (female cones).

Cycads and Ginkgos: Primitive lineage with sperm that swim to the egg, both are dioecious.
- Ginkgo biloba as a living fossil with unchanged morphology from ancient times.
- Cycads: Plant resembles ferns, prevalent during the Mesozoic era.
Conifers: Most diverse group of gymnosperms, capable of surviving in various environments due to adaptations in structure and reproductive strategies.
- Form of reproductive structures like cones facilitates survival in diverse ecological niches.

Gnetophytes share characteristics with both angiosperms and gymnosperms but have distinct reproductive features.
Traits include double fertilization and unique adaptations for dry environments, distinguishing them from other gymnosperms.
Three main genera: Ephedra, Gnetum, and Welwitschia with diverse adaptations and morphology.