Overview
This lecture covers the evolutionary significance, adaptations, reproductive strategies, and classification of seed plants, focusing on gymnosperms and their life cycles.
Seed Plant Innovations
- Seed plants transformed terrestrial ecosystems as primary producers.
- Major adaptations include reduced gametophyte size, retention in sporophyte tissue, and origin of seeds.
- Seeds protect the embryo with a seed coat and provide a food supply (endosperm).
- Seeds replaced spores as the main dispersal method due to their hardiness.
- Pollen evolution allowed fertilization without water; pollen grains carry sperm nuclei.
- Seed plants are heterosporous (produce megaspores and microspores).
Importance of Sporophyte Dominance
- The sporophyte (diploid) generation became dominant in seed plants.
- Diploidy offers protection against mutations from increased terrestrial UV radiation.
Structure and Function of the Ovule and Seed
- The ovule consists of the integument, megasporangium (nucellus), and megaspore.
- Upon fertilization, the ovule becomes a seed with an embryo, food supply, and seed coat from integuments.
- Female gametophyte provides nutritive tissue for the developing embryo.
Gymnosperms: Classification and Features
- Gymnosperms have "naked seeds" not enclosed in an ovary.
- Four main living groups: Cycadophyta (cycads), Ginkgophyta (ginkgo), Gnetophyta, and Coniferophyta (conifers).
- Cycads and Ginkgo biloba have flagellated sperm; other gymnosperms and angiosperms do not.
Gymnosperm Groups and Examples
- Cycadophyta: Palm-like, slow-growing, many endangered; Florida Coontie (Zamia integrifolia) is native.
- Ginkgophyta: Only one species (Ginkgo biloba), tolerant to cold and pollution, distinctive fan-shaped leaves.
- Gnetophyta: Includes Gnetum (trees/vines), Ephedra (produces ephedrine), and Welwitschia (desert plant).
- Coniferophyta: Cone-bearing, largest group, includes pines, firs, spruce, cypress, redwoods; adapted to dry/cold climates.
Conifer Life Cycle
- Conifers produce separate male (pollen/microsporangiate/staminate) and female (ovulate/megaspore/pistillate) cones.
- Male cones produce microspores → pollen grains (male gametophyte).
- Female cones contain ovules; fertilization occurs after pollination via pollen tube.
- Life cycle takes nearly 3 years: pollination, then fertilization after over a year, leading to seed formation and dispersal.
Key Terms & Definitions
- Sporophyte — Diploid, dominant life stage in seed plants.
- Gametophyte — Haploid, produces gametes; greatly reduced in seed plants.
- Ovule — Structure containing integument, megasporangium, and megaspore; develops into a seed after fertilization.
- Integument — Outer layer that forms the seed coat.
- Pollen grain — Male gametophyte that delivers sperm nuclei to ovule.
- Heterosporous — Produces two types of spores: megaspores (female) and microspores (male).
Action Items / Next Steps
- Review diagrams of the pine (Pinus) life cycle.
- Learn examples and characteristics of each gymnosperm group.
- Understand terminology for male and female cones.