Chapter 26 – The Colonization of Land: Evolution and Adaptations of Early Land Plants

Chapter 26 – The Colonization of Land

26.1 – Evidence of Algal Ancestry

Land plants share many characteristics with algae, indicating a common evolutionary origin. This section explores the evidence supporting the algal ancestry of plants and highlights unique plant traits.

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

• Shared Traits with Algae:

  • Plants are multicellular, eukaryotic, and autotrophic (produce their own food via photosynthesis).

  • Plant cell walls are made of cellulose, a trait shared with some algae.

  • Plants contain chlorophyll a and b as their primary photosynthetic pigments.

• Unique Traits Shared with Charophytes:

  • Rings of proteins in the plasma membrane that synthesize cellulose (not found in other algae).

  • Flagellated sperm with a similar structure.

• Charophytes: The only extant (living) group of green algae that shares these unique traits with land plants, making them the closest relatives to land plants.

Example: Chara and Coleochaete are genera of charophytes that exhibit these plant-like features.

26.1 – The Movement to Land

The transition from aquatic to terrestrial life presented both opportunities and challenges for early plants. This section discusses the environmental factors that influenced the colonization of land.

• Advantages of Living on Land:

  • Brighter sunlight for photosynthesis.

  • Plentiful carbon dioxide (CO2) in the atmosphere.

  • Abundant mineral nutrients in the soil.

• Challenges of Terrestrial Life:

  • Gravity: Need for structural support to stand upright.

  • Desiccation: Risk of drying out due to exposure to air.

  • Obtaining water and nutrients from the soil.

  • Reproduction without water as a medium for gamete dispersal.

  • Reliable water source is less predictable than in aquatic environments.

Example: Early land plants developed adaptations such as waxy cuticles and stomata to reduce water loss.

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 reproduce on land.

• Key Derived Traits:

  • Alternation of generations: A life cycle with distinct multicellular haploid and diploid stages.

  • Multicellular, dependent embryos: Embryos are retained and nourished by the parent plant.

  • Walled spores produced in sporangia: Spores have tough walls containing sporopollenin, protecting them from desiccation.

  • Multicellular gametangia: Organs that produce gametes (archegonia for eggs, antheridia for sperm).

  • Apical meristems: Regions of cell division at the tips of roots and shoots, allowing growth in length.

Example: Mosses and ferns exhibit alternation of generations and produce spores in sporangia.

26.1 – Alternation of Generations

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

• Definitions:

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

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

• Process:

  • Gametes (n) fuse to form a zygote (2n), which develops into the sporophyte.

  • The sporophyte produces spores (n) by meiosis.

  • Each spore grows into a new gametophyte, completing the cycle.

Equation for Alternation of Generations:

$\text{Gametophyte (n)} \xrightarrow{\text{mitosis}} \text{Gametes (n)} \xrightarrow{\text{fertilization}} \text{Zygote (2n)} \xrightarrow{\text{mitosis}} \text{Sporophyte (2n)} \xrightarrow{\text{meiosis}} \text{Spores (n)} \xrightarrow{\text{mitosis}} \text{Gametophyte (n)}$

Differences Among Plant Groups in Alternation of Generations

Plant groups differ in the dominance and independence of the gametophyte and sporophyte generations.

Plant Group	Dominant Generation	Relationship Between Generations
Bryophytes (mosses, liverworts, hornworts)	Gametophyte	Sporophyte is dependent on gametophyte
Seedless vascular plants (ferns, lycophytes)	Sporophyte	Both generations are independent, but sporophyte is larger and longer-lived
Seed plants (gymnosperms, angiosperms)	Sporophyte	Gametophyte is reduced and dependent on sporophyte

Example: In mosses, the green, photosynthetic structure is the gametophyte, while the sporophyte is a stalk with a capsule that grows from it.