Ch.30 - Higher Plants (Bio)

Introduction to Seed Plants
- Seed plants include gymnosperms and angiosperms.
- Characterized by the presence of seeds, which serve as a reproductive unit.
Sporophyte and Gametophyte Stages
- In seed plants, the sporophyte stage is dominant, contrasting with the gametophyte stage observed in mosses.
- Dominance shift:
- Moss: Gametophyte dominant, sporophyte dependent.
- Seed plants: Sporophyte (dominant) and gametophyte (reduced to a few cells).
- Example: Transition from gametophyte stage in moss to the heterosporous generation in seed plants.

Definition:
- Heterospory refers to the production of two distinct types of spores: microspores (male) and megaspores (female).
Microspores and Megaspores:
- Microspores give rise to male gametophytes, while megaspores develop into female gametophytes.

Pollen:
- Pollen represents the male gametophyte and is composed of few cells.
- Functions as a tool for pollination.
Female Gametophyte:
- Resides within structures called ovules, which can be found in cones or flowers.
- Eventually matures into a seed post-fertilization.

Megasporangium and Microsporangium:
- - Megasporangium: Produces a mega spore which develops into a megagametophyte (female) responsible for egg production.
- Microsporangium: Produces microspores, leading to male gametophytes.
Comparison:
- Male gametophytes often produce numerous pollen grains, contributing to pollen dispersal.

Life Cycle:
- Seed development takes considerable time, with a complete cycle potentially taking over a year for certain plant types (e.g., conifers).
- Seed development involves the maturation of the male and female structures into zygotes post-fertilization.

Pollination:
- Pollination is the process where pollen is transferred to the female structure, initiating the potential for fertilization.
- Distinction: Pollination precedes fertilization, requiring the pollen to reach the ovule.
Wind vs. Animal Pollination:
- Pollination by wind is common in gymnosperms, while angiosperms often utilize animal assistance for pollen transfer.

Key Differences:
- Gymnosperms produce naked seeds while angiosperms have seeds enclosed in fruits.
- Angiosperms are generally more diverse and dominate modern ecosystems, comprising approximately 90% of plant species.

Parts of a Flower:
- Stigma: The sticky tip that receives pollen.
- Style: The stalk between the stigma and ovary.
- Ovary: Contains ovules that will become seeds post-fertilization.
- Petals and Sepals: Modified leaf structures that are often colorful to attract pollinators.
- Stamens: Produce pollen (microspores).

Seed Structure:
- Embryo: Develops into a new plant.
- Integument: Protects the seed (becomes the seed coat).
- Female gametophyte: Sources nutrients for the embryo.

Seeds utilize various strategies to disseminate, including wind and animal interactions post-digestion of fruits.
- Example: Apples and berries are typically consumed by animals, facilitating dispersal.
Harsh Conditions: Seeds can enter dormancy, allowing them to survive adverse conditions until the environment improves.

Ephedra: Provides the active ingredient pseudoephedrine, used in decongestants.
Taxol: Extracted from the yew plant, utilized as a chemotherapy agent.

Cycads:
- Palm-like leaves, dioecious reproductive structures.
- Flagellated sperm and co-evolution with certain animal pollinators.
Ginkgo: Only one living species (Ginkgo biloba), known for its fan-shaped leaves, dioecious and flagellated sperm.
- Used for supplements and resilient to pollution.
Gnetophytes: Unique features, including variations in leaves and cones resistant to dry conditions.
Conifers: The most varied and successful group, includes pines and firs known for their woody structure and needles.

Seed plants are crucial for ecosystems and human economy through their diverse uses and roles in habitat structures.
Understanding the complex life cycles and the diversity within gymnosperms is essential for studying plant biology and ecology.