Chapter 26 – The Colonization of Land: Plant Evolution and Adaptations

Chapter 26 – The Colonization of Land

26.1 – Evidence of Algal Ancestry

Land plants evolved from photosynthetic algae, sharing many key traits with their algal ancestors. Some traits are found in all plants, while others are unique to specific groups.

• Key Plant Traits in Algae:

  • Plants are multicellular.

  • Plant cell walls are made of cellulose.

  • Plants contain chloroplasts for photosynthesis.

• Unique Plant Traits in Charophytes:

  • Rings of proteins that synthesize cellulose.

  • Flagellated sperm with similar structure.

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

26.1 – The Movement to Land

Transitioning from aquatic to terrestrial environments provided new opportunities and challenges for early plants.

• Advantages of Living on Land:

  • Brighter sunlight for photosynthesis.

  • Plentiful CO2 in the atmosphere.

  • Abundant minerals in the soil.

• Challenges of Terrestrial Life:

  • Force of gravity.

  • Reliable water source.

  • Risk of drying out (desiccation).

26.1 – Derived Traits of Plants that Facilitated Movement to Land

Land plants evolved several key traits that enabled them to survive and thrive on land, many of which are absent in their algal relatives.

• Key Derived Traits:

  • Alternation of Generations

  • Multicellular, Dependent Embryos

  • Walled Spores Produced in Sporangia

  • Apical Meristems

  • Other Derived Traits (e.g., cuticle, stomata)

Alternation of Generations

Land plants have a life cycle that alternates between two multicellular generations: the haploid gametophyte and the diploid sporophyte.

• Definitions:

  • Haploid (n): Generation that produces gametes (egg and sperm) by mitosis.

  • Diploid (2n): Generation that produces spores by meiosis.

• Alternation of Generations: The life cycle alternates between gametophyte and sporophyte generations.

Differences Among Plant Groups

Group	Dominant Generation	Vascular Tissue	Seeds
Bryophytes	Gametophyte	No	No
Seedless Vascular Plants	Sporophyte	Yes	No
Gymnosperms	Sporophyte	Yes	Yes
Angiosperms	Sporophyte	Yes	Yes

Multicellular, Dependent Embryos

Plants are embryophytes because the embryo is dependent on the parent plant. The fertilized egg develops into an embryo within the tissue of the gametophyte, where it is protected and nourished.

Walled Spores Produced in Sporangia

Sporophytes have multicellular organs called sporangia that produce spores. The walls of spores contain sporopollenin, a polymer that protects spores from harsh environments and prevents desiccation.

Apical Meristems

Apical meristems are localized regions of cell division found at the tips of roots and shoots. They enable plants to grow in length and increase exposure to resources.

Other Derived Traits

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

• Cuticle: Waxy covering that prevents water loss from the epidermis.

26.2 – Fungi Played a Key Role in the Colonization of Land

Fungi formed mutualistic relationships with early plants, aiding in nutrient absorption and adaptation to terrestrial environments.

• Mycorrhizal Relationships: Symbiotic associations between fungi and plant roots.

• Fungi as Decomposers: Fungi secrete enzymes to break down food and absorb nutrients through filaments called hyphae.

• Fungal Cell Walls: Composed of chitin, which prevents cells from bursting.

• Types of Mycorrhizal Fungi:

  • Arbuscular mycorrhizae: Hyphae extend into root cells.

  • Ectomycorrhizae: Hyphae form on the surface of roots and grow into the root cortex.

26.3 – Early Plants Radiated into a Diverse Set of Lineages

As plants adapted to terrestrial environments, they diversified into major lineages based on the presence or absence of vascular tissue.

• Vascular Tissue: Specialized tissue for transporting water and nutrients.

• Major Plant Groups:

  • Nonvascular plants (Bryophytes)

  • Vascular plants

Bryophytes

Bryophytes are nonvascular plants that dominated the first 100 million years of plant evolution. They are typically small, herbaceous plants found in moist habitats and anchored by rhizoids. The gametophyte is the dominant stage in their life cycle.

Seedless Vascular Plants

Seedless vascular plants, such as lycophytes and monilophytes, have vascular tissue but lack seeds. Their sporophyte generation is dominant, and they have adaptations for growing tall and transporting water and nutrients efficiently.

• Types of Vascular Tissue:

  • Xylem: Conducts water and minerals through tube-shaped cells.

  • Phloem: Transports sugars, amino acids, and other molecules.

• Evolution of Roots and Leaves:

  • Roots: Anchor plants and absorb water/nutrients.

  • Leaves: Primary photosynthetic organ; can be microphylls (single vein) or megaphylls (highly branched veins).

26.4 – Seeds and Pollen Grains are Key Adaptations of Life on Land

Seed plants originated around 360 million years ago and include two major clades: gymnosperms and angiosperms. Seeds and pollen grains are crucial adaptations for terrestrial life.

• Reduced Gametophytes: Gametophytes develop from spores retained in tissues of the parent sporophyte, providing protection and nourishment.

• Ovules and Pollen:

  • Ovule: Consists of an egg-producing female gametophyte surrounded by protective tissue.

  • Pollen: Male gametophyte surrounded by a protective wall.

• Seeds: Whole ovule develops into a seed, which is packaged in a protective coat made of integument. Seeds facilitate dispersal, can remain dormant, and have their own food supply.

Gymnosperms

Gymnosperms were the first seed plants, dominating terrestrial vegetation during the Permian period. They have "naked" seeds not enclosed in fruit.

Angiosperms

Angiosperms are the most diverse group of plants, making up 90% of extant species. They have specialized floral organs and adaptations for life on dry land, including flowers and fruits.

• Flower: Specialized for sexual reproduction.

• Fruit: Develops from the ovary and aids in seed dispersal.

Recap: Comparison of Major Plant Groups

Feature	Bryophytes	Seedless Vascular Plants	Gymnosperms	Angiosperms
Dominant Generation	Gametophyte	Sporophyte	Sporophyte	Sporophyte
Vascular Tissue	No	Yes	Yes	Yes
Seeds	No	No	Yes	Yes
Flowers	No	No	No	Yes

Additional info: These notes expand on the original slides by providing definitions, examples, and context for key terms and concepts, ensuring a comprehensive understanding of plant evolution and adaptation to terrestrial life.