How Plants Colonized Land — Evolution, Adaptations, and Early Plant Diversity Explained | Chapter 29 of Campbell Biology

Welcome to Last Minute Lecture! In this summary of Chapter 29 from Campbell Biology, we explore one of the most profound evolutionary leaps: the colonization of land by plants. This chapter details how ancient green algae gave rise to the first land plants, what adaptations made terrestrial life possible, and how bryophytes and early vascular plants shaped Earth's ecosystems. Watch the full podcast summary here.

From Green Algae to Forests: The Evolution of Land Plants

Land plants evolved from charophyte green algae about 470 million years ago, sharing important traits like cellulose-synthesizing complexes and flagellated sperm. The move to land presented new challenges—desiccation, nutrient uptake, and structural support—that drove the evolution of specialized plant adaptations.

• Alternation of Generations: Plant life cycles alternate between haploid (gametophyte) and diploid (sporophyte) multicellular stages, allowing for greater genetic diversity and adaptation.
• Multicellular, Dependent Embryos: Developing embryos are protected and nourished within parental tissues.
• Walled Spores in Sporangia: Durable sporopollenin coats protect spores from drying out.
• Apical Meristems: Regions at root and shoot tips enable plants to grow continuously and reach resources.
• Cuticle & Stomata: A waxy cuticle reduces water loss, while stomata control gas exchange.
• Mycorrhizae: Symbiotic fungi help plants absorb nutrients from poor soils.

Bryophytes: Pioneers of Land

The earliest land plants were bryophytes—nonvascular plants including mosses, liverworts, and hornworts. Bryophytes lack true roots and vascular tissue, absorbing water directly through their surfaces. Their life cycle is dominated by the gametophyte stage, with sperm swimming to eggs in a film of water. Bryophytes are critical in soil formation and carbon sequestration, especially Sphagnum (peat moss), which influences global climate by storing carbon.

• Liverworts (Marchantia): Early land plants, reproduce by gemmae cups.
• Mosses (Sphagnum): Store carbon, create peat bogs, aid soil formation.
• Hornworts (Anthoceros): Unique elongated sporophytes, contribute to nitrogen fixation.

Seedless Vascular Plants: Reaching New Heights

Vascular plants evolved about 425 million years ago, allowing for taller growth and larger, more complex bodies. Their sporophyte stage became dominant and independent, with specialized vascular tissues for transporting water (xylem) and nutrients (phloem).

• Xylem & Phloem: Xylem’s lignin-reinforced tracheids transport water and support tall growth; phloem distributes sugars and nutrients.
• Roots & Leaves: True roots anchor plants and absorb water; leaves (megaphylls and microphylls) maximize photosynthesis.
• Sporophylls & Spore Types: Many seedless vascular plants are homosporous (one spore type); others, like seed plants, are heterosporous (separate male and female spores).

Key Adaptations for Terrestrial Life

• Cuticle: Reduces water loss.
• Stomata: Pores that regulate gas exchange and water evaporation.
• Lignin: Structural polymer strengthening cell walls for vertical growth.
• Rhizoids: Root-like structures for anchorage (in bryophytes).

Glossary of Key Terms

• Alternation of Generations: Life cycle alternating between haploid (gametophyte) and diploid (sporophyte) stages.
• Bryophytes: Nonvascular plants—mosses, liverworts, hornworts.
• Apical Meristems: Growth regions at root and shoot tips.
• Cuticle: Waxy covering reducing water loss.
• Gametangia: Organs (archegonia, antheridia) producing gametes.
• Lignin: Compound providing rigidity to xylem.
• Rhizoids: Anchor structures in bryophytes.
• Sporangia: Structures producing spores.
• Tracheids: Water-conducting xylem cells.
• Xylem & Phloem: Vascular tissues for water and nutrient transport.

Conclusion: The Legacy of Early Land Plants

The evolution of land plants was a game changer for Earth’s ecosystems, paving the way for forests, diverse habitats, and terrestrial food webs. From humble beginnings as algae-like ancestors, plants have become the backbone of life on land. For a detailed video walkthrough and more study guides, subscribe to Last Minute Lecture!