Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 29.1 Plants Evolved from Green Algae
Land plants evolved from a group of green algae known as charophytes. This transition, which began over 470 million years ago, required key adaptations that allowed plants to survive on land. These traits—including embryos, alternation of generations, and walled spores—define the group known as embryophytes.
Shared Ancestry with Charophytes
- Plants and green algae share:
- Multicellularity
- Cellulose cell walls
- Chloroplasts with chlorophyll a and b
- Charophytes are the closest relatives to land plants:
- Circular cellulose-synthesizing proteins
- Similar flagellated sperm
- DNA evidence supports the relationship
- Zygnematophyceae (e.g., Zygnema) are the closest known relatives of land plants.
Adaptations for Life on Land
- Advantages of land:
- Unfiltered sunlight
- Abundant CO₂
- Nutrient-rich soil
- Challenges:
- Scarce water
- No structural support against gravity
Key plant adaptations include:
- Sporopollenin: Protects zygotes and spores from desiccation.
- Alternation of generations:
- Gametophyte → gametes
- Fertilization → zygote → sporophyte
- Sporophyte → meiosis → spores → gametophyte
- Multicellular embryos:
- Develop inside gametophyte
- Nourished by maternal tissues
- Walled spores in sporangia
- Apical meristems: Zones of growth at tips
- Cuticle: Waxy coating to reduce water loss
- Stomata: Regulate gas exchange and water loss
Early Plant Fossils
- Fossilized spores and tissues (~450 MYA) resemble modern bryophytes.
- First large plant structures (~425 MYA), like Cooksonia.
Major Plant Groups
- Classified by vascular tissue and seeds:
| Group | Traits |
|---|---|
| Bryophytes | Nonvascular (e.g., mosses) |
| Seedless Vascular | Vascular, no seeds (e.g., ferns) |
| Gymnosperms | Vascular, seeds, no flowers |
| Angiosperms | Vascular, seeds, flowers |
- Vascular plants represent ~93% of plant species.
In a Nutshell
Land plants evolved from charophyte algae through a series of adaptations that enabled life on land. These include multicellular embryos, spore protection, meristems, and waxy cuticles. The resulting diversity gave rise to nonvascular bryophytes, seedless vascular plants, and seed plants—shaping ecosystems across the globe.