Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 30.2 Gymnosperms Bear "Naked" Seeds, Typically on Cones
Gymnosperms are seed plants that do not produce flowers or fruits. Their "naked" seeds are exposed on cone scales rather than enclosed within ovaries. These adaptations, including seeds and pollen, allowed gymnosperms to thrive in dry environments and dominate landscapes long before angiosperms arose.
Key Adaptations in Gymnosperms
- Seeds: Develop on sporophylls (usually cones), not enclosed in ovaries.
- Pollen: Delivers sperm without water, enabling reproduction in drier conditions.
- Reduced gametophytes: Microscopic and develop within cone structures.
- These adaptations helped gymnosperms outcompete seedless plants during the drier Permian and Mesozoic eras.
Pine Life Cycle Overview
A pine tree (sporophyte) produces two types of cones:
- Pollen cones (male):
- Microsporangia contain microsporocytes → undergo meiosis → produce microspores → develop into pollen grains (male gametophytes).
- Ovulate cones (female):
- Each scale bears two ovules with megasporangia.
- Megasporocytes → meiosis → four haploid cells (only one survives as megaspore).
- Megaspore → female gametophyte with several archegonia (each producing an egg).
Pollination and Fertilization
- Wind carries pollen to the ovule.
- Pollen germinates and grows a pollen tube.
- Fertilization occurs over a year later, and one egg is fertilized to form a zygote.
- The ovule becomes a seed:
- Embryo (2n)
- Food reserves (female gametophyte, n)
- Seed coat (integument, 2n)
Evolutionary Rise of Gymnosperms
- First gymnosperm fossils date to 305 million years ago.
- Dominated during Permian and Mesozoic eras, thriving in dry conditions.
- Some gymnosperms were insect-pollinated as early as 110 million years ago.
- Replaced ferns and lycophytes in many ecosystems.
Gymnosperm Diversity (Four Phyla)
1. Cycadophyta
- Palm-like appearance; large cones
- Flagellated sperm
- Thrived in the Mesozoic ("Age of Cycads")
- Now most endangered plant group
2. Ginkgophyta
- Only species: Ginkgo biloba
- Deciduous with fan-shaped leaves
- Tolerant of pollution; seeds smell bad
- Has flagellated sperm
3. Gnetophyta
- Three genera: Gnetum, Ephedra, Welwitschia
- Diverse habitats (tropical to desert)
- Grouped by molecular evidence
- Some species produce compounds used medicinally
4. Coniferophyta (Conifers)
- Largest gymnosperm phylum (~600 species)
- Includes pines, redwoods, firs, junipers
- Mostly evergreen with needlelike or scalelike leaves
- Some examples:
- Douglas fir (timber)
- Sequoia (among largest organisms)
- Bristlecone pine (oldest living trees >5,000 years)
- Wollemi pine ("living fossil" found in 1994)
In a Nutshell
Gymnosperms reproduce using exposed seeds and wind-dispersed pollen—adaptations that allowed them to flourish in dry climates before flowering plants arose. With reduced gametophytes and protective seeds, they dominated Mesozoic landscapes and still form critical ecosystems today, especially in northern forests.