Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 30.1 Seeds and Pollen Grains Are Key Adaptations for Life on Land
Seed plants—gymnosperms and angiosperms—evolved key innovations that freed them from the water-dependent reproduction of earlier plants. Seeds and pollen grains enabled reproduction in drier environments and improved embryo protection and dispersal, driving the dominance of seed plants in terrestrial ecosystems.
Advantages of Reduced Gametophytes
- In seed plants, gametophytes are tiny and dependent on the sporophyte for nutrients and protection.
- Gametophytes develop within the sporangia of the parent plant.
- This provides:
- Protection from UV radiation
- Moisture retention
- Nutrient supply from the sporophyte
- Unlike seedless plants, where gametophytes are independent and exposed, this setup enhances embryo fitness and environmental resilience.
Heterospory: Two Types of Spores
- Seed plants are heterosporous:
- Megasporangia → Megaspores → Female gametophytes
- Microsporangia → Microspores → Male gametophytes
- Each ovule contains a megasporangium, protected by layers of tissue called integuments:
- Gymnosperms: 1 integument
- Angiosperms: 2 integuments
Ovules and the Production of Eggs
- An ovule = megasporangium + megaspore + integuments
- The female gametophyte develops inside the ovule and produces eggs.
- The micropyle is an opening for pollen entry.
Pollen and the Production of Sperm
- A pollen grain contains the male gametophyte, enclosed in a protective wall with sporopollenin.
- Pollination: Transfer of pollen to the structure housing ovules.
- Upon germination, the pollen grain grows a pollen tube, delivering sperm to the egg—no water required.
- This eliminated the need for swimming sperm, a major breakthrough for colonizing dry habitats.
Evolutionary Advantages of Seeds
Seeds offer key benefits over spores:
- Multicellular structures with:
- An embryo
- Food supply
- Protective seed coat
- Can remain dormant for extended periods, sometimes centuries.
- Seeds can be dispersed long distances by wind or animals.
- Upon germination, stored nutrients support early growth.
Scientific Highlight: Ancient Date Seeds
- 2,000-year-old date palm seeds found near the Dead Sea were tested using carbon-14 dating.
- One seed germinated successfully, showing seed viability over millennia under favorable conditions.
- Seeds' long viability and protection make them far more robust than spores in many environments.
In a Nutshell
Seed plants evolved reproductive innovations that transformed life on land. Reduced gametophytes, protective ovules, pollen, and durable seeds enabled them to thrive in diverse, often dry environments. Seeds provide longevity, nourishment, and mobility—features that allowed seed plants to become the most successful plant group on Earth.