Chapter 26 – The Colonization of Land

26.1 – Evidence of Algal Ancestry

Plants are believed to have evolved from photosynthetic algae, sharing many key traits with certain algal groups. Understanding these ancestral connections helps explain the origins of land plants and their adaptations.

• Photosynthetic Origin: Plants evolved from photosynthetic algae, specifically green algae.

• Shared Traits with Algae:

  • Plants are multicellular, eukaryotic, and autotrophic.

  • Plant cell walls are made of cellulose.

  • Plants contain chlorophyll a and b for photosynthesis.

• Charophytes: Some plant traits are only found in one extant group of green algae, the charophytes.

  • Presence of rings of proteins that synthesize cellulose in cell walls.

  • Flagellated sperm structure is similar to that of charophytes.

• Example: Charophytes are considered the closest living relatives of land plants.

26.1 – The Movement to Land

The transition from aquatic to terrestrial environments presented both opportunities and challenges for early plants. Several environmental factors influenced this evolutionary step.

• Advantages of Living on Land:

  • Brighter sunlight for photosynthesis.

  • Plentiful CO2 in the atmosphere.

  • Abundant minerals in the soil.

• Challenges of Terrestrial Life:

  • Gravity (force of support required).

  • Reliable water source is less predictable.

  • Risk of drying out (desiccation).

• Example: Early land plants had to develop structures to prevent water loss and support their bodies.

26.1 – Derived Traits of Plants that Facilitated Movement to Land

Land plants evolved several key traits that are absent in their closest algal relatives, enabling them to survive and thrive in terrestrial habitats.

• Key Derived Traits:

  • Cuticle: A waxy covering that prevents water loss.

  • Stomata: Pores that regulate gas exchange and water loss.

  • Multicellular dependent embryos: Embryos are protected and nourished by parental tissue.

  • Alternation of generations: Life cycle alternates between haploid and diploid stages.

• Example: Mosses and ferns exhibit these adaptations, allowing them to colonize land.

26.1 – Derived Traits of Plants that Facilitated Movement to Land – Alternation of Generations

Alternation of generations is a fundamental feature of the plant life cycle, involving two distinct multicellular stages: the gametophyte and the sporophyte.

• Important Definitions:

  • Gametophyte: Haploid (n) generation that produces haploid gametes (egg and sperm) by mitosis.

  • Sporophyte: Diploid (2n) generation that produces haploid (n) spores by meiosis. The sporophyte is formed by fusion of gametes (fertilization).

• Alternation of Generations: The life cycles of plants alternate between two generations of multicellular organisms: gametophytes and sporophytes.

• Example: In ferns, the dominant sporophyte produces spores that grow into gametophytes, which then produce gametes.

26.1 – Differences Among Plant Groups: Alternation of Generations

Plant groups differ in the dominance and independence of the gametophyte and sporophyte generations, which affects their life cycles and reproductive strategies.

• Key Differences:

  • Relative dominance of sporophyte and gametophyte generations.

  • Whether sporophyte and gametophyte live independently or are dependent on each other.

• Example: In mosses, the gametophyte is dominant and photosynthetic, while in flowering plants, the sporophyte is dominant.

Additional info: The alternation of generations is a key evolutionary innovation that allowed plants to adapt to terrestrial environments by separating sexual and asexual reproduction into distinct multicellular stages.