Lecture 23: Seed Plants - Gymnosperms

Seed Plants

Seed plants dominate today's flora, offering superior protection and nourishment for the next generation compared to spores, along with enhanced dispersal mechanisms.

Carboniferous Period

During the Carboniferous Period, "seed ferns" emerged, setting the stage for future plant evolution.

Key Evolutionary Developments in Seed Plants

Seed plants exhibit significant evolutionary changes, including heterospory, the development of pollen, ovules, and seeds. A variety of plant groups showcase these adaptations, including:

• Red algae

• Chlorophytes

• Charophytes

• Liverworts

• Mosses

• Hornworts

• Lycophytes

• Monilophytes

• Equisetum

• Ferns

• Psilotum

• Ginkgo

• Cycad

• Conifers (Gymnosperms)

• Gnetophytes (Gymnosperms)

• Basal Angiosperms

• Monocots (Angiosperms)

• Eudicots (Angiosperms)

Seed Plant Life Cycle

Seed plants exhibit alternation of generations between a sporophyte (2n) and a gametophyte (n) phase.

• Mega- and microsporangia are produced on strobili (cones).

• Seed plants exhibit heterospory by producing microstrobili (male cones) and megastrobili (female cones).

• Monoecious plants produce both male and female cones on the same individual.

• Dioecious plants have separate individuals producing either male or female cones.

Unlike seedless vascular plants, the spore is not dispersed. The gametophytes develop within the spore wall (endospory) and are retained within the tissues of the parent sporophyte.

Comparison of Dominant Generations

• Mosses and other nonvascular plants: Gametophyte dominant, sporophyte reduced and dependent.

• Ferns and other seedless vascular plants: Sporophyte dominant, gametophyte reduced but independent (photosynthetic and free-living).

• Seed plants (gymnosperms and angiosperms): Sporophyte dominant, gametophytes reduced (usually microscopic) and dependent on surrounding sporophyte tissue for nutrition.

Detailed Seed Plant Life Cycle

1. Mature Sporophyte (2n): Produces ovulate cones (female) and pollen cones (male).

2. Ovulate Cone: Contains an ovule with an integument (2n) surrounding a megasporangium (2n).

3. Megasporangium: Contains a megasporocyte (2n) that undergoes meiosis to produce a megaspore (n).

4. Megaspore: Develops within the megasporangium, surrounded by the integument, forming the ovule. The ovule has a micropyle for pollen entry.

5. Pollen Cone: Contains microsporangia (2n).

6. Microsporangia: Contains microsporocytes (2n) that undergo meiosis to produce microspores (n), which develop into pollen grains (n).

7. Fertilization: Pollen grain lands on the ovule, delivering sperm.

8. Post-Fertilization: The ovule develops into a seed containing the embryo (new sporophyte, 2n), food supply (n), and seed coat (2n).

Egg is within archegonium, which is within the megagametophyte, which is within the megasporangium.

Pollen

Pollen is a crucial adaptation in seed plants, eliminating the need for water for fertilization. Gymnosperm pollen grains often have